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docs: Sphinxify docs/tutorial/

Browse Source 
Sorry for the massive commit, but I just wanted to knock this one down
and it is really straightforward.

There are still a couple trivial (i.e. not related to the content)
things left to fix:

- Use of raw HTML links where :doc:`...` and :ref:`...` could be used
  instead. If you are a newbie and want to help fix this it would make
  for some good bite-sized patches; more experienced developers should
  be focusing on adding new content (to this tutorial or elsewhere, but
  please _do not_ waste your time on formatting when there is such dire
  need for documentation (see docs/SphinxQuickstartTemplate.rst to get
  started writing)).

- Highlighting of the kaleidoscope code blocks (currently left as bare
  `::`).  I will be working on writing a custom Pygments highlighter for
  this, mostly as training for maintaining the `llvm` code-block's lexer
  in-tree. I want to do this because I am extremely unhappy with how it
  just "gives up" on the slightest deviation from the expected syntax
  and leaves the whole code-block un-highlighted.

  More generally I am looking at writing some Sphinx extensions and
  keeping them in-tree as well, to support common use cases that
  currently have no good solution (like "monospace text inside a link").

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169343 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sean Silva 2012-12-05 00:26:32 +00:00
parent 4e54480531
commit ee47edfd8e
34 changed files with 17107 additions and 19075 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
docs/Makefile.sphinx
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@ -50,8 +50,6 @@ html:
@# Kind of a hack, but HTML-formatted docs are on the way out anyway.
@echo "Copying legacy HTML-formatted docs into $(BUILDDIR)/html"
@cp -a *.html $(BUILDDIR)/html
@mkdir -p $(BUILDDIR)/html/tutorial
@cp tutorial/*.html tutorial/*.png $(BUILDDIR)/html/tutorial
@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
dirhtml:

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@ -1,348 +0,0 @@
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Kaleidoscope: Tutorial Introduction and the Lexer</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="Chris Lattner">
<link rel="stylesheet" href="../_static/llvm.css" type="text/css">
</head>
<body>
<h1>Kaleidoscope: Tutorial Introduction and the Lexer</h1>
<ul>
<li><a href="index.html">Up to Tutorial Index</a></li>
<li>Chapter 1
<ol>
<li><a href="#intro">Tutorial Introduction</a></li>
<li><a href="#language">The Basic Language</a></li>
<li><a href="#lexer">The Lexer</a></li>
</ol>
</li>
<li><a href="LangImpl2.html">Chapter 2</a>: Implementing a Parser and AST</li>
</ul>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>
<!-- *********************************************************************** -->
<h2><a name="intro">Tutorial Introduction</a></h2>
<!-- *********************************************************************** -->
<div>
<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
runs through the implementation of a simple language, showing how fun and
easy it can be. This tutorial will get you up and started as well as help to
build a framework you can extend to other languages. The code in this tutorial
can also be used as a playground to hack on other LLVM specific things.
</p>
<p>
The goal of this tutorial is to progressively unveil our language, describing
how it is built up over time. This will let us cover a fairly broad range of
language design and LLVM-specific usage issues, showing and explaining the code
for it all along the way, without overwhelming you with tons of details up
front.</p>
<p>It is useful to point out ahead of time that this tutorial is really about
teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
modern and sane software engineering principles. In practice, this means that
we'll take a number of shortcuts to simplify the exposition. For example, the
code leaks memory, uses global variables all over the place, doesn't use nice
design patterns like <a
href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
is very simple. If you dig in and use the code as a basis for future projects,
fixing these deficiencies shouldn't be hard.</p>
<p>I've tried to put this tutorial together in a way that makes chapters easy to
skip over if you are already familiar with or are uninterested in the various
pieces. The structure of the tutorial is:
</p>
<ul>
<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
language, and the definition of its Lexer</b> - This shows where we are going
and the basic functionality that we want it to do. In order to make this
tutorial maximally understandable and hackable, we choose to implement
everything in C++ instead of using lexer and parser generators. LLVM obviously
works just fine with such tools, feel free to use one if you prefer.</li>
<li><b><a href="LangImpl2.html">Chapter #2</a>: Implementing a Parser and
AST</b> - With the lexer in place, we can talk about parsing techniques and
basic AST construction. This tutorial describes recursive descent parsing and
operator precedence parsing. Nothing in Chapters 1 or 2 is LLVM-specific,
the code doesn't even link in LLVM at this point. :)</li>
<li><b><a href="LangImpl3.html">Chapter #3</a>: Code generation to LLVM IR</b> -
With the AST ready, we can show off how easy generation of LLVM IR really
is.</li>
<li><b><a href="LangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
Support</b> - Because a lot of people are interested in using LLVM as a JIT,
we'll dive right into it and show you the 3 lines it takes to add JIT support.
LLVM is also useful in many other ways, but this is one simple and "sexy" way
to shows off its power. :)</li>
<li><b><a href="LangImpl5.html">Chapter #5</a>: Extending the Language: Control
Flow</b> - With the language up and running, we show how to extend it with
control flow operations (if/then/else and a 'for' loop). This gives us a chance
to talk about simple SSA construction and control flow.</li>
<li><b><a href="LangImpl6.html">Chapter #6</a>: Extending the Language:
User-defined Operators</b> - This is a silly but fun chapter that talks about
extending the language to let the user program define their own arbitrary
unary and binary operators (with assignable precedence!). This lets us build a
significant piece of the "language" as library routines.</li>
<li><b><a href="LangImpl7.html">Chapter #7</a>: Extending the Language: Mutable
Variables</b> - This chapter talks about adding user-defined local variables
along with an assignment operator. The interesting part about this is how
easy and trivial it is to construct SSA form in LLVM: no, LLVM does <em>not</em>
require your front-end to construct SSA form!</li>
<li><b><a href="LangImpl8.html">Chapter #8</a>: Conclusion and other useful LLVM
tidbits</b> - This chapter wraps up the series by talking about potential
ways to extend the language, but also includes a bunch of pointers to info about
"special topics" like adding garbage collection support, exceptions, debugging,
support for "spaghetti stacks", and a bunch of other tips and tricks.</li>
</ul>
<p>By the end of the tutorial, we'll have written a bit less than 700 lines of
non-comment, non-blank, lines of code. With this small amount of code, we'll
have built up a very reasonable compiler for a non-trivial language including
a hand-written lexer, parser, AST, as well as code generation support with a JIT
compiler. While other systems may have interesting "hello world" tutorials,
I think the breadth of this tutorial is a great testament to the strengths of
LLVM and why you should consider it if you're interested in language or compiler
design.</p>
<p>A note about this tutorial: we expect you to extend the language and play
with it on your own. Take the code and go crazy hacking away at it, compilers
don't need to be scary creatures - it can be a lot of fun to play with
languages!</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="language">The Basic Language</a></h2>
<!-- *********************************************************************** -->
<div>
<p>This tutorial will be illustrated with a toy language that we'll call
"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived
from "meaning beautiful, form, and view").
Kaleidoscope is a procedural language that allows you to define functions, use
conditionals, math, etc. Over the course of the tutorial, we'll extend
Kaleidoscope to support the if/then/else construct, a for loop, user defined
operators, JIT compilation with a simple command line interface, etc.</p>
<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
64-bit floating point type (aka 'double' in C parlance). As such, all values
are implicitly double precision and the language doesn't require type
declarations. This gives the language a very nice and simple syntax. For
example, the following simple example computes <a
href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
<div class="doc_code">
<pre>
# Compute the x'th fibonacci number.
def fib(x)
if x &lt; 3 then
1
else
fib(x-1)+fib(x-2)
# This expression will compute the 40th number.
fib(40)
</pre>
</div>
<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
JIT makes this completely trivial). This means that you can use the 'extern'
keyword to define a function before you use it (this is also useful for mutually
recursive functions). For example:</p>
<div class="doc_code">
<pre>
extern sin(arg);
extern cos(arg);
extern atan2(arg1 arg2);
atan2(sin(.4), cos(42))
</pre>
</div>
<p>A more interesting example is included in Chapter 6 where we write a little
Kaleidoscope application that <a href="LangImpl6.html#example">displays
a Mandelbrot Set</a> at various levels of magnification.</p>
<p>Lets dive into the implementation of this language!</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="lexer">The Lexer</a></h2>
<!-- *********************************************************************** -->
<div>
<p>When it comes to implementing a language, the first thing needed is
the ability to process a text file and recognize what it says. The traditional
way to do this is to use a "<a
href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
to break the input up into "tokens". Each token returned by the lexer includes
a token code and potentially some metadata (e.g. the numeric value of a number).
First, we define the possibilities:
</p>
<div class="doc_code">
<pre>
// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
// of these for known things.
enum Token {
tok_eof = -1,
// commands
tok_def = -2, tok_extern = -3,
// primary
tok_identifier = -4, tok_number = -5,
};
static std::string IdentifierStr; // Filled in if tok_identifier
static double NumVal; // Filled in if tok_number
</pre>
</div>
<p>Each token returned by our lexer will either be one of the Token enum values
or it will be an 'unknown' character like '+', which is returned as its ASCII
value. If the current token is an identifier, the <tt>IdentifierStr</tt>
global variable holds the name of the identifier. If the current token is a
numeric literal (like 1.0), <tt>NumVal</tt> holds its value. Note that we use
global variables for simplicity, this is not the best choice for a real language
implementation :).
</p>
<p>The actual implementation of the lexer is a single function named
<tt>gettok</tt>. The <tt>gettok</tt> function is called to return the next token
from standard input. Its definition starts as:</p>
<div class="doc_code">
<pre>
/// gettok - Return the next token from standard input.
static int gettok() {
static int LastChar = ' ';
// Skip any whitespace.
while (isspace(LastChar))
LastChar = getchar();
</pre>
</div>
<p>
<tt>gettok</tt> works by calling the C <tt>getchar()</tt> function to read
characters one at a time from standard input. It eats them as it recognizes
them and stores the last character read, but not processed, in LastChar. The
first thing that it has to do is ignore whitespace between tokens. This is
accomplished with the loop above.</p>
<p>The next thing <tt>gettok</tt> needs to do is recognize identifiers and
specific keywords like "def". Kaleidoscope does this with this simple loop:</p>
<div class="doc_code">
<pre>
if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
IdentifierStr = LastChar;
while (isalnum((LastChar = getchar())))
IdentifierStr += LastChar;
if (IdentifierStr == "def") return tok_def;
if (IdentifierStr == "extern") return tok_extern;
return tok_identifier;
}
</pre>
</div>
<p>Note that this code sets the '<tt>IdentifierStr</tt>' global whenever it
lexes an identifier. Also, since language keywords are matched by the same
loop, we handle them here inline. Numeric values are similar:</p>
<div class="doc_code">
<pre>
if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
std::string NumStr;
do {
NumStr += LastChar;
LastChar = getchar();
} while (isdigit(LastChar) || LastChar == '.');
NumVal = strtod(NumStr.c_str(), 0);
return tok_number;
}
</pre>
</div>
<p>This is all pretty straight-forward code for processing input. When reading
a numeric value from input, we use the C <tt>strtod</tt> function to convert it
to a numeric value that we store in <tt>NumVal</tt>. Note that this isn't doing
sufficient error checking: it will incorrectly read "1.23.45.67" and handle it as
if you typed in "1.23". Feel free to extend it :). Next we handle comments:
</p>
<div class="doc_code">
<pre>
if (LastChar == '#') {
// Comment until end of line.
do LastChar = getchar();
while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
if (LastChar != EOF)
return gettok();
}
</pre>
</div>
<p>We handle comments by skipping to the end of the line and then return the
next token. Finally, if the input doesn't match one of the above cases, it is
either an operator character like '+' or the end of the file. These are handled
with this code:</p>
<div class="doc_code">
<pre>
// Check for end of file. Don't eat the EOF.
if (LastChar == EOF)
return tok_eof;
// Otherwise, just return the character as its ascii value.
int ThisChar = LastChar;
LastChar = getchar();
return ThisChar;
}
</pre>
</div>
<p>With this, we have the complete lexer for the basic Kaleidoscope language
(the <a href="LangImpl2.html#code">full code listing</a> for the Lexer is
available in the <a href="LangImpl2.html">next chapter</a> of the tutorial).
Next we'll <a href="LangImpl2.html">build a simple parser that uses this to
build an Abstract Syntax Tree</a>. When we have that, we'll include a driver
so that you can use the lexer and parser together.
</p>
<a href="LangImpl2.html">Next: Implementing a Parser and AST</a>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
<a href="http://validator.w3.org/check/referer"><img
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
</body>
</html>

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@ -0,0 +1,280 @@
=================================================
Kaleidoscope: Tutorial Introduction and the Lexer
=================================================
.. contents::
:local:
Written by `Chris Lattner <mailto:sabre@nondot.org>`_
Tutorial Introduction
=====================
Welcome to the "Implementing a language with LLVM" tutorial. This
tutorial runs through the implementation of a simple language, showing
how fun and easy it can be. This tutorial will get you up and started as
well as help to build a framework you can extend to other languages. The
code in this tutorial can also be used as a playground to hack on other
LLVM specific things.
The goal of this tutorial is to progressively unveil our language,
describing how it is built up over time. This will let us cover a fairly
broad range of language design and LLVM-specific usage issues, showing
and explaining the code for it all along the way, without overwhelming
you with tons of details up front.
It is useful to point out ahead of time that this tutorial is really
about teaching compiler techniques and LLVM specifically, *not* about
teaching modern and sane software engineering principles. In practice,
this means that we'll take a number of shortcuts to simplify the
exposition. For example, the code leaks memory, uses global variables
all over the place, doesn't use nice design patterns like
`visitors <http://en.wikipedia.org/wiki/Visitor_pattern>`_, etc... but
it is very simple. If you dig in and use the code as a basis for future
projects, fixing these deficiencies shouldn't be hard.
I've tried to put this tutorial together in a way that makes chapters
easy to skip over if you are already familiar with or are uninterested
in the various pieces. The structure of the tutorial is:
- `Chapter #1 <#language>`_: Introduction to the Kaleidoscope
language, and the definition of its Lexer - This shows where we are
going and the basic functionality that we want it to do. In order to
make this tutorial maximally understandable and hackable, we choose
to implement everything in C++ instead of using lexer and parser
generators. LLVM obviously works just fine with such tools, feel free
to use one if you prefer.
- `Chapter #2 <LangImpl2.html>`_: Implementing a Parser and AST -
With the lexer in place, we can talk about parsing techniques and
basic AST construction. This tutorial describes recursive descent
parsing and operator precedence parsing. Nothing in Chapters 1 or 2
is LLVM-specific, the code doesn't even link in LLVM at this point.
:)
- `Chapter #3 <LangImpl3.html>`_: Code generation to LLVM IR - With
the AST ready, we can show off how easy generation of LLVM IR really
is.
- `Chapter #4 <LangImpl4.html>`_: Adding JIT and Optimizer Support
- Because a lot of people are interested in using LLVM as a JIT,
we'll dive right into it and show you the 3 lines it takes to add JIT
support. LLVM is also useful in many other ways, but this is one
simple and "sexy" way to shows off its power. :)
- `Chapter #5 <LangImpl5.html>`_: Extending the Language: Control
Flow - With the language up and running, we show how to extend it
with control flow operations (if/then/else and a 'for' loop). This
gives us a chance to talk about simple SSA construction and control
flow.
- `Chapter #6 <LangImpl6.html>`_: Extending the Language:
User-defined Operators - This is a silly but fun chapter that talks
about extending the language to let the user program define their own
arbitrary unary and binary operators (with assignable precedence!).
This lets us build a significant piece of the "language" as library
routines.
- `Chapter #7 <LangImpl7.html>`_: Extending the Language: Mutable
Variables - This chapter talks about adding user-defined local
variables along with an assignment operator. The interesting part
about this is how easy and trivial it is to construct SSA form in
LLVM: no, LLVM does *not* require your front-end to construct SSA
form!
- `Chapter #8 <LangImpl8.html>`_: Conclusion and other useful LLVM
tidbits - This chapter wraps up the series by talking about
potential ways to extend the language, but also includes a bunch of
pointers to info about "special topics" like adding garbage
collection support, exceptions, debugging, support for "spaghetti
stacks", and a bunch of other tips and tricks.
By the end of the tutorial, we'll have written a bit less than 700 lines
of non-comment, non-blank, lines of code. With this small amount of
code, we'll have built up a very reasonable compiler for a non-trivial
language including a hand-written lexer, parser, AST, as well as code
generation support with a JIT compiler. While other systems may have
interesting "hello world" tutorials, I think the breadth of this
tutorial is a great testament to the strengths of LLVM and why you
should consider it if you're interested in language or compiler design.
A note about this tutorial: we expect you to extend the language and
play with it on your own. Take the code and go crazy hacking away at it,
compilers don't need to be scary creatures - it can be a lot of fun to
play with languages!
The Basic Language
==================
This tutorial will be illustrated with a toy language that we'll call
"`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived
from "meaning beautiful, form, and view"). Kaleidoscope is a procedural
language that allows you to define functions, use conditionals, math,
etc. Over the course of the tutorial, we'll extend Kaleidoscope to
support the if/then/else construct, a for loop, user defined operators,
JIT compilation with a simple command line interface, etc.
Because we want to keep things simple, the only datatype in Kaleidoscope
is a 64-bit floating point type (aka 'double' in C parlance). As such,
all values are implicitly double precision and the language doesn't
require type declarations. This gives the language a very nice and
simple syntax. For example, the following simple example computes
`Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_
::
# Compute the x'th fibonacci number.
def fib(x)
if x < 3 then
1
else
fib(x-1)+fib(x-2)
# This expression will compute the 40th number.
fib(40)
We also allow Kaleidoscope to call into standard library functions (the
LLVM JIT makes this completely trivial). This means that you can use the
'extern' keyword to define a function before you use it (this is also
useful for mutually recursive functions). For example:
::
extern sin(arg);
extern cos(arg);
extern atan2(arg1 arg2);
atan2(sin(.4), cos(42))
A more interesting example is included in Chapter 6 where we write a
little Kaleidoscope application that `displays a Mandelbrot
Set <LangImpl6.html#example>`_ at various levels of magnification.
Lets dive into the implementation of this language!
The Lexer
=========
When it comes to implementing a language, the first thing needed is the
ability to process a text file and recognize what it says. The
traditional way to do this is to use a
"`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka
'scanner') to break the input up into "tokens". Each token returned by
the lexer includes a token code and potentially some metadata (e.g. the
numeric value of a number). First, we define the possibilities:
.. code-block:: c++
// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
// of these for known things.
enum Token {
tok_eof = -1,
// commands
tok_def = -2, tok_extern = -3,
// primary
tok_identifier = -4, tok_number = -5,
};
static std::string IdentifierStr; // Filled in if tok_identifier
static double NumVal; // Filled in if tok_number
Each token returned by our lexer will either be one of the Token enum
values or it will be an 'unknown' character like '+', which is returned
as its ASCII value. If the current token is an identifier, the
``IdentifierStr`` global variable holds the name of the identifier. If
the current token is a numeric literal (like 1.0), ``NumVal`` holds its
value. Note that we use global variables for simplicity, this is not the
best choice for a real language implementation :).
The actual implementation of the lexer is a single function named
``gettok``. The ``gettok`` function is called to return the next token
from standard input. Its definition starts as:
.. code-block:: c++
/// gettok - Return the next token from standard input.
static int gettok() {
static int LastChar = ' ';
// Skip any whitespace.
while (isspace(LastChar))
LastChar = getchar();
``gettok`` works by calling the C ``getchar()`` function to read
characters one at a time from standard input. It eats them as it
recognizes them and stores the last character read, but not processed,
in LastChar. The first thing that it has to do is ignore whitespace
between tokens. This is accomplished with the loop above.
The next thing ``gettok`` needs to do is recognize identifiers and
specific keywords like "def". Kaleidoscope does this with this simple
loop:
.. code-block:: c++
if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
IdentifierStr = LastChar;
while (isalnum((LastChar = getchar())))
IdentifierStr += LastChar;
if (IdentifierStr == "def") return tok_def;
if (IdentifierStr == "extern") return tok_extern;
return tok_identifier;
}
Note that this code sets the '``IdentifierStr``' global whenever it
lexes an identifier. Also, since language keywords are matched by the
same loop, we handle them here inline. Numeric values are similar:
.. code-block:: c++
if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
std::string NumStr;
do {
NumStr += LastChar;
LastChar = getchar();
} while (isdigit(LastChar) || LastChar == '.');
NumVal = strtod(NumStr.c_str(), 0);
return tok_number;
}
This is all pretty straight-forward code for processing input. When
reading a numeric value from input, we use the C ``strtod`` function to
convert it to a numeric value that we store in ``NumVal``. Note that
this isn't doing sufficient error checking: it will incorrectly read
"1.23.45.67" and handle it as if you typed in "1.23". Feel free to
extend it :). Next we handle comments:
.. code-block:: c++
if (LastChar == '#') {
// Comment until end of line.
do LastChar = getchar();
while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
if (LastChar != EOF)
return gettok();
}
We handle comments by skipping to the end of the line and then return
the next token. Finally, if the input doesn't match one of the above
cases, it is either an operator character like '+' or the end of the
file. These are handled with this code:
.. code-block:: c++
// Check for end of file. Don't eat the EOF.
if (LastChar == EOF)
return tok_eof;
// Otherwise, just return the character as its ascii value.
int ThisChar = LastChar;
LastChar = getchar();
return ThisChar;
}
With this, we have the complete lexer for the basic Kaleidoscope
language (the `full code listing <LangImpl2.html#code>`_ for the Lexer
is available in the `next chapter <LangImpl2.html>`_ of the tutorial).
Next we'll `build a simple parser that uses this to build an Abstract
Syntax Tree <LangImpl2.html>`_. When we have that, we'll include a
driver so that you can use the lexer and parser together.
`Next: Implementing a Parser and AST <LangImpl2.html>`_

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="Chris Lattner">
<link rel="stylesheet" href="../_static/llvm.css" type="text/css">
</head>
<body>
<h1>Kaleidoscope: Conclusion and other useful LLVM tidbits</h1>
<ul>
<li><a href="index.html">Up to Tutorial Index</a></li>
<li>Chapter 8
<ol>
<li><a href="#conclusion">Tutorial Conclusion</a></li>
<li><a href="#llvmirproperties">Properties of LLVM IR</a>
<ul>
<li><a href="#targetindep">Target Independence</a></li>
<li><a href="#safety">Safety Guarantees</a></li>
<li><a href="#langspecific">Language-Specific Optimizations</a></li>
</ul>
</li>
<li><a href="#tipsandtricks">Tips and Tricks</a>
<ul>
<li><a href="#offsetofsizeof">Implementing portable
offsetof/sizeof</a></li>
<li><a href="#gcstack">Garbage Collected Stack Frames</a></li>
</ul>
</li>
</ol>
</li>
</ul>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>
<!-- *********************************************************************** -->
<h2><a name="conclusion">Tutorial Conclusion</a></h2>
<!-- *********************************************************************** -->
<div>
<p>Welcome to the final chapter of the "<a href="index.html">Implementing a
language with LLVM</a>" tutorial. In the course of this tutorial, we have grown
our little Kaleidoscope language from being a useless toy, to being a
semi-interesting (but probably still useless) toy. :)</p>
<p>It is interesting to see how far we've come, and how little code it has
taken. We built the entire lexer, parser, AST, code generator, and an
interactive run-loop (with a JIT!) by-hand in under 700 lines of
(non-comment/non-blank) code.</p>
<p>Our little language supports a couple of interesting features: it supports
user defined binary and unary operators, it uses JIT compilation for immediate
evaluation, and it supports a few control flow constructs with SSA construction.
</p>
<p>Part of the idea of this tutorial was to show you how easy and fun it can be
to define, build, and play with languages. Building a compiler need not be a
scary or mystical process! Now that you've seen some of the basics, I strongly
encourage you to take the code and hack on it. For example, try adding:</p>
<ul>
<li><b>global variables</b> - While global variables have questional value in
modern software engineering, they are often useful when putting together quick
little hacks like the Kaleidoscope compiler itself. Fortunately, our current
setup makes it very easy to add global variables: just have value lookup check
to see if an unresolved variable is in the global variable symbol table before
rejecting it. To create a new global variable, make an instance of the LLVM
<tt>GlobalVariable</tt> class.</li>
<li><b>typed variables</b> - Kaleidoscope currently only supports variables of
type double. This gives the language a very nice elegance, because only
supporting one type means that you never have to specify types. Different
languages have different ways of handling this. The easiest way is to require
the user to specify types for every variable definition, and record the type
of the variable in the symbol table along with its Value*.</li>
<li><b>arrays, structs, vectors, etc</b> - Once you add types, you can start
extending the type system in all sorts of interesting ways. Simple arrays are
very easy and are quite useful for many different applications. Adding them is
mostly an exercise in learning how the LLVM <a
href="../LangRef.html#i_getelementptr">getelementptr</a> instruction works: it
is so nifty/unconventional, it <a
href="../GetElementPtr.html">has its own FAQ</a>! If you add support
for recursive types (e.g. linked lists), make sure to read the <a
href="../ProgrammersManual.html#TypeResolve">section in the LLVM
Programmer's Manual</a> that describes how to construct them.</li>
<li><b>standard runtime</b> - Our current language allows the user to access
arbitrary external functions, and we use it for things like "printd" and
"putchard". As you extend the language to add higher-level constructs, often
these constructs make the most sense if they are lowered to calls into a
language-supplied runtime. For example, if you add hash tables to the language,
it would probably make sense to add the routines to a runtime, instead of
inlining them all the way.</li>
<li><b>memory management</b> - Currently we can only access the stack in
Kaleidoscope. It would also be useful to be able to allocate heap memory,
either with calls to the standard libc malloc/free interface or with a garbage
collector. If you would like to use garbage collection, note that LLVM fully
supports <a href="../GarbageCollection.html">Accurate Garbage Collection</a>
including algorithms that move objects and need to scan/update the stack.</li>
<li><b>debugger support</b> - LLVM supports generation of <a
href="../SourceLevelDebugging.html">DWARF Debug info</a> which is understood by
common debuggers like GDB. Adding support for debug info is fairly
straightforward. The best way to understand it is to compile some C/C++ code
with "<tt>llvm-gcc -g -O0</tt>" and taking a look at what it produces.</li>
<li><b>exception handling support</b> - LLVM supports generation of <a
href="../ExceptionHandling.html">zero cost exceptions</a> which interoperate
with code compiled in other languages. You could also generate code by
implicitly making every function return an error value and checking it. You
could also make explicit use of setjmp/longjmp. There are many different ways
to go here.</li>
<li><b>object orientation, generics, database access, complex numbers,
geometric programming, ...</b> - Really, there is
no end of crazy features that you can add to the language.</li>
<li><b>unusual domains</b> - We've been talking about applying LLVM to a domain
that many people are interested in: building a compiler for a specific language.
However, there are many other domains that can use compiler technology that are
not typically considered. For example, LLVM has been used to implement OpenGL
graphics acceleration, translate C++ code to ActionScript, and many other
cute and clever things. Maybe you will be the first to JIT compile a regular
expression interpreter into native code with LLVM?</li>
</ul>
<p>
Have fun - try doing something crazy and unusual. Building a language like
everyone else always has, is much less fun than trying something a little crazy
or off the wall and seeing how it turns out. If you get stuck or want to talk
about it, feel free to email the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
list</a>: it has lots of people who are interested in languages and are often
willing to help out.
</p>
<p>Before we end this tutorial, I want to talk about some "tips and tricks" for generating
LLVM IR. These are some of the more subtle things that may not be obvious, but
are very useful if you want to take advantage of LLVM's capabilities.</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="llvmirproperties">Properties of the LLVM IR</a></h2>
<!-- *********************************************************************** -->
<div>
<p>We have a couple common questions about code in the LLVM IR form - lets just
get these out of the way right now, shall we?</p>
<!-- ======================================================================= -->
<h4><a name="targetindep">Target Independence</a></h4>
<!-- ======================================================================= -->
<div>
<p>Kaleidoscope is an example of a "portable language": any program written in
Kaleidoscope will work the same way on any target that it runs on. Many other
languages have this property, e.g. lisp, java, haskell, javascript, python, etc
(note that while these languages are portable, not all their libraries are).</p>
<p>One nice aspect of LLVM is that it is often capable of preserving target
independence in the IR: you can take the LLVM IR for a Kaleidoscope-compiled
program and run it on any target that LLVM supports, even emitting C code and
compiling that on targets that LLVM doesn't support natively. You can trivially
tell that the Kaleidoscope compiler generates target-independent code because it
never queries for any target-specific information when generating code.</p>
<p>The fact that LLVM provides a compact, target-independent, representation for
code gets a lot of people excited. Unfortunately, these people are usually
thinking about C or a language from the C family when they are asking questions
about language portability. I say "unfortunately", because there is really no
way to make (fully general) C code portable, other than shipping the source code
around (and of course, C source code is not actually portable in general
either - ever port a really old application from 32- to 64-bits?).</p>
<p>The problem with C (again, in its full generality) is that it is heavily
laden with target specific assumptions. As one simple example, the preprocessor
often destructively removes target-independence from the code when it processes
the input text:</p>
<div class="doc_code">
<pre>
#ifdef __i386__
int X = 1;
#else
int X = 42;
#endif
</pre>
</div>
<p>While it is possible to engineer more and more complex solutions to problems
like this, it cannot be solved in full generality in a way that is better than shipping
the actual source code.</p>
<p>That said, there are interesting subsets of C that can be made portable. If
you are willing to fix primitive types to a fixed size (say int = 32-bits,
and long = 64-bits), don't care about ABI compatibility with existing binaries,
and are willing to give up some other minor features, you can have portable
code. This can make sense for specialized domains such as an
in-kernel language.</p>
</div>
<!-- ======================================================================= -->
<h4><a name="safety">Safety Guarantees</a></h4>
<!-- ======================================================================= -->
<div>
<p>Many of the languages above are also "safe" languages: it is impossible for
a program written in Java to corrupt its address space and crash the process
(assuming the JVM has no bugs).
Safety is an interesting property that requires a combination of language
design, runtime support, and often operating system support.</p>
<p>It is certainly possible to implement a safe language in LLVM, but LLVM IR
does not itself guarantee safety. The LLVM IR allows unsafe pointer casts,
use after free bugs, buffer over-runs, and a variety of other problems. Safety
needs to be implemented as a layer on top of LLVM and, conveniently, several
groups have investigated this. Ask on the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
list</a> if you are interested in more details.</p>
</div>
<!-- ======================================================================= -->
<h4><a name="langspecific">Language-Specific Optimizations</a></h4>
<!-- ======================================================================= -->
<div>
<p>One thing about LLVM that turns off many people is that it does not solve all
the world's problems in one system (sorry 'world hunger', someone else will have
to solve you some other day). One specific complaint is that people perceive
LLVM as being incapable of performing high-level language-specific optimization:
LLVM "loses too much information".</p>
<p>Unfortunately, this is really not the place to give you a full and unified
version of "Chris Lattner's theory of compiler design". Instead, I'll make a
few observations:</p>
<p>First, you're right that LLVM does lose information. For example, as of this
writing, there is no way to distinguish in the LLVM IR whether an SSA-value came
from a C "int" or a C "long" on an ILP32 machine (other than debug info). Both
get compiled down to an 'i32' value and the information about what it came from
is lost. The more general issue here, is that the LLVM type system uses
"structural equivalence" instead of "name equivalence". Another place this
surprises people is if you have two types in a high-level language that have the
same structure (e.g. two different structs that have a single int field): these
types will compile down into a single LLVM type and it will be impossible to
tell what it came from.</p>
<p>Second, while LLVM does lose information, LLVM is not a fixed target: we
continue to enhance and improve it in many different ways. In addition to
adding new features (LLVM did not always support exceptions or debug info), we
also extend the IR to capture important information for optimization (e.g.
whether an argument is sign or zero extended, information about pointers
aliasing, etc). Many of the enhancements are user-driven: people want LLVM to
include some specific feature, so they go ahead and extend it.</p>
<p>Third, it is <em>possible and easy</em> to add language-specific
optimizations, and you have a number of choices in how to do it. As one trivial
example, it is easy to add language-specific optimization passes that
"know" things about code compiled for a language. In the case of the C family,
there is an optimization pass that "knows" about the standard C library
functions. If you call "exit(0)" in main(), it knows that it is safe to
optimize that into "return 0;" because C specifies what the 'exit'
function does.</p>
<p>In addition to simple library knowledge, it is possible to embed a variety of
other language-specific information into the LLVM IR. If you have a specific
need and run into a wall, please bring the topic up on the llvmdev list. At the
very worst, you can always treat LLVM as if it were a "dumb code generator" and
implement the high-level optimizations you desire in your front-end, on the
language-specific AST.
</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2><a name="tipsandtricks">Tips and Tricks</a></h2>
<!-- *********************************************************************** -->
<div>
<p>There is a variety of useful tips and tricks that you come to know after
working on/with LLVM that aren't obvious at first glance. Instead of letting
everyone rediscover them, this section talks about some of these issues.</p>
<!-- ======================================================================= -->
<h4><a name="offsetofsizeof">Implementing portable offsetof/sizeof</a></h4>
<!-- ======================================================================= -->
<div>
<p>One interesting thing that comes up, if you are trying to keep the code
generated by your compiler "target independent", is that you often need to know
the size of some LLVM type or the offset of some field in an llvm structure.
For example, you might need to pass the size of a type into a function that
allocates memory.</p>
<p>Unfortunately, this can vary widely across targets: for example the width of
a pointer is trivially target-specific. However, there is a <a
href="http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt">clever
way to use the getelementptr instruction</a> that allows you to compute this
in a portable way.</p>
</div>
<!-- ======================================================================= -->
<h4><a name="gcstack">Garbage Collected Stack Frames</a></h4>
<!-- ======================================================================= -->
<div>
<p>Some languages want to explicitly manage their stack frames, often so that
they are garbage collected or to allow easy implementation of closures. There
are often better ways to implement these features than explicit stack frames,
but <a
href="http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt">LLVM
does support them,</a> if you want. It requires your front-end to convert the
code into <a
href="http://en.wikipedia.org/wiki/Continuation-passing_style">Continuation
Passing Style</a> and the use of tail calls (which LLVM also supports).</p>
</div>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
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269
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View File

@ -0,0 +1,269 @@
======================================================
Kaleidoscope: Conclusion and other useful LLVM tidbits
======================================================
.. contents::
:local:
Written by `Chris Lattner <mailto:sabre@nondot.org>`_
Tutorial Conclusion
===================
Welcome to the final chapter of the "`Implementing a language with
LLVM <index.html>`_" tutorial. In the course of this tutorial, we have
grown our little Kaleidoscope language from being a useless toy, to
being a semi-interesting (but probably still useless) toy. :)
It is interesting to see how far we've come, and how little code it has
taken. We built the entire lexer, parser, AST, code generator, and an
interactive run-loop (with a JIT!) by-hand in under 700 lines of
(non-comment/non-blank) code.
Our little language supports a couple of interesting features: it
supports user defined binary and unary operators, it uses JIT
compilation for immediate evaluation, and it supports a few control flow
constructs with SSA construction.
Part of the idea of this tutorial was to show you how easy and fun it
can be to define, build, and play with languages. Building a compiler
need not be a scary or mystical process! Now that you've seen some of
the basics, I strongly encourage you to take the code and hack on it.
For example, try adding:
- **global variables** - While global variables have questional value
in modern software engineering, they are often useful when putting
together quick little hacks like the Kaleidoscope compiler itself.
Fortunately, our current setup makes it very easy to add global
variables: just have value lookup check to see if an unresolved
variable is in the global variable symbol table before rejecting it.
To create a new global variable, make an instance of the LLVM
``GlobalVariable`` class.
- **typed variables** - Kaleidoscope currently only supports variables
of type double. This gives the language a very nice elegance, because
only supporting one type means that you never have to specify types.
Different languages have different ways of handling this. The easiest
way is to require the user to specify types for every variable
definition, and record the type of the variable in the symbol table
along with its Value\*.
- **arrays, structs, vectors, etc** - Once you add types, you can start
extending the type system in all sorts of interesting ways. Simple
arrays are very easy and are quite useful for many different
applications. Adding them is mostly an exercise in learning how the
LLVM `getelementptr <../LangRef.html#i_getelementptr>`_ instruction
works: it is so nifty/unconventional, it `has its own
FAQ <../GetElementPtr.html>`_! If you add support for recursive types
(e.g. linked lists), make sure to read the `section in the LLVM
Programmer's Manual <../ProgrammersManual.html#TypeResolve>`_ that
describes how to construct them.
- **standard runtime** - Our current language allows the user to access
arbitrary external functions, and we use it for things like "printd"
and "putchard". As you extend the language to add higher-level
constructs, often these constructs make the most sense if they are
lowered to calls into a language-supplied runtime. For example, if
you add hash tables to the language, it would probably make sense to
add the routines to a runtime, instead of inlining them all the way.
- **memory management** - Currently we can only access the stack in
Kaleidoscope. It would also be useful to be able to allocate heap
memory, either with calls to the standard libc malloc/free interface
or with a garbage collector. If you would like to use garbage
collection, note that LLVM fully supports `Accurate Garbage
Collection <../GarbageCollection.html>`_ including algorithms that
move objects and need to scan/update the stack.
- **debugger support** - LLVM supports generation of `DWARF Debug
info <../SourceLevelDebugging.html>`_ which is understood by common
debuggers like GDB. Adding support for debug info is fairly
straightforward. The best way to understand it is to compile some
C/C++ code with "``llvm-gcc -g -O0``" and taking a look at what it
produces.
- **exception handling support** - LLVM supports generation of `zero
cost exceptions <../ExceptionHandling.html>`_ which interoperate with
code compiled in other languages. You could also generate code by
implicitly making every function return an error value and checking
it. You could also make explicit use of setjmp/longjmp. There are
many different ways to go here.
- **object orientation, generics, database access, complex numbers,
geometric programming, ...** - Really, there is no end of crazy
features that you can add to the language.
- **unusual domains** - We've been talking about applying LLVM to a
domain that many people are interested in: building a compiler for a
specific language. However, there are many other domains that can use
compiler technology that are not typically considered. For example,
LLVM has been used to implement OpenGL graphics acceleration,
translate C++ code to ActionScript, and many other cute and clever
things. Maybe you will be the first to JIT compile a regular
expression interpreter into native code with LLVM?
Have fun - try doing something crazy and unusual. Building a language
like everyone else always has, is much less fun than trying something a
little crazy or off the wall and seeing how it turns out. If you get
stuck or want to talk about it, feel free to email the `llvmdev mailing
list <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_: it has lots
of people who are interested in languages and are often willing to help
out.
Before we end this tutorial, I want to talk about some "tips and tricks"
for generating LLVM IR. These are some of the more subtle things that
may not be obvious, but are very useful if you want to take advantage of
LLVM's capabilities.
Properties of the LLVM IR
=========================
We have a couple common questions about code in the LLVM IR form - lets
just get these out of the way right now, shall we?
Target Independence
-------------------
Kaleidoscope is an example of a "portable language": any program written
in Kaleidoscope will work the same way on any target that it runs on.
Many other languages have this property, e.g. lisp, java, haskell,
javascript, python, etc (note that while these languages are portable,
not all their libraries are).
One nice aspect of LLVM is that it is often capable of preserving target
independence in the IR: you can take the LLVM IR for a
Kaleidoscope-compiled program and run it on any target that LLVM
supports, even emitting C code and compiling that on targets that LLVM
doesn't support natively. You can trivially tell that the Kaleidoscope
compiler generates target-independent code because it never queries for
any target-specific information when generating code.
The fact that LLVM provides a compact, target-independent,
representation for code gets a lot of people excited. Unfortunately,
these people are usually thinking about C or a language from the C
family when they are asking questions about language portability. I say
"unfortunately", because there is really no way to make (fully general)
C code portable, other than shipping the source code around (and of
course, C source code is not actually portable in general either - ever
port a really old application from 32- to 64-bits?).
The problem with C (again, in its full generality) is that it is heavily
laden with target specific assumptions. As one simple example, the
preprocessor often destructively removes target-independence from the
code when it processes the input text:
.. code-block:: c
#ifdef __i386__
int X = 1;
#else
int X = 42;
#endif
While it is possible to engineer more and more complex solutions to
problems like this, it cannot be solved in full generality in a way that
is better than shipping the actual source code.
That said, there are interesting subsets of C that can be made portable.
If you are willing to fix primitive types to a fixed size (say int =
32-bits, and long = 64-bits), don't care about ABI compatibility with
existing binaries, and are willing to give up some other minor features,
you can have portable code. This can make sense for specialized domains
such as an in-kernel language.
Safety Guarantees
-----------------
Many of the languages above are also "safe" languages: it is impossible
for a program written in Java to corrupt its address space and crash the
process (assuming the JVM has no bugs). Safety is an interesting
property that requires a combination of language design, runtime
support, and often operating system support.
It is certainly possible to implement a safe language in LLVM, but LLVM
IR does not itself guarantee safety. The LLVM IR allows unsafe pointer
casts, use after free bugs, buffer over-runs, and a variety of other
problems. Safety needs to be implemented as a layer on top of LLVM and,
conveniently, several groups have investigated this. Ask on the `llvmdev
mailing list <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_ if
you are interested in more details.
Language-Specific Optimizations
-------------------------------
One thing about LLVM that turns off many people is that it does not
solve all the world's problems in one system (sorry 'world hunger',
someone else will have to solve you some other day). One specific
complaint is that people perceive LLVM as being incapable of performing
high-level language-specific optimization: LLVM "loses too much
information".
Unfortunately, this is really not the place to give you a full and
unified version of "Chris Lattner's theory of compiler design". Instead,
I'll make a few observations:
First, you're right that LLVM does lose information. For example, as of
this writing, there is no way to distinguish in the LLVM IR whether an
SSA-value came from a C "int" or a C "long" on an ILP32 machine (other
than debug info). Both get compiled down to an 'i32' value and the
information about what it came from is lost. The more general issue
here, is that the LLVM type system uses "structural equivalence" instead
of "name equivalence". Another place this surprises people is if you
have two types in a high-level language that have the same structure
(e.g. two different structs that have a single int field): these types
will compile down into a single LLVM type and it will be impossible to
tell what it came from.
Second, while LLVM does lose information, LLVM is not a fixed target: we
continue to enhance and improve it in many different ways. In addition
to adding new features (LLVM did not always support exceptions or debug
info), we also extend the IR to capture important information for
optimization (e.g. whether an argument is sign or zero extended,
information about pointers aliasing, etc). Many of the enhancements are
user-driven: people want LLVM to include some specific feature, so they
go ahead and extend it.
Third, it is *possible and easy* to add language-specific optimizations,
and you have a number of choices in how to do it. As one trivial
example, it is easy to add language-specific optimization passes that
"know" things about code compiled for a language. In the case of the C
family, there is an optimization pass that "knows" about the standard C
library functions. If you call "exit(0)" in main(), it knows that it is
safe to optimize that into "return 0;" because C specifies what the
'exit' function does.
In addition to simple library knowledge, it is possible to embed a
variety of other language-specific information into the LLVM IR. If you
have a specific need and run into a wall, please bring the topic up on
the llvmdev list. At the very worst, you can always treat LLVM as if it
were a "dumb code generator" and implement the high-level optimizations
you desire in your front-end, on the language-specific AST.
Tips and Tricks
===============
There is a variety of useful tips and tricks that you come to know after
working on/with LLVM that aren't obvious at first glance. Instead of
letting everyone rediscover them, this section talks about some of these
issues.
Implementing portable offsetof/sizeof
-------------------------------------
One interesting thing that comes up, if you are trying to keep the code
generated by your compiler "target independent", is that you often need
to know the size of some LLVM type or the offset of some field in an
llvm structure. For example, you might need to pass the size of a type
into a function that allocates memory.
Unfortunately, this can vary widely across targets: for example the
width of a pointer is trivially target-specific. However, there is a
`clever way to use the getelementptr
instruction <http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt>`_
that allows you to compute this in a portable way.
Garbage Collected Stack Frames
------------------------------
Some languages want to explicitly manage their stack frames, often so
that they are garbage collected or to allow easy implementation of
closures. There are often better ways to implement these features than
explicit stack frames, but `LLVM does support
them, <http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt>`_
if you want. It requires your front-end to convert the code into
`Continuation Passing
Style <http://en.wikipedia.org/wiki/Continuation-passing_style>`_ and
the use of tail calls (which LLVM also supports).

365
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@ -1,365 +0,0 @@
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Kaleidoscope: Tutorial Introduction and the Lexer</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="Chris Lattner">
<meta name="author" content="Erick Tryzelaar">
<link rel="stylesheet" href="../_static/llvm.css" type="text/css">
</head>
<body>
<h1>Kaleidoscope: Tutorial Introduction and the Lexer</h1>
<ul>
<li><a href="index.html">Up to Tutorial Index</a></li>
<li>Chapter 1
<ol>
<li><a href="#intro">Tutorial Introduction</a></li>
<li><a href="#language">The Basic Language</a></li>
<li><a href="#lexer">The Lexer</a></li>
</ol>
</li>
<li><a href="OCamlLangImpl2.html">Chapter 2</a>: Implementing a Parser and
AST</li>
</ul>
<div class="doc_author">
<p>
Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="intro">Tutorial Introduction</a></h2>
<!-- *********************************************************************** -->
<div>
<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
runs through the implementation of a simple language, showing how fun and
easy it can be. This tutorial will get you up and started as well as help to
build a framework you can extend to other languages. The code in this tutorial
can also be used as a playground to hack on other LLVM specific things.
</p>
<p>
The goal of this tutorial is to progressively unveil our language, describing
how it is built up over time. This will let us cover a fairly broad range of
language design and LLVM-specific usage issues, showing and explaining the code
for it all along the way, without overwhelming you with tons of details up
front.</p>
<p>It is useful to point out ahead of time that this tutorial is really about
teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
modern and sane software engineering principles. In practice, this means that
we'll take a number of shortcuts to simplify the exposition. For example, the
code leaks memory, uses global variables all over the place, doesn't use nice
design patterns like <a
href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
is very simple. If you dig in and use the code as a basis for future projects,
fixing these deficiencies shouldn't be hard.</p>
<p>I've tried to put this tutorial together in a way that makes chapters easy to
skip over if you are already familiar with or are uninterested in the various
pieces. The structure of the tutorial is:
</p>
<ul>
<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
language, and the definition of its Lexer</b> - This shows where we are going
and the basic functionality that we want it to do. In order to make this
tutorial maximally understandable and hackable, we choose to implement
everything in Objective Caml instead of using lexer and parser generators.
LLVM obviously works just fine with such tools, feel free to use one if you
prefer.</li>
<li><b><a href="OCamlLangImpl2.html">Chapter #2</a>: Implementing a Parser and
AST</b> - With the lexer in place, we can talk about parsing techniques and
basic AST construction. This tutorial describes recursive descent parsing and
operator precedence parsing. Nothing in Chapters 1 or 2 is LLVM-specific,
the code doesn't even link in LLVM at this point. :)</li>
<li><b><a href="OCamlLangImpl3.html">Chapter #3</a>: Code generation to LLVM
IR</b> - With the AST ready, we can show off how easy generation of LLVM IR
really is.</li>
<li><b><a href="OCamlLangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
Support</b> - Because a lot of people are interested in using LLVM as a JIT,
we'll dive right into it and show you the 3 lines it takes to add JIT support.
LLVM is also useful in many other ways, but this is one simple and "sexy" way
to shows off its power. :)</li>
<li><b><a href="OCamlLangImpl5.html">Chapter #5</a>: Extending the Language:
Control Flow</b> - With the language up and running, we show how to extend it
with control flow operations (if/then/else and a 'for' loop). This gives us a
chance to talk about simple SSA construction and control flow.</li>
<li><b><a href="OCamlLangImpl6.html">Chapter #6</a>: Extending the Language:
User-defined Operators</b> - This is a silly but fun chapter that talks about
extending the language to let the user program define their own arbitrary
unary and binary operators (with assignable precedence!). This lets us build a
significant piece of the "language" as library routines.</li>
<li><b><a href="OCamlLangImpl7.html">Chapter #7</a>: Extending the Language:
Mutable Variables</b> - This chapter talks about adding user-defined local
variables along with an assignment operator. The interesting part about this
is how easy and trivial it is to construct SSA form in LLVM: no, LLVM does
<em>not</em> require your front-end to construct SSA form!</li>
<li><b><a href="OCamlLangImpl8.html">Chapter #8</a>: Conclusion and other
useful LLVM tidbits</b> - This chapter wraps up the series by talking about
potential ways to extend the language, but also includes a bunch of pointers to
info about "special topics" like adding garbage collection support, exceptions,
debugging, support for "spaghetti stacks", and a bunch of other tips and
tricks.</li>
</ul>
<p>By the end of the tutorial, we'll have written a bit less than 700 lines of
non-comment, non-blank, lines of code. With this small amount of code, we'll
have built up a very reasonable compiler for a non-trivial language including
a hand-written lexer, parser, AST, as well as code generation support with a JIT
compiler. While other systems may have interesting "hello world" tutorials,
I think the breadth of this tutorial is a great testament to the strengths of
LLVM and why you should consider it if you're interested in language or compiler
design.</p>
<p>A note about this tutorial: we expect you to extend the language and play
with it on your own. Take the code and go crazy hacking away at it, compilers
don't need to be scary creatures - it can be a lot of fun to play with
languages!</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="language">The Basic Language</a></h2>
<!-- *********************************************************************** -->
<div>
<p>This tutorial will be illustrated with a toy language that we'll call
"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived
from "meaning beautiful, form, and view").
Kaleidoscope is a procedural language that allows you to define functions, use
conditionals, math, etc. Over the course of the tutorial, we'll extend
Kaleidoscope to support the if/then/else construct, a for loop, user defined
operators, JIT compilation with a simple command line interface, etc.</p>
<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
64-bit floating point type (aka 'float' in O'Caml parlance). As such, all
values are implicitly double precision and the language doesn't require type
declarations. This gives the language a very nice and simple syntax. For
example, the following simple example computes <a
href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
<div class="doc_code">
<pre>
# Compute the x'th fibonacci number.
def fib(x)
if x &lt; 3 then
1
else
fib(x-1)+fib(x-2)
# This expression will compute the 40th number.
fib(40)
</pre>
</div>
<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
JIT makes this completely trivial). This means that you can use the 'extern'
keyword to define a function before you use it (this is also useful for mutually
recursive functions). For example:</p>
<div class="doc_code">
<pre>
extern sin(arg);
extern cos(arg);
extern atan2(arg1 arg2);
atan2(sin(.4), cos(42))
</pre>
</div>
<p>A more interesting example is included in Chapter 6 where we write a little
Kaleidoscope application that <a href="OCamlLangImpl6.html#example">displays
a Mandelbrot Set</a> at various levels of magnification.</p>
<p>Lets dive into the implementation of this language!</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="lexer">The Lexer</a></h2>
<!-- *********************************************************************** -->
<div>
<p>When it comes to implementing a language, the first thing needed is
the ability to process a text file and recognize what it says. The traditional
way to do this is to use a "<a
href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
to break the input up into "tokens". Each token returned by the lexer includes
a token code and potentially some metadata (e.g. the numeric value of a number).
First, we define the possibilities:
</p>
<div class="doc_code">
<pre>
(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
* these others for known things. *)
type token =
(* commands *)
| Def | Extern
(* primary *)
| Ident of string | Number of float
(* unknown *)
| Kwd of char
</pre>
</div>
<p>Each token returned by our lexer will be one of the token variant values.
An unknown character like '+' will be returned as <tt>Token.Kwd '+'</tt>. If
the curr token is an identifier, the value will be <tt>Token.Ident s</tt>. If
the current token is a numeric literal (like 1.0), the value will be
<tt>Token.Number 1.0</tt>.
</p>
<p>The actual implementation of the lexer is a collection of functions driven
by a function named <tt>Lexer.lex</tt>. The <tt>Lexer.lex</tt> function is
called to return the next token from standard input. We will use
<a href="http://caml.inria.fr/pub/docs/manual-camlp4/index.html">Camlp4</a>
to simplify the tokenization of the standard input. Its definition starts
as:</p>
<div class="doc_code">
<pre>
(*===----------------------------------------------------------------------===
* Lexer
*===----------------------------------------------------------------------===*)
let rec lex = parser
(* Skip any whitespace. *)
| [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
</pre>
</div>
<p>
<tt>Lexer.lex</tt> works by recursing over a <tt>char Stream.t</tt> to read
characters one at a time from the standard input. It eats them as it recognizes
them and stores them in in a <tt>Token.token</tt> variant. The first thing that
it has to do is ignore whitespace between tokens. This is accomplished with the
recursive call above.</p>
<p>The next thing <tt>Lexer.lex</tt> needs to do is recognize identifiers and
specific keywords like "def". Kaleidoscope does this with a pattern match
and a helper function.<p>
<div class="doc_code">
<pre>
(* identifier: [a-zA-Z][a-zA-Z0-9] *)
| [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_ident buffer stream
...
and lex_ident buffer = parser
| [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
Buffer.add_char buffer c;
lex_ident buffer stream
| [&lt; stream=lex &gt;] -&gt;
match Buffer.contents buffer with
| "def" -&gt; [&lt; 'Token.Def; stream &gt;]
| "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
| id -&gt; [&lt; 'Token.Ident id; stream &gt;]
</pre>
</div>
<p>Numeric values are similar:</p>
<div class="doc_code">
<pre>
(* number: [0-9.]+ *)
| [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_number buffer stream
...
and lex_number buffer = parser
| [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
Buffer.add_char buffer c;
lex_number buffer stream
| [&lt; stream=lex &gt;] -&gt;
[&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
</pre>
</div>
<p>This is all pretty straight-forward code for processing input. When reading
a numeric value from input, we use the ocaml <tt>float_of_string</tt> function
to convert it to a numeric value that we store in <tt>Token.Number</tt>. Note
that this isn't doing sufficient error checking: it will raise <tt>Failure</tt>
if the string "1.23.45.67". Feel free to extend it :). Next we handle
comments:
</p>
<div class="doc_code">
<pre>
(* Comment until end of line. *)
| [&lt; ' ('#'); stream &gt;] -&gt;
lex_comment stream
...
and lex_comment = parser
| [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
| [&lt; 'c; e=lex_comment &gt;] -&gt; e
| [&lt; &gt;] -&gt; [&lt; &gt;]
</pre>
</div>
<p>We handle comments by skipping to the end of the line and then return the
next token. Finally, if the input doesn't match one of the above cases, it is
either an operator character like '+' or the end of the file. These are handled
with this code:</p>
<div class="doc_code">
<pre>
(* Otherwise, just return the character as its ascii value. *)
| [&lt; 'c; stream &gt;] -&gt;
[&lt; 'Token.Kwd c; lex stream &gt;]
(* end of stream. *)
| [&lt; &gt;] -&gt; [&lt; &gt;]
</pre>
</div>
<p>With this, we have the complete lexer for the basic Kaleidoscope language
(the <a href="OCamlLangImpl2.html#code">full code listing</a> for the Lexer is
available in the <a href="OCamlLangImpl2.html">next chapter</a> of the
tutorial). Next we'll <a href="OCamlLangImpl2.html">build a simple parser that
uses this to build an Abstract Syntax Tree</a>. When we have that, we'll
include a driver so that you can use the lexer and parser together.
</p>
<a href="OCamlLangImpl2.html">Next: Implementing a Parser and AST</a>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
<a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
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=================================================
Kaleidoscope: Tutorial Introduction and the Lexer
=================================================
.. contents::
:local:
Written by `Chris Lattner <mailto:sabre@nondot.org>`_ and `Erick
Tryzelaar <mailto:idadesub@users.sourceforge.net>`_
Tutorial Introduction
=====================
Welcome to the "Implementing a language with LLVM" tutorial. This
tutorial runs through the implementation of a simple language, showing
how fun and easy it can be. This tutorial will get you up and started as
well as help to build a framework you can extend to other languages. The
code in this tutorial can also be used as a playground to hack on other
LLVM specific things.
The goal of this tutorial is to progressively unveil our language,
describing how it is built up over time. This will let us cover a fairly
broad range of language design and LLVM-specific usage issues, showing
and explaining the code for it all along the way, without overwhelming
you with tons of details up front.
It is useful to point out ahead of time that this tutorial is really
about teaching compiler techniques and LLVM specifically, *not* about
teaching modern and sane software engineering principles. In practice,
this means that we'll take a number of shortcuts to simplify the
exposition. For example, the code leaks memory, uses global variables
all over the place, doesn't use nice design patterns like
`visitors <http://en.wikipedia.org/wiki/Visitor_pattern>`_, etc... but
it is very simple. If you dig in and use the code as a basis for future
projects, fixing these deficiencies shouldn't be hard.
I've tried to put this tutorial together in a way that makes chapters
easy to skip over if you are already familiar with or are uninterested
in the various pieces. The structure of the tutorial is:
- `Chapter #1 <#language>`_: Introduction to the Kaleidoscope
language, and the definition of its Lexer - This shows where we are
going and the basic functionality that we want it to do. In order to
make this tutorial maximally understandable and hackable, we choose
to implement everything in Objective Caml instead of using lexer and
parser generators. LLVM obviously works just fine with such tools,
feel free to use one if you prefer.
- `Chapter #2 <OCamlLangImpl2.html>`_: Implementing a Parser and
AST - With the lexer in place, we can talk about parsing techniques
and basic AST construction. This tutorial describes recursive descent
parsing and operator precedence parsing. Nothing in Chapters 1 or 2
is LLVM-specific, the code doesn't even link in LLVM at this point.
:)
- `Chapter #3 <OCamlLangImpl3.html>`_: Code generation to LLVM IR -
With the AST ready, we can show off how easy generation of LLVM IR
really is.
- `Chapter #4 <OCamlLangImpl4.html>`_: Adding JIT and Optimizer
Support - Because a lot of people are interested in using LLVM as a
JIT, we'll dive right into it and show you the 3 lines it takes to
add JIT support. LLVM is also useful in many other ways, but this is
one simple and "sexy" way to shows off its power. :)
- `Chapter #5 <OCamlLangImpl5.html>`_: Extending the Language:
Control Flow - With the language up and running, we show how to
extend it with control flow operations (if/then/else and a 'for'
loop). This gives us a chance to talk about simple SSA construction
and control flow.
- `Chapter #6 <OCamlLangImpl6.html>`_: Extending the Language:
User-defined Operators - This is a silly but fun chapter that talks
about extending the language to let the user program define their own
arbitrary unary and binary operators (with assignable precedence!).
This lets us build a significant piece of the "language" as library
routines.
- `Chapter #7 <OCamlLangImpl7.html>`_: Extending the Language:
Mutable Variables - This chapter talks about adding user-defined
local variables along with an assignment operator. The interesting
part about this is how easy and trivial it is to construct SSA form
in LLVM: no, LLVM does *not* require your front-end to construct SSA
form!
- `Chapter #8 <OCamlLangImpl8.html>`_: Conclusion and other useful
LLVM tidbits - This chapter wraps up the series by talking about
potential ways to extend the language, but also includes a bunch of
pointers to info about "special topics" like adding garbage
collection support, exceptions, debugging, support for "spaghetti
stacks", and a bunch of other tips and tricks.
By the end of the tutorial, we'll have written a bit less than 700 lines
of non-comment, non-blank, lines of code. With this small amount of
code, we'll have built up a very reasonable compiler for a non-trivial
language including a hand-written lexer, parser, AST, as well as code
generation support with a JIT compiler. While other systems may have
interesting "hello world" tutorials, I think the breadth of this
tutorial is a great testament to the strengths of LLVM and why you
should consider it if you're interested in language or compiler design.
A note about this tutorial: we expect you to extend the language and
play with it on your own. Take the code and go crazy hacking away at it,
compilers don't need to be scary creatures - it can be a lot of fun to
play with languages!
The Basic Language
==================
This tutorial will be illustrated with a toy language that we'll call
"`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived
from "meaning beautiful, form, and view"). Kaleidoscope is a procedural
language that allows you to define functions, use conditionals, math,
etc. Over the course of the tutorial, we'll extend Kaleidoscope to
support the if/then/else construct, a for loop, user defined operators,
JIT compilation with a simple command line interface, etc.
Because we want to keep things simple, the only datatype in Kaleidoscope
is a 64-bit floating point type (aka 'float' in O'Caml parlance). As
such, all values are implicitly double precision and the language
doesn't require type declarations. This gives the language a very nice
and simple syntax. For example, the following simple example computes
`Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_
::
# Compute the x'th fibonacci number.
def fib(x)
if x < 3 then
1
else
fib(x-1)+fib(x-2)
# This expression will compute the 40th number.
fib(40)
We also allow Kaleidoscope to call into standard library functions (the
LLVM JIT makes this completely trivial). This means that you can use the
'extern' keyword to define a function before you use it (this is also
useful for mutually recursive functions). For example:
::
extern sin(arg);
extern cos(arg);
extern atan2(arg1 arg2);
atan2(sin(.4), cos(42))
A more interesting example is included in Chapter 6 where we write a
little Kaleidoscope application that `displays a Mandelbrot
Set <OCamlLangImpl6.html#example>`_ at various levels of magnification.
Lets dive into the implementation of this language!
The Lexer
=========
When it comes to implementing a language, the first thing needed is the
ability to process a text file and recognize what it says. The
traditional way to do this is to use a
"`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka
'scanner') to break the input up into "tokens". Each token returned by
the lexer includes a token code and potentially some metadata (e.g. the
numeric value of a number). First, we define the possibilities:
.. code-block:: ocaml
(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
* these others for known things. *)
type token =
(* commands *)
| Def | Extern
(* primary *)
| Ident of string | Number of float
(* unknown *)
| Kwd of char
Each token returned by our lexer will be one of the token variant
values. An unknown character like '+' will be returned as
``Token.Kwd '+'``. If the curr token is an identifier, the value will be
``Token.Ident s``. If the current token is a numeric literal (like 1.0),
the value will be ``Token.Number 1.0``.
The actual implementation of the lexer is a collection of functions
driven by a function named ``Lexer.lex``. The ``Lexer.lex`` function is
called to return the next token from standard input. We will use
`Camlp4 <http://caml.inria.fr/pub/docs/manual-camlp4/index.html>`_ to
simplify the tokenization of the standard input. Its definition starts
as:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Lexer
*===----------------------------------------------------------------------===*)
let rec lex = parser
(* Skip any whitespace. *)
| [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
``Lexer.lex`` works by recursing over a ``char Stream.t`` to read
characters one at a time from the standard input. It eats them as it
recognizes them and stores them in in a ``Token.token`` variant. The
first thing that it has to do is ignore whitespace between tokens. This
is accomplished with the recursive call above.
The next thing ``Lexer.lex`` needs to do is recognize identifiers and
specific keywords like "def". Kaleidoscope does this with a pattern
match and a helper function.
.. code-block:: ocaml
(* identifier: [a-zA-Z][a-zA-Z0-9] *)
| [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_ident buffer stream
...
and lex_ident buffer = parser
| [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
Buffer.add_char buffer c;
lex_ident buffer stream
| [< stream=lex >] ->
match Buffer.contents buffer with
| "def" -> [< 'Token.Def; stream >]
| "extern" -> [< 'Token.Extern; stream >]
| id -> [< 'Token.Ident id; stream >]
Numeric values are similar:
.. code-block:: ocaml
(* number: [0-9.]+ *)
| [< ' ('0' .. '9' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_number buffer stream
...
and lex_number buffer = parser
| [< ' ('0' .. '9' | '.' as c); stream >] ->
Buffer.add_char buffer c;
lex_number buffer stream
| [< stream=lex >] ->
[< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
This is all pretty straight-forward code for processing input. When
reading a numeric value from input, we use the ocaml ``float_of_string``
function to convert it to a numeric value that we store in
``Token.Number``. Note that this isn't doing sufficient error checking:
it will raise ``Failure`` if the string "1.23.45.67". Feel free to
extend it :). Next we handle comments:
.. code-block:: ocaml
(* Comment until end of line. *)
| [< ' ('#'); stream >] ->
lex_comment stream
...
and lex_comment = parser
| [< ' ('\n'); stream=lex >] -> stream
| [< 'c; e=lex_comment >] -> e
| [< >] -> [< >]
We handle comments by skipping to the end of the line and then return
the next token. Finally, if the input doesn't match one of the above
cases, it is either an operator character like '+' or the end of the
file. These are handled with this code:
.. code-block:: ocaml
(* Otherwise, just return the character as its ascii value. *)
| [< 'c; stream >] ->
[< 'Token.Kwd c; lex stream >]
(* end of stream. *)
| [< >] -> [< >]
With this, we have the complete lexer for the basic Kaleidoscope
language (the `full code listing <OCamlLangImpl2.html#code>`_ for the
Lexer is available in the `next chapter <OCamlLangImpl2.html>`_ of the
tutorial). Next we'll `build a simple parser that uses this to build an
Abstract Syntax Tree <OCamlLangImpl2.html>`_. When we have that, we'll
include a driver so that you can use the lexer and parser together.
`Next: Implementing a Parser and AST <OCamlLangImpl2.html>`_

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===========================================
Kaleidoscope: Implementing a Parser and AST
===========================================
.. contents::
:local:
Written by `Chris Lattner <mailto:sabre@nondot.org>`_ and `Erick
Tryzelaar <mailto:idadesub@users.sourceforge.net>`_
Chapter 2 Introduction
======================
Welcome to Chapter 2 of the "`Implementing a language with LLVM in
Objective Caml <index.html>`_" tutorial. This chapter shows you how to
use the lexer, built in `Chapter 1 <OCamlLangImpl1.html>`_, to build a
full `parser <http://en.wikipedia.org/wiki/Parsing>`_ for our
Kaleidoscope language. Once we have a parser, we'll define and build an
`Abstract Syntax
Tree <http://en.wikipedia.org/wiki/Abstract_syntax_tree>`_ (AST).
The parser we will build uses a combination of `Recursive Descent
Parsing <http://en.wikipedia.org/wiki/Recursive_descent_parser>`_ and
`Operator-Precedence
Parsing <http://en.wikipedia.org/wiki/Operator-precedence_parser>`_ to
parse the Kaleidoscope language (the latter for binary expressions and
the former for everything else). Before we get to parsing though, lets
talk about the output of the parser: the Abstract Syntax Tree.
The Abstract Syntax Tree (AST)
==============================
The AST for a program captures its behavior in such a way that it is
easy for later stages of the compiler (e.g. code generation) to
interpret. We basically want one object for each construct in the
language, and the AST should closely model the language. In
Kaleidoscope, we have expressions, a prototype, and a function object.
We'll start with expressions first:
.. code-block:: ocaml
(* expr - Base type for all expression nodes. *)
type expr =
(* variant for numeric literals like "1.0". *)
| Number of float
The code above shows the definition of the base ExprAST class and one
subclass which we use for numeric literals. The important thing to note
about this code is that the Number variant captures the numeric value of
the literal as an instance variable. This allows later phases of the
compiler to know what the stored numeric value is.
Right now we only create the AST, so there are no useful functions on
them. It would be very easy to add a function to pretty print the code,
for example. Here are the other expression AST node definitions that
we'll use in the basic form of the Kaleidoscope language:
.. code-block:: ocaml
(* variant for referencing a variable, like "a". *)
| Variable of string
(* variant for a binary operator. *)
| Binary of char * expr * expr
(* variant for function calls. *)
| Call of string * expr array
This is all (intentionally) rather straight-forward: variables capture
the variable name, binary operators capture their opcode (e.g. '+'), and
calls capture a function name as well as a list of any argument
expressions. One thing that is nice about our AST is that it captures
the language features without talking about the syntax of the language.
Note that there is no discussion about precedence of binary operators,
lexical structure, etc.
For our basic language, these are all of the expression nodes we'll
define. Because it doesn't have conditional control flow, it isn't
Turing-complete; we'll fix that in a later installment. The two things
we need next are a way to talk about the interface to a function, and a
way to talk about functions themselves:
.. code-block:: ocaml
(* proto - This type represents the "prototype" for a function, which captures
* its name, and its argument names (thus implicitly the number of arguments the
* function takes). *)
type proto = Prototype of string * string array
(* func - This type represents a function definition itself. *)
type func = Function of proto * expr
In Kaleidoscope, functions are typed with just a count of their
arguments. Since all values are double precision floating point, the
type of each argument doesn't need to be stored anywhere. In a more
aggressive and realistic language, the "expr" variants would probably
have a type field.
With this scaffolding, we can now talk about parsing expressions and
function bodies in Kaleidoscope.
Parser Basics
=============
Now that we have an AST to build, we need to define the parser code to
build it. The idea here is that we want to parse something like "x+y"
(which is returned as three tokens by the lexer) into an AST that could
be generated with calls like this:
.. code-block:: ocaml
let x = Variable "x" in
let y = Variable "y" in
let result = Binary ('+', x, y) in
...
The error handling routines make use of the builtin ``Stream.Failure``
and ``Stream.Error``s. ``Stream.Failure`` is raised when the parser is
unable to find any matching token in the first position of a pattern.
``Stream.Error`` is raised when the first token matches, but the rest do
not. The error recovery in our parser will not be the best and is not
particular user-friendly, but it will be enough for our tutorial. These
exceptions make it easier to handle errors in routines that have various
return types.
With these basic types and exceptions, we can implement the first piece
of our grammar: numeric literals.
Basic Expression Parsing
========================
We start with numeric literals, because they are the simplest to
process. For each production in our grammar, we'll define a function
which parses that production. We call this class of expressions
"primary" expressions, for reasons that will become more clear `later in
the tutorial <OCamlLangImpl6.html#unary>`_. In order to parse an
arbitrary primary expression, we need to determine what sort of
expression it is. For numeric literals, we have:
.. code-block:: ocaml
(* primary
* ::= identifier
* ::= numberexpr
* ::= parenexpr *)
parse_primary = parser
(* numberexpr ::= number *)
| [< 'Token.Number n >] -> Ast.Number n
This routine is very simple: it expects to be called when the current
token is a ``Token.Number`` token. It takes the current number value,
creates a ``Ast.Number`` node, advances the lexer to the next token, and
finally returns.
There are some interesting aspects to this. The most important one is
that this routine eats all of the tokens that correspond to the
production and returns the lexer buffer with the next token (which is
not part of the grammar production) ready to go. This is a fairly
standard way to go for recursive descent parsers. For a better example,
the parenthesis operator is defined like this:
.. code-block:: ocaml
(* parenexpr ::= '(' expression ')' *)
| [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
This function illustrates a number of interesting things about the
parser:
1) It shows how we use the ``Stream.Error`` exception. When called, this
function expects that the current token is a '(' token, but after
parsing the subexpression, it is possible that there is no ')' waiting.
For example, if the user types in "(4 x" instead of "(4)", the parser
should emit an error. Because errors can occur, the parser needs a way
to indicate that they happened. In our parser, we use the camlp4
shortcut syntax ``token ?? "parse error"``, where if the token before
the ``??`` does not match, then ``Stream.Error "parse error"`` will be
raised.
2) Another interesting aspect of this function is that it uses recursion
by calling ``Parser.parse_primary`` (we will soon see that
``Parser.parse_primary`` can call ``Parser.parse_primary``). This is
powerful because it allows us to handle recursive grammars, and keeps
each production very simple. Note that parentheses do not cause
construction of AST nodes themselves. While we could do it this way, the
most important role of parentheses are to guide the parser and provide
grouping. Once the parser constructs the AST, parentheses are not
needed.
The next simple production is for handling variable references and
function calls:
.. code-block:: ocaml
(* identifierexpr
* ::= identifier
* ::= identifier '(' argumentexpr ')' *)
| [< 'Token.Ident id; stream >] ->
let rec parse_args accumulator = parser
| [< e=parse_expr; stream >] ->
begin parser
| [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
| [< >] -> e :: accumulator
end stream
| [< >] -> accumulator
in
let rec parse_ident id = parser
(* Call. *)
| [< 'Token.Kwd '(';
args=parse_args [];
'Token.Kwd ')' ?? "expected ')'">] ->
Ast.Call (id, Array.of_list (List.rev args))
(* Simple variable ref. *)
| [< >] -> Ast.Variable id
in
parse_ident id stream
This routine follows the same style as the other routines. (It expects
to be called if the current token is a ``Token.Ident`` token). It also
has recursion and error handling. One interesting aspect of this is that
it uses *look-ahead* to determine if the current identifier is a stand
alone variable reference or if it is a function call expression. It
handles this by checking to see if the token after the identifier is a
'(' token, constructing either a ``Ast.Variable`` or ``Ast.Call`` node
as appropriate.
We finish up by raising an exception if we received a token we didn't
expect:
.. code-block:: ocaml
| [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
Now that basic expressions are handled, we need to handle binary
expressions. They are a bit more complex.
Binary Expression Parsing
=========================
Binary expressions are significantly harder to parse because they are
often ambiguous. For example, when given the string "x+y\*z", the parser
can choose to parse it as either "(x+y)\*z" or "x+(y\*z)". With common
definitions from mathematics, we expect the later parse, because "\*"
(multiplication) has higher *precedence* than "+" (addition).
There are many ways to handle this, but an elegant and efficient way is
to use `Operator-Precedence
Parsing <http://en.wikipedia.org/wiki/Operator-precedence_parser>`_.
This parsing technique uses the precedence of binary operators to guide
recursion. To start with, we need a table of precedences:
.. code-block:: ocaml
(* binop_precedence - This holds the precedence for each binary operator that is
* defined *)
let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
(* precedence - Get the precedence of the pending binary operator token. *)
let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
...
let main () =
(* Install standard binary operators.
* 1 is the lowest precedence. *)
Hashtbl.add Parser.binop_precedence '<' 10;
Hashtbl.add Parser.binop_precedence '+' 20;
Hashtbl.add Parser.binop_precedence '-' 20;
Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *)
...
For the basic form of Kaleidoscope, we will only support 4 binary
operators (this can obviously be extended by you, our brave and intrepid
reader). The ``Parser.precedence`` function returns the precedence for
the current token, or -1 if the token is not a binary operator. Having a
``Hashtbl.t`` makes it easy to add new operators and makes it clear that
the algorithm doesn't depend on the specific operators involved, but it
would be easy enough to eliminate the ``Hashtbl.t`` and do the
comparisons in the ``Parser.precedence`` function. (Or just use a
fixed-size array).
With the helper above defined, we can now start parsing binary
expressions. The basic idea of operator precedence parsing is to break
down an expression with potentially ambiguous binary operators into
pieces. Consider ,for example, the expression "a+b+(c+d)\*e\*f+g".
Operator precedence parsing considers this as a stream of primary
expressions separated by binary operators. As such, it will first parse
the leading primary expression "a", then it will see the pairs [+, b]
[+, (c+d)] [\*, e] [\*, f] and [+, g]. Note that because parentheses are
primary expressions, the binary expression parser doesn't need to worry
about nested subexpressions like (c+d) at all.
To start, an expression is a primary expression potentially followed by
a sequence of [binop,primaryexpr] pairs:
.. code-block:: ocaml
(* expression
* ::= primary binoprhs *)
and parse_expr = parser
| [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream
``Parser.parse_bin_rhs`` is the function that parses the sequence of
pairs for us. It takes a precedence and a pointer to an expression for
the part that has been parsed so far. Note that "x" is a perfectly valid
expression: As such, "binoprhs" is allowed to be empty, in which case it
returns the expression that is passed into it. In our example above, the
code passes the expression for "a" into ``Parser.parse_bin_rhs`` and the
current token is "+".
The precedence value passed into ``Parser.parse_bin_rhs`` indicates the
*minimal operator precedence* that the function is allowed to eat. For
example, if the current pair stream is [+, x] and
``Parser.parse_bin_rhs`` is passed in a precedence of 40, it will not
consume any tokens (because the precedence of '+' is only 20). With this
in mind, ``Parser.parse_bin_rhs`` starts with:
.. code-block:: ocaml
(* binoprhs
* ::= ('+' primary)* *)
and parse_bin_rhs expr_prec lhs stream =
match Stream.peek stream with
(* If this is a binop, find its precedence. *)
| Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
let token_prec = precedence c in
(* If this is a binop that binds at least as tightly as the current binop,
* consume it, otherwise we are done. *)
if token_prec < expr_prec then lhs else begin
This code gets the precedence of the current token and checks to see if
if is too low. Because we defined invalid tokens to have a precedence of
-1, this check implicitly knows that the pair-stream ends when the token
stream runs out of binary operators. If this check succeeds, we know
that the token is a binary operator and that it will be included in this
expression:
.. code-block:: ocaml
(* Eat the binop. *)
Stream.junk stream;
(* Okay, we know this is a binop. *)
let rhs =
match Stream.peek stream with
| Some (Token.Kwd c2) ->
As such, this code eats (and remembers) the binary operator and then
parses the primary expression that follows. This builds up the whole
pair, the first of which is [+, b] for the running example.
Now that we parsed the left-hand side of an expression and one pair of
the RHS sequence, we have to decide which way the expression associates.
In particular, we could have "(a+b) binop unparsed" or "a + (b binop
unparsed)". To determine this, we look ahead at "binop" to determine its
precedence and compare it to BinOp's precedence (which is '+' in this
case):
.. code-block:: ocaml
(* If BinOp binds less tightly with rhs than the operator after
* rhs, let the pending operator take rhs as its lhs. *)
let next_prec = precedence c2 in
if token_prec < next_prec
If the precedence of the binop to the right of "RHS" is lower or equal
to the precedence of our current operator, then we know that the
parentheses associate as "(a+b) binop ...". In our example, the current
operator is "+" and the next operator is "+", we know that they have the
same precedence. In this case we'll create the AST node for "a+b", and
then continue parsing:
.. code-block:: ocaml
... if body omitted ...
in
(* Merge lhs/rhs. *)
let lhs = Ast.Binary (c, lhs, rhs) in
parse_bin_rhs expr_prec lhs stream
end
In our example above, this will turn "a+b+" into "(a+b)" and execute the
next iteration of the loop, with "+" as the current token. The code
above will eat, remember, and parse "(c+d)" as the primary expression,
which makes the current pair equal to [+, (c+d)]. It will then evaluate
the 'if' conditional above with "\*" as the binop to the right of the
primary. In this case, the precedence of "\*" is higher than the
precedence of "+" so the if condition will be entered.
The critical question left here is "how can the if condition parse the
right hand side in full"? In particular, to build the AST correctly for
our example, it needs to get all of "(c+d)\*e\*f" as the RHS expression
variable. The code to do this is surprisingly simple (code from the
above two blocks duplicated for context):
.. code-block:: ocaml
match Stream.peek stream with
| Some (Token.Kwd c2) ->
(* If BinOp binds less tightly with rhs than the operator after
* rhs, let the pending operator take rhs as its lhs. *)
if token_prec < precedence c2
then parse_bin_rhs (token_prec + 1) rhs stream
else rhs
| _ -> rhs
in
(* Merge lhs/rhs. *)
let lhs = Ast.Binary (c, lhs, rhs) in
parse_bin_rhs expr_prec lhs stream
end
At this point, we know that the binary operator to the RHS of our
primary has higher precedence than the binop we are currently parsing.
As such, we know that any sequence of pairs whose operators are all
higher precedence than "+" should be parsed together and returned as
"RHS". To do this, we recursively invoke the ``Parser.parse_bin_rhs``
function specifying "token\_prec+1" as the minimum precedence required
for it to continue. In our example above, this will cause it to return
the AST node for "(c+d)\*e\*f" as RHS, which is then set as the RHS of
the '+' expression.
Finally, on the next iteration of the while loop, the "+g" piece is
parsed and added to the AST. With this little bit of code (14
non-trivial lines), we correctly handle fully general binary expression
parsing in a very elegant way. This was a whirlwind tour of this code,
and it is somewhat subtle. I recommend running through it with a few
tough examples to see how it works.
This wraps up handling of expressions. At this point, we can point the
parser at an arbitrary token stream and build an expression from it,
stopping at the first token that is not part of the expression. Next up
we need to handle function definitions, etc.
Parsing the Rest
================
The next thing missing is handling of function prototypes. In
Kaleidoscope, these are used both for 'extern' function declarations as
well as function body definitions. The code to do this is
straight-forward and not very interesting (once you've survived
expressions):
.. code-block:: ocaml
(* prototype
* ::= id '(' id* ')' *)
let parse_prototype =
let rec parse_args accumulator = parser
| [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
| [< >] -> accumulator
in
parser
| [< 'Token.Ident id;
'Token.Kwd '(' ?? "expected '(' in prototype";
args=parse_args [];
'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
(* success. *)
Ast.Prototype (id, Array.of_list (List.rev args))
| [< >] ->
raise (Stream.Error "expected function name in prototype")
Given this, a function definition is very simple, just a prototype plus
an expression to implement the body:
.. code-block:: ocaml
(* definition ::= 'def' prototype expression *)
let parse_definition = parser
| [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
Ast.Function (p, e)
In addition, we support 'extern' to declare functions like 'sin' and
'cos' as well as to support forward declaration of user functions. These
'extern's are just prototypes with no body:
.. code-block:: ocaml
(* external ::= 'extern' prototype *)
let parse_extern = parser
| [< 'Token.Extern; e=parse_prototype >] -> e
Finally, we'll also let the user type in arbitrary top-level expressions
and evaluate them on the fly. We will handle this by defining anonymous
nullary (zero argument) functions for them:
.. code-block:: ocaml
(* toplevelexpr ::= expression *)
let parse_toplevel = parser
| [< e=parse_expr >] ->
(* Make an anonymous proto. *)
Ast.Function (Ast.Prototype ("", [||]), e)
Now that we have all the pieces, let's build a little driver that will
let us actually *execute* this code we've built!
The Driver
==========
The driver for this simply invokes all of the parsing pieces with a
top-level dispatch loop. There isn't much interesting here, so I'll just
include the top-level loop. See `below <#code>`_ for full code in the
"Top-Level Parsing" section.
.. code-block:: ocaml
(* top ::= definition | external | expression | ';' *)
let rec main_loop stream =
match Stream.peek stream with
| None -> ()
(* ignore top-level semicolons. *)
| Some (Token.Kwd ';') ->
Stream.junk stream;
main_loop stream
| Some token ->
begin
try match token with
| Token.Def ->
ignore(Parser.parse_definition stream);
print_endline "parsed a function definition.";
| Token.Extern ->
ignore(Parser.parse_extern stream);
print_endline "parsed an extern.";
| _ ->
(* Evaluate a top-level expression into an anonymous function. *)
ignore(Parser.parse_toplevel stream);
print_endline "parsed a top-level expr";
with Stream.Error s ->
(* Skip token for error recovery. *)
Stream.junk stream;
print_endline s;
end;
print_string "ready> "; flush stdout;
main_loop stream
The most interesting part of this is that we ignore top-level
semicolons. Why is this, you ask? The basic reason is that if you type
"4 + 5" at the command line, the parser doesn't know whether that is the
end of what you will type or not. For example, on the next line you
could type "def foo..." in which case 4+5 is the end of a top-level
expression. Alternatively you could type "\* 6", which would continue
the expression. Having top-level semicolons allows you to type "4+5;",
and the parser will know you are done.
Conclusions
===========
With just under 300 lines of commented code (240 lines of non-comment,
non-blank code), we fully defined our minimal language, including a
lexer, parser, and AST builder. With this done, the executable will
validate Kaleidoscope code and tell us if it is grammatically invalid.
For example, here is a sample interaction:
.. code-block:: bash
$ ./toy.byte
ready> def foo(x y) x+foo(y, 4.0);
Parsed a function definition.
ready> def foo(x y) x+y y;
Parsed a function definition.
Parsed a top-level expr
ready> def foo(x y) x+y );
Parsed a function definition.
Error: unknown token when expecting an expression
ready> extern sin(a);
ready> Parsed an extern
ready> ^D
$
There is a lot of room for extension here. You can define new AST nodes,
extend the language in many ways, etc. In the `next
installment <OCamlLangImpl3.html>`_, we will describe how to generate
LLVM Intermediate Representation (IR) from the AST.
Full Code Listing
=================
Here is the complete code listing for this and the previous chapter.
Note that it is fully self-contained: you don't need LLVM or any
external libraries at all for this. (Besides the ocaml standard
libraries, of course.) To build this, just compile with:
.. code-block:: bash
# Compile
ocamlbuild toy.byte
# Run
./toy.byte
Here is the code:
\_tags:
::
<{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
token.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Lexer Tokens
*===----------------------------------------------------------------------===*)
(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
* these others for known things. *)
type token =
(* commands *)
| Def | Extern
(* primary *)
| Ident of string | Number of float
(* unknown *)
| Kwd of char
lexer.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Lexer
*===----------------------------------------------------------------------===*)
let rec lex = parser
(* Skip any whitespace. *)
| [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
(* identifier: [a-zA-Z][a-zA-Z0-9] *)
| [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_ident buffer stream
(* number: [0-9.]+ *)
| [< ' ('0' .. '9' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_number buffer stream
(* Comment until end of line. *)
| [< ' ('#'); stream >] ->
lex_comment stream
(* Otherwise, just return the character as its ascii value. *)
| [< 'c; stream >] ->
[< 'Token.Kwd c; lex stream >]
(* end of stream. *)
| [< >] -> [< >]
and lex_number buffer = parser
| [< ' ('0' .. '9' | '.' as c); stream >] ->
Buffer.add_char buffer c;
lex_number buffer stream
| [< stream=lex >] ->
[< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
and lex_ident buffer = parser
| [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
Buffer.add_char buffer c;
lex_ident buffer stream
| [< stream=lex >] ->
match Buffer.contents buffer with
| "def" -> [< 'Token.Def; stream >]
| "extern" -> [< 'Token.Extern; stream >]
| id -> [< 'Token.Ident id; stream >]
and lex_comment = parser
| [< ' ('\n'); stream=lex >] -> stream
| [< 'c; e=lex_comment >] -> e
| [< >] -> [< >]
ast.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Abstract Syntax Tree (aka Parse Tree)
*===----------------------------------------------------------------------===*)
(* expr - Base type for all expression nodes. *)
type expr =
(* variant for numeric literals like "1.0". *)
| Number of float
(* variant for referencing a variable, like "a". *)
| Variable of string
(* variant for a binary operator. *)
| Binary of char * expr * expr
(* variant for function calls. *)
| Call of string * expr array
(* proto - This type represents the "prototype" for a function, which captures
* its name, and its argument names (thus implicitly the number of arguments the
* function takes). *)
type proto = Prototype of string * string array
(* func - This type represents a function definition itself. *)
type func = Function of proto * expr
parser.ml:
.. code-block:: ocaml
(*===---------------------------------------------------------------------===
* Parser
*===---------------------------------------------------------------------===*)
(* binop_precedence - This holds the precedence for each binary operator that is
* defined *)
let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
(* precedence - Get the precedence of the pending binary operator token. *)
let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
(* primary
* ::= identifier
* ::= numberexpr
* ::= parenexpr *)
let rec parse_primary = parser
(* numberexpr ::= number *)
| [< 'Token.Number n >] -> Ast.Number n
(* parenexpr ::= '(' expression ')' *)
| [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
(* identifierexpr
* ::= identifier
* ::= identifier '(' argumentexpr ')' *)
| [< 'Token.Ident id; stream >] ->
let rec parse_args accumulator = parser
| [< e=parse_expr; stream >] ->
begin parser
| [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
| [< >] -> e :: accumulator
end stream
| [< >] -> accumulator
in
let rec parse_ident id = parser
(* Call. *)
| [< 'Token.Kwd '(';
args=parse_args [];
'Token.Kwd ')' ?? "expected ')'">] ->
Ast.Call (id, Array.of_list (List.rev args))
(* Simple variable ref. *)
| [< >] -> Ast.Variable id
in
parse_ident id stream
| [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
(* binoprhs
* ::= ('+' primary)* *)
and parse_bin_rhs expr_prec lhs stream =
match Stream.peek stream with
(* If this is a binop, find its precedence. *)
| Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
let token_prec = precedence c in
(* If this is a binop that binds at least as tightly as the current binop,
* consume it, otherwise we are done. *)
if token_prec < expr_prec then lhs else begin
(* Eat the binop. *)
Stream.junk stream;
(* Parse the primary expression after the binary operator. *)
let rhs = parse_primary stream in
(* Okay, we know this is a binop. *)
let rhs =
match Stream.peek stream with
| Some (Token.Kwd c2) ->
(* If BinOp binds less tightly with rhs than the operator after
* rhs, let the pending operator take rhs as its lhs. *)
let next_prec = precedence c2 in
if token_prec < next_prec
then parse_bin_rhs (token_prec + 1) rhs stream
else rhs
| _ -> rhs
in
(* Merge lhs/rhs. *)
let lhs = Ast.Binary (c, lhs, rhs) in
parse_bin_rhs expr_prec lhs stream
end
| _ -> lhs
(* expression
* ::= primary binoprhs *)
and parse_expr = parser
| [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream
(* prototype
* ::= id '(' id* ')' *)
let parse_prototype =
let rec parse_args accumulator = parser
| [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
| [< >] -> accumulator
in
parser
| [< 'Token.Ident id;
'Token.Kwd '(' ?? "expected '(' in prototype";
args=parse_args [];
'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
(* success. *)
Ast.Prototype (id, Array.of_list (List.rev args))
| [< >] ->
raise (Stream.Error "expected function name in prototype")
(* definition ::= 'def' prototype expression *)
let parse_definition = parser
| [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
Ast.Function (p, e)
(* toplevelexpr ::= expression *)
let parse_toplevel = parser
| [< e=parse_expr >] ->
(* Make an anonymous proto. *)
Ast.Function (Ast.Prototype ("", [||]), e)
(* external ::= 'extern' prototype *)
let parse_extern = parser
| [< 'Token.Extern; e=parse_prototype >] -> e
toplevel.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Top-Level parsing and JIT Driver
*===----------------------------------------------------------------------===*)
(* top ::= definition | external | expression | ';' *)
let rec main_loop stream =
match Stream.peek stream with
| None -> ()
(* ignore top-level semicolons. *)
| Some (Token.Kwd ';') ->
Stream.junk stream;
main_loop stream
| Some token ->
begin
try match token with
| Token.Def ->
ignore(Parser.parse_definition stream);
print_endline "parsed a function definition.";
| Token.Extern ->
ignore(Parser.parse_extern stream);
print_endline "parsed an extern.";
| _ ->
(* Evaluate a top-level expression into an anonymous function. *)
ignore(Parser.parse_toplevel stream);
print_endline "parsed a top-level expr";
with Stream.Error s ->
(* Skip token for error recovery. *)
Stream.junk stream;
print_endline s;
end;
print_string "ready> "; flush stdout;
main_loop stream
toy.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Main driver code.
*===----------------------------------------------------------------------===*)
let main () =
(* Install standard binary operators.
* 1 is the lowest precedence. *)
Hashtbl.add Parser.binop_precedence '<' 10;
Hashtbl.add Parser.binop_precedence '+' 20;
Hashtbl.add Parser.binop_precedence '-' 20;
Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *)
(* Prime the first token. *)
print_string "ready> "; flush stdout;
let stream = Lexer.lex (Stream.of_channel stdin) in
(* Run the main "interpreter loop" now. *)
Toplevel.main_loop stream;
;;
main ()
`Next: Implementing Code Generation to LLVM IR <OCamlLangImpl3.html>`_

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========================================
Kaleidoscope: Code generation to LLVM IR
========================================
.. contents::
:local:
Written by `Chris Lattner <mailto:sabre@nondot.org>`_ and `Erick
Tryzelaar <mailto:idadesub@users.sourceforge.net>`_
Chapter 3 Introduction
======================
Welcome to Chapter 3 of the "`Implementing a language with
LLVM <index.html>`_" tutorial. This chapter shows you how to transform
the `Abstract Syntax Tree <OCamlLangImpl2.html>`_, built in Chapter 2,
into LLVM IR. This will teach you a little bit about how LLVM does
things, as well as demonstrate how easy it is to use. It's much more
work to build a lexer and parser than it is to generate LLVM IR code. :)
**Please note**: the code in this chapter and later require LLVM 2.3 or
LLVM SVN to work. LLVM 2.2 and before will not work with it.
Code Generation Setup
=====================
In order to generate LLVM IR, we want some simple setup to get started.
First we define virtual code generation (codegen) methods in each AST
class:
.. code-block:: ocaml
let rec codegen_expr = function
| Ast.Number n -> ...
| Ast.Variable name -> ...
The ``Codegen.codegen_expr`` function says to emit IR for that AST node
along with all the things it depends on, and they all return an LLVM
Value object. "Value" is the class used to represent a "`Static Single
Assignment
(SSA) <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_
register" or "SSA value" in LLVM. The most distinct aspect of SSA values
is that their value is computed as the related instruction executes, and
it does not get a new value until (and if) the instruction re-executes.
In other words, there is no way to "change" an SSA value. For more
information, please read up on `Static Single
Assignment <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_
- the concepts are really quite natural once you grok them.
The second thing we want is an "Error" exception like we used for the
parser, which will be used to report errors found during code generation
(for example, use of an undeclared parameter):
.. code-block:: ocaml
exception Error of string
let context = global_context ()
let the_module = create_module context "my cool jit"
let builder = builder context
let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
let double_type = double_type context
The static variables will be used during code generation.
``Codgen.the_module`` is the LLVM construct that contains all of the
functions and global variables in a chunk of code. In many ways, it is
the top-level structure that the LLVM IR uses to contain code.
The ``Codegen.builder`` object is a helper object that makes it easy to
generate LLVM instructions. Instances of the
```IRBuilder`` <http://llvm.org/doxygen/IRBuilder_8h-source.html>`_
class keep track of the current place to insert instructions and has
methods to create new instructions.
The ``Codegen.named_values`` map keeps track of which values are defined
in the current scope and what their LLVM representation is. (In other
words, it is a symbol table for the code). In this form of Kaleidoscope,
the only things that can be referenced are function parameters. As such,
function parameters will be in this map when generating code for their
function body.
With these basics in place, we can start talking about how to generate
code for each expression. Note that this assumes that the
``Codgen.builder`` has been set up to generate code *into* something.
For now, we'll assume that this has already been done, and we'll just
use it to emit code.
Expression Code Generation
==========================
Generating LLVM code for expression nodes is very straightforward: less
than 30 lines of commented code for all four of our expression nodes.
First we'll do numeric literals:
.. code-block:: ocaml
| Ast.Number n -> const_float double_type n
In the LLVM IR, numeric constants are represented with the
``ConstantFP`` class, which holds the numeric value in an ``APFloat``
internally (``APFloat`` has the capability of holding floating point
constants of Arbitrary Precision). This code basically just creates
and returns a ``ConstantFP``. Note that in the LLVM IR that constants
are all uniqued together and shared. For this reason, the API uses "the
foo::get(..)" idiom instead of "new foo(..)" or "foo::Create(..)".
.. code-block:: ocaml
| Ast.Variable name ->
(try Hashtbl.find named_values name with
| Not_found -> raise (Error "unknown variable name"))
References to variables are also quite simple using LLVM. In the simple
version of Kaleidoscope, we assume that the variable has already been
emitted somewhere and its value is available. In practice, the only
values that can be in the ``Codegen.named_values`` map are function
arguments. This code simply checks to see that the specified name is in
the map (if not, an unknown variable is being referenced) and returns
the value for it. In future chapters, we'll add support for `loop
induction variables <LangImpl5.html#for>`_ in the symbol table, and for
`local variables <LangImpl7.html#localvars>`_.
.. code-block:: ocaml
| Ast.Binary (op, lhs, rhs) ->
let lhs_val = codegen_expr lhs in
let rhs_val = codegen_expr rhs in
begin
match op with
| '+' -> build_fadd lhs_val rhs_val "addtmp" builder
| '-' -> build_fsub lhs_val rhs_val "subtmp" builder
| '*' -> build_fmul lhs_val rhs_val "multmp" builder
| '<' ->
(* Convert bool 0/1 to double 0.0 or 1.0 *)
let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
build_uitofp i double_type "booltmp" builder
| _ -> raise (Error "invalid binary operator")
end
Binary operators start to get more interesting. The basic idea here is
that we recursively emit code for the left-hand side of the expression,
then the right-hand side, then we compute the result of the binary
expression. In this code, we do a simple switch on the opcode to create
the right LLVM instruction.
In the example above, the LLVM builder class is starting to show its
value. IRBuilder knows where to insert the newly created instruction,
all you have to do is specify what instruction to create (e.g. with
``Llvm.create_add``), which operands to use (``lhs`` and ``rhs`` here)
and optionally provide a name for the generated instruction.
One nice thing about LLVM is that the name is just a hint. For instance,
if the code above emits multiple "addtmp" variables, LLVM will
automatically provide each one with an increasing, unique numeric
suffix. Local value names for instructions are purely optional, but it
makes it much easier to read the IR dumps.
`LLVM instructions <../LangRef.html#instref>`_ are constrained by strict
rules: for example, the Left and Right operators of an `add
instruction <../LangRef.html#i_add>`_ must have the same type, and the
result type of the add must match the operand types. Because all values
in Kaleidoscope are doubles, this makes for very simple code for add,
sub and mul.
On the other hand, LLVM specifies that the `fcmp
instruction <../LangRef.html#i_fcmp>`_ always returns an 'i1' value (a
one bit integer). The problem with this is that Kaleidoscope wants the
value to be a 0.0 or 1.0 value. In order to get these semantics, we
combine the fcmp instruction with a `uitofp
instruction <../LangRef.html#i_uitofp>`_. This instruction converts its
input integer into a floating point value by treating the input as an
unsigned value. In contrast, if we used the `sitofp
instruction <../LangRef.html#i_sitofp>`_, the Kaleidoscope '<' operator
would return 0.0 and -1.0, depending on the input value.
.. code-block:: ocaml
| Ast.Call (callee, args) ->
(* Look up the name in the module table. *)
let callee =
match lookup_function callee the_module with
| Some callee -> callee
| None -> raise (Error "unknown function referenced")
in
let params = params callee in
(* If argument mismatch error. *)
if Array.length params == Array.length args then () else
raise (Error "incorrect # arguments passed");
let args = Array.map codegen_expr args in
build_call callee args "calltmp" builder
Code generation for function calls is quite straightforward with LLVM.
The code above initially does a function name lookup in the LLVM
Module's symbol table. Recall that the LLVM Module is the container that
holds all of the functions we are JIT'ing. By giving each function the
same name as what the user specifies, we can use the LLVM symbol table
to resolve function names for us.
Once we have the function to call, we recursively codegen each argument
that is to be passed in, and create an LLVM `call
instruction <../LangRef.html#i_call>`_. Note that LLVM uses the native C
calling conventions by default, allowing these calls to also call into
standard library functions like "sin" and "cos", with no additional
effort.
This wraps up our handling of the four basic expressions that we have so
far in Kaleidoscope. Feel free to go in and add some more. For example,
by browsing the `LLVM language reference <../LangRef.html>`_ you'll find
several other interesting instructions that are really easy to plug into
our basic framework.
Function Code Generation
========================
Code generation for prototypes and functions must handle a number of
details, which make their code less beautiful than expression code
generation, but allows us to illustrate some important points. First,
lets talk about code generation for prototypes: they are used both for
function bodies and external function declarations. The code starts
with:
.. code-block:: ocaml
let codegen_proto = function
| Ast.Prototype (name, args) ->
(* Make the function type: double(double,double) etc. *)
let doubles = Array.make (Array.length args) double_type in
let ft = function_type double_type doubles in
let f =
match lookup_function name the_module with
This code packs a lot of power into a few lines. Note first that this
function returns a "Function\*" instead of a "Value\*" (although at the
moment they both are modeled by ``llvalue`` in ocaml). Because a
"prototype" really talks about the external interface for a function
(not the value computed by an expression), it makes sense for it to
return the LLVM Function it corresponds to when codegen'd.
The call to ``Llvm.function_type`` creates the ``Llvm.llvalue`` that
should be used for a given Prototype. Since all function arguments in
Kaleidoscope are of type double, the first line creates a vector of "N"
LLVM double types. It then uses the ``Llvm.function_type`` method to
create a function type that takes "N" doubles as arguments, returns one
double as a result, and that is not vararg (that uses the function
``Llvm.var_arg_function_type``). Note that Types in LLVM are uniqued
just like ``Constant``'s are, so you don't "new" a type, you "get" it.
The final line above checks if the function has already been defined in
``Codegen.the_module``. If not, we will create it.
.. code-block:: ocaml
| None -> declare_function name ft the_module
This indicates the type and name to use, as well as which module to
insert into. By default we assume a function has
``Llvm.Linkage.ExternalLinkage``. "`external
linkage <LangRef.html#linkage>`_" means that the function may be defined
outside the current module and/or that it is callable by functions
outside the module. The "``name``" passed in is the name the user
specified: this name is registered in "``Codegen.the_module``"s symbol
table, which is used by the function call code above.
In Kaleidoscope, I choose to allow redefinitions of functions in two
cases: first, we want to allow 'extern'ing a function more than once, as
long as the prototypes for the externs match (since all arguments have
the same type, we just have to check that the number of arguments
match). Second, we want to allow 'extern'ing a function and then
defining a body for it. This is useful when defining mutually recursive
functions.
.. code-block:: ocaml
(* If 'f' conflicted, there was already something named 'name'. If it
* has a body, don't allow redefinition or reextern. *)
| Some f ->
(* If 'f' already has a body, reject this. *)
if Array.length (basic_blocks f) == 0 then () else
raise (Error "redefinition of function");
(* If 'f' took a different number of arguments, reject. *)
if Array.length (params f) == Array.length args then () else
raise (Error "redefinition of function with different # args");
f
in
In order to verify the logic above, we first check to see if the
pre-existing function is "empty". In this case, empty means that it has
no basic blocks in it, which means it has no body. If it has no body, it
is a forward declaration. Since we don't allow anything after a full
definition of the function, the code rejects this case. If the previous
reference to a function was an 'extern', we simply verify that the
number of arguments for that definition and this one match up. If not,
we emit an error.
.. code-block:: ocaml
(* Set names for all arguments. *)
Array.iteri (fun i a ->
let n = args.(i) in
set_value_name n a;
Hashtbl.add named_values n a;
) (params f);
f
The last bit of code for prototypes loops over all of the arguments in
the function, setting the name of the LLVM Argument objects to match,
and registering the arguments in the ``Codegen.named_values`` map for
future use by the ``Ast.Variable`` variant. Once this is set up, it
returns the Function object to the caller. Note that we don't check for
conflicting argument names here (e.g. "extern foo(a b a)"). Doing so
would be very straight-forward with the mechanics we have already used
above.
.. code-block:: ocaml
let codegen_func = function
| Ast.Function (proto, body) ->
Hashtbl.clear named_values;
let the_function = codegen_proto proto in
Code generation for function definitions starts out simply enough: we
just codegen the prototype (Proto) and verify that it is ok. We then
clear out the ``Codegen.named_values`` map to make sure that there isn't
anything in it from the last function we compiled. Code generation of
the prototype ensures that there is an LLVM Function object that is
ready to go for us.
.. code-block:: ocaml
(* Create a new basic block to start insertion into. *)
let bb = append_block context "entry" the_function in
position_at_end bb builder;
try
let ret_val = codegen_expr body in
Now we get to the point where the ``Codegen.builder`` is set up. The
first line creates a new `basic
block <http://en.wikipedia.org/wiki/Basic_block>`_ (named "entry"),
which is inserted into ``the_function``. The second line then tells the
builder that new instructions should be inserted into the end of the new
basic block. Basic blocks in LLVM are an important part of functions
that define the `Control Flow
Graph <http://en.wikipedia.org/wiki/Control_flow_graph>`_. Since we
don't have any control flow, our functions will only contain one block
at this point. We'll fix this in `Chapter 5 <OCamlLangImpl5.html>`_ :).
.. code-block:: ocaml
let ret_val = codegen_expr body in
(* Finish off the function. *)
let _ = build_ret ret_val builder in
(* Validate the generated code, checking for consistency. *)
Llvm_analysis.assert_valid_function the_function;
the_function
Once the insertion point is set up, we call the ``Codegen.codegen_func``
method for the root expression of the function. If no error happens,
this emits code to compute the expression into the entry block and
returns the value that was computed. Assuming no error, we then create
an LLVM `ret instruction <../LangRef.html#i_ret>`_, which completes the
function. Once the function is built, we call
``Llvm_analysis.assert_valid_function``, which is provided by LLVM. This
function does a variety of consistency checks on the generated code, to
determine if our compiler is doing everything right. Using this is
important: it can catch a lot of bugs. Once the function is finished and
validated, we return it.
.. code-block:: ocaml
with e ->
delete_function the_function;
raise e
The only piece left here is handling of the error case. For simplicity,
we handle this by merely deleting the function we produced with the
``Llvm.delete_function`` method. This allows the user to redefine a
function that they incorrectly typed in before: if we didn't delete it,
it would live in the symbol table, with a body, preventing future
redefinition.
This code does have a bug, though. Since the ``Codegen.codegen_proto``
can return a previously defined forward declaration, our code can
actually delete a forward declaration. There are a number of ways to fix
this bug, see what you can come up with! Here is a testcase:
::
extern foo(a b); # ok, defines foo.
def foo(a b) c; # error, 'c' is invalid.
def bar() foo(1, 2); # error, unknown function "foo"
Driver Changes and Closing Thoughts
===================================
For now, code generation to LLVM doesn't really get us much, except that
we can look at the pretty IR calls. The sample code inserts calls to
Codegen into the "``Toplevel.main_loop``", and then dumps out the LLVM
IR. This gives a nice way to look at the LLVM IR for simple functions.
For example:
::
ready> 4+5;
Read top-level expression:
define double @""() {
entry:
%addtmp = fadd double 4.000000e+00, 5.000000e+00
ret double %addtmp
}
Note how the parser turns the top-level expression into anonymous
functions for us. This will be handy when we add `JIT
support <OCamlLangImpl4.html#jit>`_ in the next chapter. Also note that
the code is very literally transcribed, no optimizations are being
performed. We will `add
optimizations <OCamlLangImpl4.html#trivialconstfold>`_ explicitly in the
next chapter.
::
ready> def foo(a b) a*a + 2*a*b + b*b;
Read function definition:
define double @foo(double %a, double %b) {
entry:
%multmp = fmul double %a, %a
%multmp1 = fmul double 2.000000e+00, %a
%multmp2 = fmul double %multmp1, %b
%addtmp = fadd double %multmp, %multmp2
%multmp3 = fmul double %b, %b
%addtmp4 = fadd double %addtmp, %multmp3
ret double %addtmp4
}
This shows some simple arithmetic. Notice the striking similarity to the
LLVM builder calls that we use to create the instructions.
::
ready> def bar(a) foo(a, 4.0) + bar(31337);
Read function definition:
define double @bar(double %a) {
entry:
%calltmp = call double @foo(double %a, double 4.000000e+00)
%calltmp1 = call double @bar(double 3.133700e+04)
%addtmp = fadd double %calltmp, %calltmp1
ret double %addtmp
}
This shows some function calls. Note that this function will take a long
time to execute if you call it. In the future we'll add conditional
control flow to actually make recursion useful :).
::
ready> extern cos(x);
Read extern:
declare double @cos(double)
ready> cos(1.234);
Read top-level expression:
define double @""() {
entry:
%calltmp = call double @cos(double 1.234000e+00)
ret double %calltmp
}
This shows an extern for the libm "cos" function, and a call to it.
::
ready> ^D
; ModuleID = 'my cool jit'
define double @""() {
entry:
%addtmp = fadd double 4.000000e+00, 5.000000e+00
ret double %addtmp
}
define double @foo(double %a, double %b) {
entry:
%multmp = fmul double %a, %a
%multmp1 = fmul double 2.000000e+00, %a
%multmp2 = fmul double %multmp1, %b
%addtmp = fadd double %multmp, %multmp2
%multmp3 = fmul double %b, %b
%addtmp4 = fadd double %addtmp, %multmp3
ret double %addtmp4
}
define double @bar(double %a) {
entry:
%calltmp = call double @foo(double %a, double 4.000000e+00)
%calltmp1 = call double @bar(double 3.133700e+04)
%addtmp = fadd double %calltmp, %calltmp1
ret double %addtmp
}
declare double @cos(double)
define double @""() {
entry:
%calltmp = call double @cos(double 1.234000e+00)
ret double %calltmp
}
When you quit the current demo, it dumps out the IR for the entire
module generated. Here you can see the big picture with all the
functions referencing each other.
This wraps up the third chapter of the Kaleidoscope tutorial. Up next,
we'll describe how to `add JIT codegen and optimizer
support <OCamlLangImpl4.html>`_ to this so we can actually start running
code!
Full Code Listing
=================
Here is the complete code listing for our running example, enhanced with
the LLVM code generator. Because this uses the LLVM libraries, we need
to link them in. To do this, we use the
`llvm-config <http://llvm.org/cmds/llvm-config.html>`_ tool to inform
our makefile/command line about which options to use:
.. code-block:: bash
# Compile
ocamlbuild toy.byte
# Run
./toy.byte
Here is the code:
\_tags:
::
<{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
<*.{byte,native}>: g++, use_llvm, use_llvm_analysis
myocamlbuild.ml:
.. code-block:: ocaml
open Ocamlbuild_plugin;;
ocaml_lib ~extern:true "llvm";;
ocaml_lib ~extern:true "llvm_analysis";;
flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;
token.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Lexer Tokens
*===----------------------------------------------------------------------===*)
(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
* these others for known things. *)
type token =
(* commands *)
| Def | Extern
(* primary *)
| Ident of string | Number of float
(* unknown *)
| Kwd of char
lexer.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Lexer
*===----------------------------------------------------------------------===*)
let rec lex = parser
(* Skip any whitespace. *)
| [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
(* identifier: [a-zA-Z][a-zA-Z0-9] *)
| [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_ident buffer stream
(* number: [0-9.]+ *)
| [< ' ('0' .. '9' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_number buffer stream
(* Comment until end of line. *)
| [< ' ('#'); stream >] ->
lex_comment stream
(* Otherwise, just return the character as its ascii value. *)
| [< 'c; stream >] ->
[< 'Token.Kwd c; lex stream >]
(* end of stream. *)
| [< >] -> [< >]
and lex_number buffer = parser
| [< ' ('0' .. '9' | '.' as c); stream >] ->
Buffer.add_char buffer c;
lex_number buffer stream
| [< stream=lex >] ->
[< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
and lex_ident buffer = parser
| [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
Buffer.add_char buffer c;
lex_ident buffer stream
| [< stream=lex >] ->
match Buffer.contents buffer with
| "def" -> [< 'Token.Def; stream >]
| "extern" -> [< 'Token.Extern; stream >]
| id -> [< 'Token.Ident id; stream >]
and lex_comment = parser
| [< ' ('\n'); stream=lex >] -> stream
| [< 'c; e=lex_comment >] -> e
| [< >] -> [< >]
ast.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Abstract Syntax Tree (aka Parse Tree)
*===----------------------------------------------------------------------===*)
(* expr - Base type for all expression nodes. *)
type expr =
(* variant for numeric literals like "1.0". *)
| Number of float
(* variant for referencing a variable, like "a". *)
| Variable of string
(* variant for a binary operator. *)
| Binary of char * expr * expr
(* variant for function calls. *)
| Call of string * expr array
(* proto - This type represents the "prototype" for a function, which captures
* its name, and its argument names (thus implicitly the number of arguments the
* function takes). *)
type proto = Prototype of string * string array
(* func - This type represents a function definition itself. *)
type func = Function of proto * expr
parser.ml:
.. code-block:: ocaml
(*===---------------------------------------------------------------------===
* Parser
*===---------------------------------------------------------------------===*)
(* binop_precedence - This holds the precedence for each binary operator that is
* defined *)
let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
(* precedence - Get the precedence of the pending binary operator token. *)
let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
(* primary
* ::= identifier
* ::= numberexpr
* ::= parenexpr *)
let rec parse_primary = parser
(* numberexpr ::= number *)
| [< 'Token.Number n >] -> Ast.Number n
(* parenexpr ::= '(' expression ')' *)
| [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
(* identifierexpr
* ::= identifier
* ::= identifier '(' argumentexpr ')' *)
| [< 'Token.Ident id; stream >] ->
let rec parse_args accumulator = parser
| [< e=parse_expr; stream >] ->
begin parser
| [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
| [< >] -> e :: accumulator
end stream
| [< >] -> accumulator
in
let rec parse_ident id = parser
(* Call. *)
| [< 'Token.Kwd '(';
args=parse_args [];
'Token.Kwd ')' ?? "expected ')'">] ->
Ast.Call (id, Array.of_list (List.rev args))
(* Simple variable ref. *)
| [< >] -> Ast.Variable id
in
parse_ident id stream
| [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
(* binoprhs
* ::= ('+' primary)* *)
and parse_bin_rhs expr_prec lhs stream =
match Stream.peek stream with
(* If this is a binop, find its precedence. *)
| Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
let token_prec = precedence c in
(* If this is a binop that binds at least as tightly as the current binop,
* consume it, otherwise we are done. *)
if token_prec < expr_prec then lhs else begin
(* Eat the binop. *)
Stream.junk stream;
(* Parse the primary expression after the binary operator. *)
let rhs = parse_primary stream in
(* Okay, we know this is a binop. *)
let rhs =
match Stream.peek stream with
| Some (Token.Kwd c2) ->
(* If BinOp binds less tightly with rhs than the operator after
* rhs, let the pending operator take rhs as its lhs. *)
let next_prec = precedence c2 in
if token_prec < next_prec
then parse_bin_rhs (token_prec + 1) rhs stream
else rhs
| _ -> rhs
in
(* Merge lhs/rhs. *)
let lhs = Ast.Binary (c, lhs, rhs) in
parse_bin_rhs expr_prec lhs stream
end
| _ -> lhs
(* expression
* ::= primary binoprhs *)
and parse_expr = parser
| [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream
(* prototype
* ::= id '(' id* ')' *)
let parse_prototype =
let rec parse_args accumulator = parser
| [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
| [< >] -> accumulator
in
parser
| [< 'Token.Ident id;
'Token.Kwd '(' ?? "expected '(' in prototype";
args=parse_args [];
'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
(* success. *)
Ast.Prototype (id, Array.of_list (List.rev args))
| [< >] ->
raise (Stream.Error "expected function name in prototype")
(* definition ::= 'def' prototype expression *)
let parse_definition = parser
| [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
Ast.Function (p, e)
(* toplevelexpr ::= expression *)
let parse_toplevel = parser
| [< e=parse_expr >] ->
(* Make an anonymous proto. *)
Ast.Function (Ast.Prototype ("", [||]), e)
(* external ::= 'extern' prototype *)
let parse_extern = parser
| [< 'Token.Extern; e=parse_prototype >] -> e
codegen.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Code Generation
*===----------------------------------------------------------------------===*)
open Llvm
exception Error of string
let context = global_context ()
let the_module = create_module context "my cool jit"
let builder = builder context
let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
let double_type = double_type context
let rec codegen_expr = function
| Ast.Number n -> const_float double_type n
| Ast.Variable name ->
(try Hashtbl.find named_values name with
| Not_found -> raise (Error "unknown variable name"))
| Ast.Binary (op, lhs, rhs) ->
let lhs_val = codegen_expr lhs in
let rhs_val = codegen_expr rhs in
begin
match op with
| '+' -> build_add lhs_val rhs_val "addtmp" builder
| '-' -> build_sub lhs_val rhs_val "subtmp" builder
| '*' -> build_mul lhs_val rhs_val "multmp" builder
| '<' ->
(* Convert bool 0/1 to double 0.0 or 1.0 *)
let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
build_uitofp i double_type "booltmp" builder
| _ -> raise (Error "invalid binary operator")
end
| Ast.Call (callee, args) ->
(* Look up the name in the module table. *)
let callee =
match lookup_function callee the_module with
| Some callee -> callee
| None -> raise (Error "unknown function referenced")
in
let params = params callee in
(* If argument mismatch error. *)
if Array.length params == Array.length args then () else
raise (Error "incorrect # arguments passed");
let args = Array.map codegen_expr args in
build_call callee args "calltmp" builder
let codegen_proto = function
| Ast.Prototype (name, args) ->
(* Make the function type: double(double,double) etc. *)
let doubles = Array.make (Array.length args) double_type in
let ft = function_type double_type doubles in
let f =
match lookup_function name the_module with
| None -> declare_function name ft the_module
(* If 'f' conflicted, there was already something named 'name'. If it
* has a body, don't allow redefinition or reextern. *)
| Some f ->
(* If 'f' already has a body, reject this. *)
if block_begin f <> At_end f then
raise (Error "redefinition of function");
(* If 'f' took a different number of arguments, reject. *)
if element_type (type_of f) <> ft then
raise (Error "redefinition of function with different # args");
f
in
(* Set names for all arguments. *)
Array.iteri (fun i a ->
let n = args.(i) in
set_value_name n a;
Hashtbl.add named_values n a;
) (params f);
f
let codegen_func = function
| Ast.Function (proto, body) ->
Hashtbl.clear named_values;
let the_function = codegen_proto proto in
(* Create a new basic block to start insertion into. *)
let bb = append_block context "entry" the_function in
position_at_end bb builder;
try
let ret_val = codegen_expr body in
(* Finish off the function. *)
let _ = build_ret ret_val builder in
(* Validate the generated code, checking for consistency. *)
Llvm_analysis.assert_valid_function the_function;
the_function
with e ->
delete_function the_function;
raise e
toplevel.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Top-Level parsing and JIT Driver
*===----------------------------------------------------------------------===*)
open Llvm
(* top ::= definition | external | expression | ';' *)
let rec main_loop stream =
match Stream.peek stream with
| None -> ()
(* ignore top-level semicolons. *)
| Some (Token.Kwd ';') ->
Stream.junk stream;
main_loop stream
| Some token ->
begin
try match token with
| Token.Def ->
let e = Parser.parse_definition stream in
print_endline "parsed a function definition.";
dump_value (Codegen.codegen_func e);
| Token.Extern ->
let e = Parser.parse_extern stream in
print_endline "parsed an extern.";
dump_value (Codegen.codegen_proto e);
| _ ->
(* Evaluate a top-level expression into an anonymous function. *)
let e = Parser.parse_toplevel stream in
print_endline "parsed a top-level expr";
dump_value (Codegen.codegen_func e);
with Stream.Error s | Codegen.Error s ->
(* Skip token for error recovery. *)
Stream.junk stream;
print_endline s;
end;
print_string "ready> "; flush stdout;
main_loop stream
toy.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Main driver code.
*===----------------------------------------------------------------------===*)
open Llvm
let main () =
(* Install standard binary operators.
* 1 is the lowest precedence. *)
Hashtbl.add Parser.binop_precedence '<' 10;
Hashtbl.add Parser.binop_precedence '+' 20;
Hashtbl.add Parser.binop_precedence '-' 20;
Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *)
(* Prime the first token. *)
print_string "ready> "; flush stdout;
let stream = Lexer.lex (Stream.of_channel stdin) in
(* Run the main "interpreter loop" now. *)
Toplevel.main_loop stream;
(* Print out all the generated code. *)
dump_module Codegen.the_module
;;
main ()
`Next: Adding JIT and Optimizer Support <OCamlLangImpl4.html>`_

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==============================================
Kaleidoscope: Adding JIT and Optimizer Support
==============================================
.. contents::
:local:
Written by `Chris Lattner <mailto:sabre@nondot.org>`_ and `Erick
Tryzelaar <mailto:idadesub@users.sourceforge.net>`_
Chapter 4 Introduction
======================
Welcome to Chapter 4 of the "`Implementing a language with
LLVM <index.html>`_" tutorial. Chapters 1-3 described the implementation
of a simple language and added support for generating LLVM IR. This
chapter describes two new techniques: adding optimizer support to your
language, and adding JIT compiler support. These additions will
demonstrate how to get nice, efficient code for the Kaleidoscope
language.
Trivial Constant Folding
========================
**Note:** the default ``IRBuilder`` now always includes the constant
folding optimisations below.
Our demonstration for Chapter 3 is elegant and easy to extend.
Unfortunately, it does not produce wonderful code. For example, when
compiling simple code, we don't get obvious optimizations:
::
ready> def test(x) 1+2+x;
Read function definition:
define double @test(double %x) {
entry:
%addtmp = fadd double 1.000000e+00, 2.000000e+00
%addtmp1 = fadd double %addtmp, %x
ret double %addtmp1
}
This code is a very, very literal transcription of the AST built by
parsing the input. As such, this transcription lacks optimizations like
constant folding (we'd like to get "``add x, 3.0``" in the example
above) as well as other more important optimizations. Constant folding,
in particular, is a very common and very important optimization: so much
so that many language implementors implement constant folding support in
their AST representation.
With LLVM, you don't need this support in the AST. Since all calls to
build LLVM IR go through the LLVM builder, it would be nice if the
builder itself checked to see if there was a constant folding
opportunity when you call it. If so, it could just do the constant fold
and return the constant instead of creating an instruction. This is
exactly what the ``LLVMFoldingBuilder`` class does.
All we did was switch from ``LLVMBuilder`` to ``LLVMFoldingBuilder``.
Though we change no other code, we now have all of our instructions
implicitly constant folded without us having to do anything about it.
For example, the input above now compiles to:
::
ready> def test(x) 1+2+x;
Read function definition:
define double @test(double %x) {
entry:
%addtmp = fadd double 3.000000e+00, %x
ret double %addtmp
}
Well, that was easy :). In practice, we recommend always using
``LLVMFoldingBuilder`` when generating code like this. It has no
"syntactic overhead" for its use (you don't have to uglify your compiler
with constant checks everywhere) and it can dramatically reduce the
amount of LLVM IR that is generated in some cases (particular for
languages with a macro preprocessor or that use a lot of constants).
On the other hand, the ``LLVMFoldingBuilder`` is limited by the fact
that it does all of its analysis inline with the code as it is built. If
you take a slightly more complex example:
::
ready> def test(x) (1+2+x)*(x+(1+2));
ready> Read function definition:
define double @test(double %x) {
entry:
%addtmp = fadd double 3.000000e+00, %x
%addtmp1 = fadd double %x, 3.000000e+00
%multmp = fmul double %addtmp, %addtmp1
ret double %multmp
}
In this case, the LHS and RHS of the multiplication are the same value.
We'd really like to see this generate "``tmp = x+3; result = tmp*tmp;``"
instead of computing "``x*3``" twice.
Unfortunately, no amount of local analysis will be able to detect and
correct this. This requires two transformations: reassociation of
expressions (to make the add's lexically identical) and Common
Subexpression Elimination (CSE) to delete the redundant add instruction.
Fortunately, LLVM provides a broad range of optimizations that you can
use, in the form of "passes".
LLVM Optimization Passes
========================
LLVM provides many optimization passes, which do many different sorts of
things and have different tradeoffs. Unlike other systems, LLVM doesn't
hold to the mistaken notion that one set of optimizations is right for
all languages and for all situations. LLVM allows a compiler implementor
to make complete decisions about what optimizations to use, in which
order, and in what situation.
As a concrete example, LLVM supports both "whole module" passes, which
look across as large of body of code as they can (often a whole file,
but if run at link time, this can be a substantial portion of the whole
program). It also supports and includes "per-function" passes which just
operate on a single function at a time, without looking at other
functions. For more information on passes and how they are run, see the
`How to Write a Pass <../WritingAnLLVMPass.html>`_ document and the
`List of LLVM Passes <../Passes.html>`_.
For Kaleidoscope, we are currently generating functions on the fly, one
at a time, as the user types them in. We aren't shooting for the
ultimate optimization experience in this setting, but we also want to
catch the easy and quick stuff where possible. As such, we will choose
to run a few per-function optimizations as the user types the function
in. If we wanted to make a "static Kaleidoscope compiler", we would use
exactly the code we have now, except that we would defer running the
optimizer until the entire file has been parsed.
In order to get per-function optimizations going, we need to set up a
`Llvm.PassManager <../WritingAnLLVMPass.html#passmanager>`_ to hold and
organize the LLVM optimizations that we want to run. Once we have that,
we can add a set of optimizations to run. The code looks like this:
.. code-block:: ocaml
(* Create the JIT. *)
let the_execution_engine = ExecutionEngine.create Codegen.the_module in
let the_fpm = PassManager.create_function Codegen.the_module in
(* Set up the optimizer pipeline. Start with registering info about how the
* target lays out data structures. *)
DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
(* Do simple "peephole" optimizations and bit-twiddling optzn. *)
add_instruction_combining the_fpm;
(* reassociate expressions. *)
add_reassociation the_fpm;
(* Eliminate Common SubExpressions. *)
add_gvn the_fpm;
(* Simplify the control flow graph (deleting unreachable blocks, etc). *)
add_cfg_simplification the_fpm;
ignore (PassManager.initialize the_fpm);
(* Run the main "interpreter loop" now. *)
Toplevel.main_loop the_fpm the_execution_engine stream;
The meat of the matter here, is the definition of "``the_fpm``". It
requires a pointer to the ``the_module`` to construct itself. Once it is
set up, we use a series of "add" calls to add a bunch of LLVM passes.
The first pass is basically boilerplate, it adds a pass so that later
optimizations know how the data structures in the program are laid out.
The "``the_execution_engine``" variable is related to the JIT, which we
will get to in the next section.
In this case, we choose to add 4 optimization passes. The passes we
chose here are a pretty standard set of "cleanup" optimizations that are
useful for a wide variety of code. I won't delve into what they do but,
believe me, they are a good starting place :).
Once the ``Llvm.PassManager.`` is set up, we need to make use of it. We
do this by running it after our newly created function is constructed
(in ``Codegen.codegen_func``), but before it is returned to the client:
.. code-block:: ocaml
let codegen_func the_fpm = function
...
try
let ret_val = codegen_expr body in
(* Finish off the function. *)
let _ = build_ret ret_val builder in
(* Validate the generated code, checking for consistency. *)
Llvm_analysis.assert_valid_function the_function;
(* Optimize the function. *)
let _ = PassManager.run_function the_function the_fpm in
the_function
As you can see, this is pretty straightforward. The ``the_fpm``
optimizes and updates the LLVM Function\* in place, improving
(hopefully) its body. With this in place, we can try our test above
again:
::
ready> def test(x) (1+2+x)*(x+(1+2));
ready> Read function definition:
define double @test(double %x) {
entry:
%addtmp = fadd double %x, 3.000000e+00
%multmp = fmul double %addtmp, %addtmp
ret double %multmp
}
As expected, we now get our nicely optimized code, saving a floating
point add instruction from every execution of this function.
LLVM provides a wide variety of optimizations that can be used in
certain circumstances. Some `documentation about the various
passes <../Passes.html>`_ is available, but it isn't very complete.
Another good source of ideas can come from looking at the passes that
``Clang`` runs to get started. The "``opt``" tool allows you to
experiment with passes from the command line, so you can see if they do
anything.
Now that we have reasonable code coming out of our front-end, lets talk
about executing it!
Adding a JIT Compiler
=====================
Code that is available in LLVM IR can have a wide variety of tools
applied to it. For example, you can run optimizations on it (as we did
above), you can dump it out in textual or binary forms, you can compile
the code to an assembly file (.s) for some target, or you can JIT
compile it. The nice thing about the LLVM IR representation is that it
is the "common currency" between many different parts of the compiler.
In this section, we'll add JIT compiler support to our interpreter. The
basic idea that we want for Kaleidoscope is to have the user enter
function bodies as they do now, but immediately evaluate the top-level
expressions they type in. For example, if they type in "1 + 2;", we
should evaluate and print out 3. If they define a function, they should
be able to call it from the command line.
In order to do this, we first declare and initialize the JIT. This is
done by adding a global variable and a call in ``main``:
.. code-block:: ocaml
...
let main () =
...
(* Create the JIT. *)
let the_execution_engine = ExecutionEngine.create Codegen.the_module in
...
This creates an abstract "Execution Engine" which can be either a JIT
compiler or the LLVM interpreter. LLVM will automatically pick a JIT
compiler for you if one is available for your platform, otherwise it
will fall back to the interpreter.
Once the ``Llvm_executionengine.ExecutionEngine.t`` is created, the JIT
is ready to be used. There are a variety of APIs that are useful, but
the simplest one is the
"``Llvm_executionengine.ExecutionEngine.run_function``" function. This
method JIT compiles the specified LLVM Function and returns a function
pointer to the generated machine code. In our case, this means that we
can change the code that parses a top-level expression to look like
this:
.. code-block:: ocaml
(* Evaluate a top-level expression into an anonymous function. *)
let e = Parser.parse_toplevel stream in
print_endline "parsed a top-level expr";
let the_function = Codegen.codegen_func the_fpm e in
dump_value the_function;
(* JIT the function, returning a function pointer. *)
let result = ExecutionEngine.run_function the_function [||]
the_execution_engine in
print_string "Evaluated to ";
print_float (GenericValue.as_float Codegen.double_type result);
print_newline ();
Recall that we compile top-level expressions into a self-contained LLVM
function that takes no arguments and returns the computed double.
Because the LLVM JIT compiler matches the native platform ABI, this
means that you can just cast the result pointer to a function pointer of
that type and call it directly. This means, there is no difference
between JIT compiled code and native machine code that is statically
linked into your application.
With just these two changes, lets see how Kaleidoscope works now!
::
ready> 4+5;
define double @""() {
entry:
ret double 9.000000e+00
}
Evaluated to 9.000000
Well this looks like it is basically working. The dump of the function
shows the "no argument function that always returns double" that we
synthesize for each top level expression that is typed in. This
demonstrates very basic functionality, but can we do more?
::
ready> def testfunc(x y) x + y*2;
Read function definition:
define double @testfunc(double %x, double %y) {
entry:
%multmp = fmul double %y, 2.000000e+00
%addtmp = fadd double %multmp, %x
ret double %addtmp
}
ready> testfunc(4, 10);
define double @""() {
entry:
%calltmp = call double @testfunc(double 4.000000e+00, double 1.000000e+01)
ret double %calltmp
}
Evaluated to 24.000000
This illustrates that we can now call user code, but there is something
a bit subtle going on here. Note that we only invoke the JIT on the
anonymous functions that *call testfunc*, but we never invoked it on
*testfunc* itself. What actually happened here is that the JIT scanned
for all non-JIT'd functions transitively called from the anonymous
function and compiled all of them before returning from
``run_function``.
The JIT provides a number of other more advanced interfaces for things
like freeing allocated machine code, rejit'ing functions to update them,
etc. However, even with this simple code, we get some surprisingly
powerful capabilities - check this out (I removed the dump of the
anonymous functions, you should get the idea by now :) :
::
ready> extern sin(x);
Read extern:
declare double @sin(double)
ready> extern cos(x);
Read extern:
declare double @cos(double)
ready> sin(1.0);
Evaluated to 0.841471
ready> def foo(x) sin(x)*sin(x) + cos(x)*cos(x);
Read function definition:
define double @foo(double %x) {
entry:
%calltmp = call double @sin(double %x)
%multmp = fmul double %calltmp, %calltmp
%calltmp2 = call double @cos(double %x)
%multmp4 = fmul double %calltmp2, %calltmp2
%addtmp = fadd double %multmp, %multmp4
ret double %addtmp
}
ready> foo(4.0);
Evaluated to 1.000000
Whoa, how does the JIT know about sin and cos? The answer is
surprisingly simple: in this example, the JIT started execution of a
function and got to a function call. It realized that the function was
not yet JIT compiled and invoked the standard set of routines to resolve
the function. In this case, there is no body defined for the function,
so the JIT ended up calling "``dlsym("sin")``" on the Kaleidoscope
process itself. Since "``sin``" is defined within the JIT's address
space, it simply patches up calls in the module to call the libm version
of ``sin`` directly.
The LLVM JIT provides a number of interfaces (look in the
``llvm_executionengine.mli`` file) for controlling how unknown functions
get resolved. It allows you to establish explicit mappings between IR
objects and addresses (useful for LLVM global variables that you want to
map to static tables, for example), allows you to dynamically decide on
the fly based on the function name, and even allows you to have the JIT
compile functions lazily the first time they're called.
One interesting application of this is that we can now extend the
language by writing arbitrary C code to implement operations. For
example, if we add:
.. code-block:: c++
/* putchard - putchar that takes a double and returns 0. */
extern "C"
double putchard(double X) {
putchar((char)X);
return 0;
}
Now we can produce simple output to the console by using things like:
"``extern putchard(x); putchard(120);``", which prints a lowercase 'x'
on the console (120 is the ASCII code for 'x'). Similar code could be
used to implement file I/O, console input, and many other capabilities
in Kaleidoscope.
This completes the JIT and optimizer chapter of the Kaleidoscope
tutorial. At this point, we can compile a non-Turing-complete
programming language, optimize and JIT compile it in a user-driven way.
Next up we'll look into `extending the language with control flow
constructs <OCamlLangImpl5.html>`_, tackling some interesting LLVM IR
issues along the way.
Full Code Listing
=================
Here is the complete code listing for our running example, enhanced with
the LLVM JIT and optimizer. To build this example, use:
.. code-block:: bash
# Compile
ocamlbuild toy.byte
# Run
./toy.byte
Here is the code:
\_tags:
::
<{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
<*.{byte,native}>: g++, use_llvm, use_llvm_analysis
<*.{byte,native}>: use_llvm_executionengine, use_llvm_target
<*.{byte,native}>: use_llvm_scalar_opts, use_bindings
myocamlbuild.ml:
.. code-block:: ocaml
open Ocamlbuild_plugin;;
ocaml_lib ~extern:true "llvm";;
ocaml_lib ~extern:true "llvm_analysis";;
ocaml_lib ~extern:true "llvm_executionengine";;
ocaml_lib ~extern:true "llvm_target";;
ocaml_lib ~extern:true "llvm_scalar_opts";;
flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;
dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
token.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Lexer Tokens
*===----------------------------------------------------------------------===*)
(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
* these others for known things. *)
type token =
(* commands *)
| Def | Extern
(* primary *)
| Ident of string | Number of float
(* unknown *)
| Kwd of char
lexer.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Lexer
*===----------------------------------------------------------------------===*)
let rec lex = parser
(* Skip any whitespace. *)
| [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
(* identifier: [a-zA-Z][a-zA-Z0-9] *)
| [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_ident buffer stream
(* number: [0-9.]+ *)
| [< ' ('0' .. '9' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_number buffer stream
(* Comment until end of line. *)
| [< ' ('#'); stream >] ->
lex_comment stream
(* Otherwise, just return the character as its ascii value. *)
| [< 'c; stream >] ->
[< 'Token.Kwd c; lex stream >]
(* end of stream. *)
| [< >] -> [< >]
and lex_number buffer = parser
| [< ' ('0' .. '9' | '.' as c); stream >] ->
Buffer.add_char buffer c;
lex_number buffer stream
| [< stream=lex >] ->
[< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
and lex_ident buffer = parser
| [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
Buffer.add_char buffer c;
lex_ident buffer stream
| [< stream=lex >] ->
match Buffer.contents buffer with
| "def" -> [< 'Token.Def; stream >]
| "extern" -> [< 'Token.Extern; stream >]
| id -> [< 'Token.Ident id; stream >]
and lex_comment = parser
| [< ' ('\n'); stream=lex >] -> stream
| [< 'c; e=lex_comment >] -> e
| [< >] -> [< >]
ast.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Abstract Syntax Tree (aka Parse Tree)
*===----------------------------------------------------------------------===*)
(* expr - Base type for all expression nodes. *)
type expr =
(* variant for numeric literals like "1.0". *)
| Number of float
(* variant for referencing a variable, like "a". *)
| Variable of string
(* variant for a binary operator. *)
| Binary of char * expr * expr
(* variant for function calls. *)
| Call of string * expr array
(* proto - This type represents the "prototype" for a function, which captures
* its name, and its argument names (thus implicitly the number of arguments the
* function takes). *)
type proto = Prototype of string * string array
(* func - This type represents a function definition itself. *)
type func = Function of proto * expr
parser.ml:
.. code-block:: ocaml
(*===---------------------------------------------------------------------===
* Parser
*===---------------------------------------------------------------------===*)
(* binop_precedence - This holds the precedence for each binary operator that is
* defined *)
let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
(* precedence - Get the precedence of the pending binary operator token. *)
let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
(* primary
* ::= identifier
* ::= numberexpr
* ::= parenexpr *)
let rec parse_primary = parser
(* numberexpr ::= number *)
| [< 'Token.Number n >] -> Ast.Number n
(* parenexpr ::= '(' expression ')' *)
| [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
(* identifierexpr
* ::= identifier
* ::= identifier '(' argumentexpr ')' *)
| [< 'Token.Ident id; stream >] ->
let rec parse_args accumulator = parser
| [< e=parse_expr; stream >] ->
begin parser
| [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
| [< >] -> e :: accumulator
end stream
| [< >] -> accumulator
in
let rec parse_ident id = parser
(* Call. *)
| [< 'Token.Kwd '(';
args=parse_args [];
'Token.Kwd ')' ?? "expected ')'">] ->
Ast.Call (id, Array.of_list (List.rev args))
(* Simple variable ref. *)
| [< >] -> Ast.Variable id
in
parse_ident id stream
| [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
(* binoprhs
* ::= ('+' primary)* *)
and parse_bin_rhs expr_prec lhs stream =
match Stream.peek stream with
(* If this is a binop, find its precedence. *)
| Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
let token_prec = precedence c in
(* If this is a binop that binds at least as tightly as the current binop,
* consume it, otherwise we are done. *)
if token_prec < expr_prec then lhs else begin
(* Eat the binop. *)
Stream.junk stream;
(* Parse the primary expression after the binary operator. *)
let rhs = parse_primary stream in
(* Okay, we know this is a binop. *)
let rhs =
match Stream.peek stream with
| Some (Token.Kwd c2) ->
(* If BinOp binds less tightly with rhs than the operator after
* rhs, let the pending operator take rhs as its lhs. *)
let next_prec = precedence c2 in
if token_prec < next_prec
then parse_bin_rhs (token_prec + 1) rhs stream
else rhs
| _ -> rhs
in
(* Merge lhs/rhs. *)
let lhs = Ast.Binary (c, lhs, rhs) in
parse_bin_rhs expr_prec lhs stream
end
| _ -> lhs
(* expression
* ::= primary binoprhs *)
and parse_expr = parser
| [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream
(* prototype
* ::= id '(' id* ')' *)
let parse_prototype =
let rec parse_args accumulator = parser
| [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
| [< >] -> accumulator
in
parser
| [< 'Token.Ident id;
'Token.Kwd '(' ?? "expected '(' in prototype";
args=parse_args [];
'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
(* success. *)
Ast.Prototype (id, Array.of_list (List.rev args))
| [< >] ->
raise (Stream.Error "expected function name in prototype")
(* definition ::= 'def' prototype expression *)
let parse_definition = parser
| [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
Ast.Function (p, e)
(* toplevelexpr ::= expression *)
let parse_toplevel = parser
| [< e=parse_expr >] ->
(* Make an anonymous proto. *)
Ast.Function (Ast.Prototype ("", [||]), e)
(* external ::= 'extern' prototype *)
let parse_extern = parser
| [< 'Token.Extern; e=parse_prototype >] -> e
codegen.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Code Generation
*===----------------------------------------------------------------------===*)
open Llvm
exception Error of string
let context = global_context ()
let the_module = create_module context "my cool jit"
let builder = builder context
let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
let double_type = double_type context
let rec codegen_expr = function
| Ast.Number n -> const_float double_type n
| Ast.Variable name ->
(try Hashtbl.find named_values name with
| Not_found -> raise (Error "unknown variable name"))
| Ast.Binary (op, lhs, rhs) ->
let lhs_val = codegen_expr lhs in
let rhs_val = codegen_expr rhs in
begin
match op with
| '+' -> build_add lhs_val rhs_val "addtmp" builder
| '-' -> build_sub lhs_val rhs_val "subtmp" builder
| '*' -> build_mul lhs_val rhs_val "multmp" builder
| '<' ->
(* Convert bool 0/1 to double 0.0 or 1.0 *)
let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
build_uitofp i double_type "booltmp" builder
| _ -> raise (Error "invalid binary operator")
end
| Ast.Call (callee, args) ->
(* Look up the name in the module table. *)
let callee =
match lookup_function callee the_module with
| Some callee -> callee
| None -> raise (Error "unknown function referenced")
in
let params = params callee in
(* If argument mismatch error. *)
if Array.length params == Array.length args then () else
raise (Error "incorrect # arguments passed");
let args = Array.map codegen_expr args in
build_call callee args "calltmp" builder
let codegen_proto = function
| Ast.Prototype (name, args) ->
(* Make the function type: double(double,double) etc. *)
let doubles = Array.make (Array.length args) double_type in
let ft = function_type double_type doubles in
let f =
match lookup_function name the_module with
| None -> declare_function name ft the_module
(* If 'f' conflicted, there was already something named 'name'. If it
* has a body, don't allow redefinition or reextern. *)
| Some f ->
(* If 'f' already has a body, reject this. *)
if block_begin f <> At_end f then
raise (Error "redefinition of function");
(* If 'f' took a different number of arguments, reject. *)
if element_type (type_of f) <> ft then
raise (Error "redefinition of function with different # args");
f
in
(* Set names for all arguments. *)
Array.iteri (fun i a ->
let n = args.(i) in
set_value_name n a;
Hashtbl.add named_values n a;
) (params f);
f
let codegen_func the_fpm = function
| Ast.Function (proto, body) ->
Hashtbl.clear named_values;
let the_function = codegen_proto proto in
(* Create a new basic block to start insertion into. *)
let bb = append_block context "entry" the_function in
position_at_end bb builder;
try
let ret_val = codegen_expr body in
(* Finish off the function. *)
let _ = build_ret ret_val builder in
(* Validate the generated code, checking for consistency. *)
Llvm_analysis.assert_valid_function the_function;
(* Optimize the function. *)
let _ = PassManager.run_function the_function the_fpm in
the_function
with e ->
delete_function the_function;
raise e
toplevel.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Top-Level parsing and JIT Driver
*===----------------------------------------------------------------------===*)
open Llvm
open Llvm_executionengine
(* top ::= definition | external | expression | ';' *)
let rec main_loop the_fpm the_execution_engine stream =
match Stream.peek stream with
| None -> ()
(* ignore top-level semicolons. *)
| Some (Token.Kwd ';') ->
Stream.junk stream;
main_loop the_fpm the_execution_engine stream
| Some token ->
begin
try match token with
| Token.Def ->
let e = Parser.parse_definition stream in
print_endline "parsed a function definition.";
dump_value (Codegen.codegen_func the_fpm e);
| Token.Extern ->
let e = Parser.parse_extern stream in
print_endline "parsed an extern.";
dump_value (Codegen.codegen_proto e);
| _ ->
(* Evaluate a top-level expression into an anonymous function. *)
let e = Parser.parse_toplevel stream in
print_endline "parsed a top-level expr";
let the_function = Codegen.codegen_func the_fpm e in
dump_value the_function;
(* JIT the function, returning a function pointer. *)
let result = ExecutionEngine.run_function the_function [||]
the_execution_engine in
print_string "Evaluated to ";
print_float (GenericValue.as_float Codegen.double_type result);
print_newline ();
with Stream.Error s | Codegen.Error s ->
(* Skip token for error recovery. *)
Stream.junk stream;
print_endline s;
end;
print_string "ready> "; flush stdout;
main_loop the_fpm the_execution_engine stream
toy.ml:
.. code-block:: ocaml
(*===----------------------------------------------------------------------===
* Main driver code.
*===----------------------------------------------------------------------===*)
open Llvm
open Llvm_executionengine
open Llvm_target
open Llvm_scalar_opts
let main () =
ignore (initialize_native_target ());
(* Install standard binary operators.
* 1 is the lowest precedence. *)
Hashtbl.add Parser.binop_precedence '<' 10;
Hashtbl.add Parser.binop_precedence '+' 20;
Hashtbl.add Parser.binop_precedence '-' 20;
Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *)
(* Prime the first token. *)
print_string "ready> "; flush stdout;
let stream = Lexer.lex (Stream.of_channel stdin) in
(* Create the JIT. *)
let the_execution_engine = ExecutionEngine.create Codegen.the_module in
let the_fpm = PassManager.create_function Codegen.the_module in
(* Set up the optimizer pipeline. Start with registering info about how the
* target lays out data structures. *)
DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
(* Do simple "peephole" optimizations and bit-twiddling optzn. *)
add_instruction_combination the_fpm;
(* reassociate expressions. *)
add_reassociation the_fpm;
(* Eliminate Common SubExpressions. *)
add_gvn the_fpm;
(* Simplify the control flow graph (deleting unreachable blocks, etc). *)
add_cfg_simplification the_fpm;
ignore (PassManager.initialize the_fpm);
(* Run the main "interpreter loop" now. *)
Toplevel.main_loop the_fpm the_execution_engine stream;
(* Print out all the generated code. *)
dump_module Codegen.the_module
;;
main ()
bindings.c
.. code-block:: c
#include <stdio.h>
/* putchard - putchar that takes a double and returns 0. */
extern double putchard(double X) {
putchar((char)X);
return 0;
}
`Next: Extending the language: control flow <OCamlLangImpl5.html>`_

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="Chris Lattner">
<link rel="stylesheet" href="../_static/llvm.css" type="text/css">
</head>
<body>
<h1>Kaleidoscope: Conclusion and other useful LLVM tidbits</h1>
<ul>
<li><a href="index.html">Up to Tutorial Index</a></li>
<li>Chapter 8
<ol>
<li><a href="#conclusion">Tutorial Conclusion</a></li>
<li><a href="#llvmirproperties">Properties of LLVM IR</a>
<ul>
<li><a href="#targetindep">Target Independence</a></li>
<li><a href="#safety">Safety Guarantees</a></li>
<li><a href="#langspecific">Language-Specific Optimizations</a></li>
</ul>
</li>
<li><a href="#tipsandtricks">Tips and Tricks</a>
<ul>
<li><a href="#offsetofsizeof">Implementing portable
offsetof/sizeof</a></li>
<li><a href="#gcstack">Garbage Collected Stack Frames</a></li>
</ul>
</li>
</ol>
</li>
</ul>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>
<!-- *********************************************************************** -->
<h2><a name="conclusion">Tutorial Conclusion</a></h2>
<!-- *********************************************************************** -->
<div>
<p>Welcome to the final chapter of the "<a href="index.html">Implementing a
language with LLVM</a>" tutorial. In the course of this tutorial, we have grown
our little Kaleidoscope language from being a useless toy, to being a
semi-interesting (but probably still useless) toy. :)</p>
<p>It is interesting to see how far we've come, and how little code it has
taken. We built the entire lexer, parser, AST, code generator, and an
interactive run-loop (with a JIT!) by-hand in under 700 lines of
(non-comment/non-blank) code.</p>
<p>Our little language supports a couple of interesting features: it supports
user defined binary and unary operators, it uses JIT compilation for immediate
evaluation, and it supports a few control flow constructs with SSA construction.
</p>
<p>Part of the idea of this tutorial was to show you how easy and fun it can be
to define, build, and play with languages. Building a compiler need not be a
scary or mystical process! Now that you've seen some of the basics, I strongly
encourage you to take the code and hack on it. For example, try adding:</p>
<ul>
<li><b>global variables</b> - While global variables have questional value in
modern software engineering, they are often useful when putting together quick
little hacks like the Kaleidoscope compiler itself. Fortunately, our current
setup makes it very easy to add global variables: just have value lookup check
to see if an unresolved variable is in the global variable symbol table before
rejecting it. To create a new global variable, make an instance of the LLVM
<tt>GlobalVariable</tt> class.</li>
<li><b>typed variables</b> - Kaleidoscope currently only supports variables of
type double. This gives the language a very nice elegance, because only
supporting one type means that you never have to specify types. Different
languages have different ways of handling this. The easiest way is to require
the user to specify types for every variable definition, and record the type
of the variable in the symbol table along with its Value*.</li>
<li><b>arrays, structs, vectors, etc</b> - Once you add types, you can start
extending the type system in all sorts of interesting ways. Simple arrays are
very easy and are quite useful for many different applications. Adding them is
mostly an exercise in learning how the LLVM <a
href="../LangRef.html#i_getelementptr">getelementptr</a> instruction works: it
is so nifty/unconventional, it <a
href="../GetElementPtr.html">has its own FAQ</a>! If you add support
for recursive types (e.g. linked lists), make sure to read the <a
href="../ProgrammersManual.html#TypeResolve">section in the LLVM
Programmer's Manual</a> that describes how to construct them.</li>
<li><b>standard runtime</b> - Our current language allows the user to access
arbitrary external functions, and we use it for things like "printd" and
"putchard". As you extend the language to add higher-level constructs, often
these constructs make the most sense if they are lowered to calls into a
language-supplied runtime. For example, if you add hash tables to the language,
it would probably make sense to add the routines to a runtime, instead of
inlining them all the way.</li>
<li><b>memory management</b> - Currently we can only access the stack in
Kaleidoscope. It would also be useful to be able to allocate heap memory,
either with calls to the standard libc malloc/free interface or with a garbage
collector. If you would like to use garbage collection, note that LLVM fully
supports <a href="../GarbageCollection.html">Accurate Garbage Collection</a>
including algorithms that move objects and need to scan/update the stack.</li>
<li><b>debugger support</b> - LLVM supports generation of <a
href="../SourceLevelDebugging.html">DWARF Debug info</a> which is understood by
common debuggers like GDB. Adding support for debug info is fairly
straightforward. The best way to understand it is to compile some C/C++ code
with "<tt>llvm-gcc -g -O0</tt>" and taking a look at what it produces.</li>
<li><b>exception handling support</b> - LLVM supports generation of <a
href="../ExceptionHandling.html">zero cost exceptions</a> which interoperate
with code compiled in other languages. You could also generate code by
implicitly making every function return an error value and checking it. You
could also make explicit use of setjmp/longjmp. There are many different ways
to go here.</li>
<li><b>object orientation, generics, database access, complex numbers,
geometric programming, ...</b> - Really, there is
no end of crazy features that you can add to the language.</li>
<li><b>unusual domains</b> - We've been talking about applying LLVM to a domain
that many people are interested in: building a compiler for a specific language.
However, there are many other domains that can use compiler technology that are
not typically considered. For example, LLVM has been used to implement OpenGL
graphics acceleration, translate C++ code to ActionScript, and many other
cute and clever things. Maybe you will be the first to JIT compile a regular
expression interpreter into native code with LLVM?</li>
</ul>
<p>
Have fun - try doing something crazy and unusual. Building a language like
everyone else always has, is much less fun than trying something a little crazy
or off the wall and seeing how it turns out. If you get stuck or want to talk
about it, feel free to email the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
list</a>: it has lots of people who are interested in languages and are often
willing to help out.
</p>
<p>Before we end this tutorial, I want to talk about some "tips and tricks" for generating
LLVM IR. These are some of the more subtle things that may not be obvious, but
are very useful if you want to take advantage of LLVM's capabilities.</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="llvmirproperties">Properties of the LLVM IR</a></h2>
<!-- *********************************************************************** -->
<div>
<p>We have a couple common questions about code in the LLVM IR form - lets just
get these out of the way right now, shall we?</p>
<!-- ======================================================================= -->
<h4><a name="targetindep">Target Independence</a></h4>
<!-- ======================================================================= -->
<div>
<p>Kaleidoscope is an example of a "portable language": any program written in
Kaleidoscope will work the same way on any target that it runs on. Many other
languages have this property, e.g. lisp, java, haskell, javascript, python, etc
(note that while these languages are portable, not all their libraries are).</p>
<p>One nice aspect of LLVM is that it is often capable of preserving target
independence in the IR: you can take the LLVM IR for a Kaleidoscope-compiled
program and run it on any target that LLVM supports, even emitting C code and
compiling that on targets that LLVM doesn't support natively. You can trivially
tell that the Kaleidoscope compiler generates target-independent code because it
never queries for any target-specific information when generating code.</p>
<p>The fact that LLVM provides a compact, target-independent, representation for
code gets a lot of people excited. Unfortunately, these people are usually
thinking about C or a language from the C family when they are asking questions
about language portability. I say "unfortunately", because there is really no
way to make (fully general) C code portable, other than shipping the source code
around (and of course, C source code is not actually portable in general
either - ever port a really old application from 32- to 64-bits?).</p>
<p>The problem with C (again, in its full generality) is that it is heavily
laden with target specific assumptions. As one simple example, the preprocessor
often destructively removes target-independence from the code when it processes
the input text:</p>
<div class="doc_code">
<pre>
#ifdef __i386__
int X = 1;
#else
int X = 42;
#endif
</pre>
</div>
<p>While it is possible to engineer more and more complex solutions to problems
like this, it cannot be solved in full generality in a way that is better than shipping
the actual source code.</p>
<p>That said, there are interesting subsets of C that can be made portable. If
you are willing to fix primitive types to a fixed size (say int = 32-bits,
and long = 64-bits), don't care about ABI compatibility with existing binaries,
and are willing to give up some other minor features, you can have portable
code. This can make sense for specialized domains such as an
in-kernel language.</p>
</div>
<!-- ======================================================================= -->
<h4><a name="safety">Safety Guarantees</a></h4>
<!-- ======================================================================= -->
<div>
<p>Many of the languages above are also "safe" languages: it is impossible for
a program written in Java to corrupt its address space and crash the process
(assuming the JVM has no bugs).
Safety is an interesting property that requires a combination of language
design, runtime support, and often operating system support.</p>
<p>It is certainly possible to implement a safe language in LLVM, but LLVM IR
does not itself guarantee safety. The LLVM IR allows unsafe pointer casts,
use after free bugs, buffer over-runs, and a variety of other problems. Safety
needs to be implemented as a layer on top of LLVM and, conveniently, several
groups have investigated this. Ask on the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
list</a> if you are interested in more details.</p>
</div>
<!-- ======================================================================= -->
<h4><a name="langspecific">Language-Specific Optimizations</a></h4>
<!-- ======================================================================= -->
<div>
<p>One thing about LLVM that turns off many people is that it does not solve all
the world's problems in one system (sorry 'world hunger', someone else will have
to solve you some other day). One specific complaint is that people perceive
LLVM as being incapable of performing high-level language-specific optimization:
LLVM "loses too much information".</p>
<p>Unfortunately, this is really not the place to give you a full and unified
version of "Chris Lattner's theory of compiler design". Instead, I'll make a
few observations:</p>
<p>First, you're right that LLVM does lose information. For example, as of this
writing, there is no way to distinguish in the LLVM IR whether an SSA-value came
from a C "int" or a C "long" on an ILP32 machine (other than debug info). Both
get compiled down to an 'i32' value and the information about what it came from
is lost. The more general issue here, is that the LLVM type system uses
"structural equivalence" instead of "name equivalence". Another place this
surprises people is if you have two types in a high-level language that have the
same structure (e.g. two different structs that have a single int field): these
types will compile down into a single LLVM type and it will be impossible to
tell what it came from.</p>
<p>Second, while LLVM does lose information, LLVM is not a fixed target: we
continue to enhance and improve it in many different ways. In addition to
adding new features (LLVM did not always support exceptions or debug info), we
also extend the IR to capture important information for optimization (e.g.
whether an argument is sign or zero extended, information about pointers
aliasing, etc). Many of the enhancements are user-driven: people want LLVM to
include some specific feature, so they go ahead and extend it.</p>
<p>Third, it is <em>possible and easy</em> to add language-specific
optimizations, and you have a number of choices in how to do it. As one trivial
example, it is easy to add language-specific optimization passes that
"know" things about code compiled for a language. In the case of the C family,
there is an optimization pass that "knows" about the standard C library
functions. If you call "exit(0)" in main(), it knows that it is safe to
optimize that into "return 0;" because C specifies what the 'exit'
function does.</p>
<p>In addition to simple library knowledge, it is possible to embed a variety of
other language-specific information into the LLVM IR. If you have a specific
need and run into a wall, please bring the topic up on the llvmdev list. At the
very worst, you can always treat LLVM as if it were a "dumb code generator" and
implement the high-level optimizations you desire in your front-end, on the
language-specific AST.
</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2><a name="tipsandtricks">Tips and Tricks</a></h2>
<!-- *********************************************************************** -->
<div>
<p>There is a variety of useful tips and tricks that you come to know after
working on/with LLVM that aren't obvious at first glance. Instead of letting
everyone rediscover them, this section talks about some of these issues.</p>
<!-- ======================================================================= -->
<h4><a name="offsetofsizeof">Implementing portable offsetof/sizeof</a></h4>
<!-- ======================================================================= -->
<div>
<p>One interesting thing that comes up, if you are trying to keep the code
generated by your compiler "target independent", is that you often need to know
the size of some LLVM type or the offset of some field in an llvm structure.
For example, you might need to pass the size of a type into a function that
allocates memory.</p>
<p>Unfortunately, this can vary widely across targets: for example the width of
a pointer is trivially target-specific. However, there is a <a
href="http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt">clever
way to use the getelementptr instruction</a> that allows you to compute this
in a portable way.</p>
</div>
<!-- ======================================================================= -->
<h4><a name="gcstack">Garbage Collected Stack Frames</a></h4>
<!-- ======================================================================= -->
<div>
<p>Some languages want to explicitly manage their stack frames, often so that
they are garbage collected or to allow easy implementation of closures. There
are often better ways to implement these features than explicit stack frames,
but <a
href="http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt">LLVM
does support them,</a> if you want. It requires your front-end to convert the
code into <a
href="http://en.wikipedia.org/wiki/Continuation-passing_style">Continuation
Passing Style</a> and the use of tail calls (which LLVM also supports).</p>
</div>
</div>
<!-- *********************************************************************** -->
<hr>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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======================================================
Kaleidoscope: Conclusion and other useful LLVM tidbits
======================================================
.. contents::
:local:
Written by `Chris Lattner <mailto:sabre@nondot.org>`_
Tutorial Conclusion
===================
Welcome to the final chapter of the "`Implementing a language with
LLVM <index.html>`_" tutorial. In the course of this tutorial, we have
grown our little Kaleidoscope language from being a useless toy, to
being a semi-interesting (but probably still useless) toy. :)
It is interesting to see how far we've come, and how little code it has
taken. We built the entire lexer, parser, AST, code generator, and an
interactive run-loop (with a JIT!) by-hand in under 700 lines of
(non-comment/non-blank) code.
Our little language supports a couple of interesting features: it
supports user defined binary and unary operators, it uses JIT
compilation for immediate evaluation, and it supports a few control flow
constructs with SSA construction.
Part of the idea of this tutorial was to show you how easy and fun it
can be to define, build, and play with languages. Building a compiler
need not be a scary or mystical process! Now that you've seen some of
the basics, I strongly encourage you to take the code and hack on it.
For example, try adding:
- **global variables** - While global variables have questional value
in modern software engineering, they are often useful when putting
together quick little hacks like the Kaleidoscope compiler itself.
Fortunately, our current setup makes it very easy to add global
variables: just have value lookup check to see if an unresolved
variable is in the global variable symbol table before rejecting it.
To create a new global variable, make an instance of the LLVM
``GlobalVariable`` class.
- **typed variables** - Kaleidoscope currently only supports variables
of type double. This gives the language a very nice elegance, because
only supporting one type means that you never have to specify types.
Different languages have different ways of handling this. The easiest
way is to require the user to specify types for every variable
definition, and record the type of the variable in the symbol table
along with its Value\*.
- **arrays, structs, vectors, etc** - Once you add types, you can start
extending the type system in all sorts of interesting ways. Simple
arrays are very easy and are quite useful for many different
applications. Adding them is mostly an exercise in learning how the
LLVM `getelementptr <../LangRef.html#i_getelementptr>`_ instruction
works: it is so nifty/unconventional, it `has its own
FAQ <../GetElementPtr.html>`_! If you add support for recursive types
(e.g. linked lists), make sure to read the `section in the LLVM
Programmer's Manual <../ProgrammersManual.html#TypeResolve>`_ that
describes how to construct them.
- **standard runtime** - Our current language allows the user to access
arbitrary external functions, and we use it for things like "printd"
and "putchard". As you extend the language to add higher-level
constructs, often these constructs make the most sense if they are
lowered to calls into a language-supplied runtime. For example, if
you add hash tables to the language, it would probably make sense to
add the routines to a runtime, instead of inlining them all the way.
- **memory management** - Currently we can only access the stack in
Kaleidoscope. It would also be useful to be able to allocate heap
memory, either with calls to the standard libc malloc/free interface
or with a garbage collector. If you would like to use garbage
collection, note that LLVM fully supports `Accurate Garbage
Collection <../GarbageCollection.html>`_ including algorithms that
move objects and need to scan/update the stack.
- **debugger support** - LLVM supports generation of `DWARF Debug
info <../SourceLevelDebugging.html>`_ which is understood by common
debuggers like GDB. Adding support for debug info is fairly
straightforward. The best way to understand it is to compile some
C/C++ code with "``llvm-gcc -g -O0``" and taking a look at what it
produces.
- **exception handling support** - LLVM supports generation of `zero
cost exceptions <../ExceptionHandling.html>`_ which interoperate with
code compiled in other languages. You could also generate code by
implicitly making every function return an error value and checking
it. You could also make explicit use of setjmp/longjmp. There are
many different ways to go here.
- **object orientation, generics, database access, complex numbers,
geometric programming, ...** - Really, there is no end of crazy
features that you can add to the language.
- **unusual domains** - We've been talking about applying LLVM to a
domain that many people are interested in: building a compiler for a
specific language. However, there are many other domains that can use
compiler technology that are not typically considered. For example,
LLVM has been used to implement OpenGL graphics acceleration,
translate C++ code to ActionScript, and many other cute and clever
things. Maybe you will be the first to JIT compile a regular
expression interpreter into native code with LLVM?
Have fun - try doing something crazy and unusual. Building a language
like everyone else always has, is much less fun than trying something a
little crazy or off the wall and seeing how it turns out. If you get
stuck or want to talk about it, feel free to email the `llvmdev mailing
list <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_: it has lots
of people who are interested in languages and are often willing to help
out.
Before we end this tutorial, I want to talk about some "tips and tricks"
for generating LLVM IR. These are some of the more subtle things that
may not be obvious, but are very useful if you want to take advantage of
LLVM's capabilities.
Properties of the LLVM IR
=========================
We have a couple common questions about code in the LLVM IR form - lets
just get these out of the way right now, shall we?
Target Independence
-------------------
Kaleidoscope is an example of a "portable language": any program written
in Kaleidoscope will work the same way on any target that it runs on.
Many other languages have this property, e.g. lisp, java, haskell,
javascript, python, etc (note that while these languages are portable,
not all their libraries are).
One nice aspect of LLVM is that it is often capable of preserving target
independence in the IR: you can take the LLVM IR for a
Kaleidoscope-compiled program and run it on any target that LLVM
supports, even emitting C code and compiling that on targets that LLVM
doesn't support natively. You can trivially tell that the Kaleidoscope
compiler generates target-independent code because it never queries for
any target-specific information when generating code.
The fact that LLVM provides a compact, target-independent,
representation for code gets a lot of people excited. Unfortunately,
these people are usually thinking about C or a language from the C
family when they are asking questions about language portability. I say
"unfortunately", because there is really no way to make (fully general)
C code portable, other than shipping the source code around (and of
course, C source code is not actually portable in general either - ever
port a really old application from 32- to 64-bits?).
The problem with C (again, in its full generality) is that it is heavily
laden with target specific assumptions. As one simple example, the
preprocessor often destructively removes target-independence from the
code when it processes the input text:
.. code-block:: c
#ifdef __i386__
int X = 1;
#else
int X = 42;
#endif
While it is possible to engineer more and more complex solutions to
problems like this, it cannot be solved in full generality in a way that
is better than shipping the actual source code.
That said, there are interesting subsets of C that can be made portable.
If you are willing to fix primitive types to a fixed size (say int =
32-bits, and long = 64-bits), don't care about ABI compatibility with
existing binaries, and are willing to give up some other minor features,
you can have portable code. This can make sense for specialized domains
such as an in-kernel language.
Safety Guarantees
-----------------
Many of the languages above are also "safe" languages: it is impossible
for a program written in Java to corrupt its address space and crash the
process (assuming the JVM has no bugs). Safety is an interesting
property that requires a combination of language design, runtime
support, and often operating system support.
It is certainly possible to implement a safe language in LLVM, but LLVM
IR does not itself guarantee safety. The LLVM IR allows unsafe pointer
casts, use after free bugs, buffer over-runs, and a variety of other
problems. Safety needs to be implemented as a layer on top of LLVM and,
conveniently, several groups have investigated this. Ask on the `llvmdev
mailing list <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_ if
you are interested in more details.
Language-Specific Optimizations
-------------------------------
One thing about LLVM that turns off many people is that it does not
solve all the world's problems in one system (sorry 'world hunger',
someone else will have to solve you some other day). One specific
complaint is that people perceive LLVM as being incapable of performing
high-level language-specific optimization: LLVM "loses too much
information".
Unfortunately, this is really not the place to give you a full and
unified version of "Chris Lattner's theory of compiler design". Instead,
I'll make a few observations:
First, you're right that LLVM does lose information. For example, as of
this writing, there is no way to distinguish in the LLVM IR whether an
SSA-value came from a C "int" or a C "long" on an ILP32 machine (other
than debug info). Both get compiled down to an 'i32' value and the
information about what it came from is lost. The more general issue
here, is that the LLVM type system uses "structural equivalence" instead
of "name equivalence". Another place this surprises people is if you
have two types in a high-level language that have the same structure
(e.g. two different structs that have a single int field): these types
will compile down into a single LLVM type and it will be impossible to
tell what it came from.
Second, while LLVM does lose information, LLVM is not a fixed target: we
continue to enhance and improve it in many different ways. In addition
to adding new features (LLVM did not always support exceptions or debug
info), we also extend the IR to capture important information for
optimization (e.g. whether an argument is sign or zero extended,
information about pointers aliasing, etc). Many of the enhancements are
user-driven: people want LLVM to include some specific feature, so they
go ahead and extend it.
Third, it is *possible and easy* to add language-specific optimizations,
and you have a number of choices in how to do it. As one trivial
example, it is easy to add language-specific optimization passes that
"know" things about code compiled for a language. In the case of the C
family, there is an optimization pass that "knows" about the standard C
library functions. If you call "exit(0)" in main(), it knows that it is
safe to optimize that into "return 0;" because C specifies what the
'exit' function does.
In addition to simple library knowledge, it is possible to embed a
variety of other language-specific information into the LLVM IR. If you
have a specific need and run into a wall, please bring the topic up on
the llvmdev list. At the very worst, you can always treat LLVM as if it
were a "dumb code generator" and implement the high-level optimizations
you desire in your front-end, on the language-specific AST.
Tips and Tricks
===============
There is a variety of useful tips and tricks that you come to know after
working on/with LLVM that aren't obvious at first glance. Instead of
letting everyone rediscover them, this section talks about some of these
issues.
Implementing portable offsetof/sizeof
-------------------------------------
One interesting thing that comes up, if you are trying to keep the code
generated by your compiler "target independent", is that you often need
to know the size of some LLVM type or the offset of some field in an
llvm structure. For example, you might need to pass the size of a type
into a function that allocates memory.
Unfortunately, this can vary widely across targets: for example the
width of a pointer is trivially target-specific. However, there is a
`clever way to use the getelementptr
instruction <http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt>`_
that allows you to compute this in a portable way.
Garbage Collected Stack Frames
------------------------------
Some languages want to explicitly manage their stack frames, often so
that they are garbage collected or to allow easy implementation of
closures. There are often better ways to implement these features than
explicit stack frames, but `LLVM does support
them, <http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt>`_
if you want. It requires your front-end to convert the code into
`Continuation Passing
Style <http://en.wikipedia.org/wiki/Continuation-passing_style>`_ and
the use of tail calls (which LLVM also supports).

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================================
LLVM Tutorial: Table of Contents
================================
.. TODO:: Use Sphinx toctree once all of these pages are converted.
Kaleidoscope: Implementing a Language with LLVM
===============================================
#. Kaleidoscope: Implementing a Language with LLVM
.. toctree::
:titlesonly:
:glob:
:numbered:
#. `Tutorial Introduction and the Lexer <LangImpl1.html>`__
#. `Implementing a Parser and AST <LangImpl2.html>`__
#. `Implementing Code Generation to LLVM IR <LangImpl3.html>`__
#. `Adding JIT and Optimizer Support <LangImpl4.html>`__
#. `Extending the language: control flow <LangImpl5.html>`__
#. `Extending the language: user-defined operators <LangImpl6.html>`__
#. `Extending the language: mutable variables / SSA
construction <LangImpl7.html>`__
#. `Conclusion and other useful LLVM tidbits <LangImpl8.html>`__
LangImpl*
#. Kaleidoscope: Implementing a Language with LLVM in Objective Caml
Kaleidoscope: Implementing a Language with LLVM in Objective Caml
=================================================================
#. `Tutorial Introduction and the Lexer <OCamlLangImpl1.html>`__
#. `Implementing a Parser and AST <OCamlLangImpl2.html>`__
#. `Implementing Code Generation to LLVM IR <OCamlLangImpl3.html>`__
#. `Adding JIT and Optimizer Support <OCamlLangImpl4.html>`__
#. `Extending the language: control flow <OCamlLangImpl5.html>`__
#. `Extending the language: user-defined
operators <OCamlLangImpl6.html>`__
#. `Extending the language: mutable variables / SSA
construction <OCamlLangImpl7.html>`__
#. `Conclusion and other useful LLVM tidbits <OCamlLangImpl8.html>`__
.. toctree::
:titlesonly:
:glob:
:numbered:
#. Advanced Topics
#. `Writing an Optimization for
LLVM <http://llvm.org/pubs/2004-09-22-LCPCLLVMTutorial.html>`_
OCamlLangImpl*
Advanced Topics
===============
#. `Writing an Optimization for LLVM <http://llvm.org/pubs/2004-09-22-LCPCLLVMTutorial.html>`_