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finish the tutorial, yaay.

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comments and feedback welcome.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43701 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2007-11-05 07:00:54 +00:00
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docs/tutorial/LangImpl8.html
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@ -3,7 +3,8 @@
<html>
<head>
<title>Kaleidoscope: Conclusion, ideas for extensions, and other useful tidbits</title>
<title>Kaleidoscope: Conclusion, ideas for extensions, and other useful
tidbits</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="Chris Lattner">
<link rel="stylesheet" href="../llvm.css" type="text/css">
@ -88,7 +89,7 @@ common debuggers like GDB. Adding support for debug info is fairly
straight-forward. 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 handlingsupport</b> - LLVM supports generation of <a
<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
@ -99,6 +100,14 @@ to go here.</li>
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>
@ -117,13 +126,198 @@ are very useful if you want to take advantage of LLVM's capabilities.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="llvmirproperties">Properties of LLVM
IR</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="targetindep">Target
Independence</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<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 language
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 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 even make real sense for specialized domains such as an
in-kernel language.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="safety">Safety Guarantees</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<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.
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>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="langspecific">Language-Specific
Optimizations</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<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
do some specific feature, so they go ahead and extend it to do so.</p>
<p>Third, it <em>is certainly possible</em> to add language-specific
optimizations, and you have a number of choices in how to do it. As one trivial
example, it is possible to add language-specific optimization passes that
"known" things about code compiled for a language. In the case of the C family,
there is an optimziation 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;" for example, 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 class="doc_section"><a name="tipsandtricks">Tips and Tricks</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p></p>
<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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="offsetofsizeof">Implementing portable
offsetof/sizeof</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<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>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="gcstack">Garbage Collected
Stack Frames</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<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 if you want</a>. 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 use of tail calls (which LLVM also supports).</p>
</div>