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Many typos, grammaro, and wording fixes. Patch by

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Kelly Wilson, thanks!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44043 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2007-11-13 07:06:30 +00:00
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59
docs/tutorial/LangImpl3.html
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@ -41,7 +41,7 @@ Support</li>
<p>Welcome to Chapter 3 of the "<a href="index.html">Implementing a language
with LLVM</a>" tutorial. This chapter shows you how to transform the <a
href="LangImpl2.html">Abstract Syntax Tree built in Chapter 2</a> into LLVM IR.
href="LangImpl2.html">Abstract Syntax Tree</a>, 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. :)
@ -79,14 +79,14 @@ public:
</pre>
</div>
<p>The Codegen() method says to emit IR for that AST node and all things it
<p>The Codegen() method 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 "<a
href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
Assignment (SSA)</a> 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 order words, there is no way to "change" an SSA value. For
re-executes. In other words, there is no way to "change" an SSA value. For
more information, please read up on <a
href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
Assignment</a> - the concepts are really quite natural once you grok them.</p>
@ -97,7 +97,7 @@ this. Again, this tutorial won't dwell on good software engineering practices:
for our purposes, adding a virtual method is simplest.</p>
<p>The
second thing we want is an "Error" method like we used for parser, which will
second thing we want is an "Error" method like we used for the parser, which will
be used to report errors found during code generation (for example, use of an
undeclared parameter):</p>
@ -144,7 +144,7 @@ has already been done, and we'll just use it to emit code.
<div class="doc_text">
<p>Generating LLVM code for expression nodes is very straight-forward: less
<p>Generating LLVM code for expression nodes is very straightforward: less
than 45 lines of commented code for all four of our expression nodes. First,
we'll do numeric literals:</p>
@ -174,7 +174,7 @@ Value *VariableExprAST::Codegen() {
</pre>
</div>
<p>References to variables are also quite simple here. In the simple version
<p>References to variables are also quite simple using LLVM. In the simple version
of Kaleidoscope, we assume that the variable has already been emited somewhere
and its value is available. In practice, the only values that can be in the
<tt>NamedValues</tt> map are function arguments. This
@ -211,9 +211,9 @@ 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.
</p>
<p>In this example, the LLVM builder class is starting to show its value.
Because it knows where to insert the newly created instruction, you just have to
specify what instruction to create (e.g. with <tt>CreateAdd</tt>), which
<p>In the example above, the LLVM builder class is starting to show its value.
LLVMBuilder knows where to insert the newly created instruction, all you have to
do is specify what instruction to create (e.g. with <tt>CreateAdd</tt>), which
operands to use (<tt>L</tt> and <tt>R</tt> here) and optionally provide a name
for the generated instruction. One nice thing about LLVM is that the name is
just a hint: if there are multiple additions in a single function, the first
@ -221,17 +221,16 @@ will be named "addtmp" and the second will be "autorenamed" by adding a suffix,
giving it a name like "addtmp42". Local value names for instructions are purely
optional, but it makes it much easier to read the IR dumps.</p>
<p><a href="../LangRef.html#instref">LLVM instructions</a> are constrained with
<p><a href="../LangRef.html#instref">LLVM instructions</a> are constrained by
strict rules: for example, the Left and Right operators of
an <a href="../LangRef.html#i_add">add instruction</a> have to have the same
type, and that the result type of the add must match the operand types. Because
an <a href="../LangRef.html#i_add">add instruction</a> 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.</p>
<p>On the other hand, LLVM specifies that the <a
href="../LangRef.html#i_fcmp">fcmp instruction</a> always returns an 'i1' value
(a one bit integer). However, 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 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 <a href="../LangRef.html#i_uitofp">uitofp instruction</a>. 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 <a
@ -261,8 +260,8 @@ Value *CallExprAST::Codegen() {
</pre>
</div>
<p>Code generation for function calls is quite straight-forward with LLVM. The
code above first looks the name of the function up in the LLVM Module's symbol
<p>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
@ -271,8 +270,8 @@ us.</p>
<p>Once we have the function to call, we recursively codegen each argument that
is to be passed in, and create an LLVM <a href="../LangRef.html#i_call">call
instruction</a>. Note that LLVM uses the native C calling conventions by
default, allowing these calls to call into standard library functions like
"sin" and "cos" with no additional effort.</p>
default, allowing these calls to also call into standard library functions like
"sin" and "cos", with no additional effort.</p>
<p>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
@ -321,7 +320,7 @@ this). Note that Types in LLVM are uniqued just like Constants are, so you
don't "new" a type, you "get" it.</p>
<p>The final line above actually creates the function that the prototype will
correspond to. This indicates which type, linkage, and name to use, and which
correspond to. This indicates the type, linkage and name to use, as well as which
module to insert into. "<a href="LangRef.html#linkage">external linkage</a>"
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
@ -343,7 +342,7 @@ above.</p>
<p>The Module symbol table works just like the Function symbol table when it
comes to name conflicts: if a new function is created with a name was previously
added to the symbol table, it will get implicitly renamed when added to the
Module. The code above exploits this fact to tell if there was a previous
Module. The code above exploits this fact to determine if there was a previous
definition of this function.</p>
<p>In Kaleidoscope, I choose to allow redefinitions of functions in two cases:
@ -403,7 +402,7 @@ definition and this one match up. If not, we emit an error.</p>
</div>
<p>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
function, setting the name of the LLVM Argument objects to match, and registering
the arguments in the <tt>NamedValues</tt> map for future use by the
<tt>VariableExprAST</tt> AST node. Once this is set up, it returns the Function
object to the caller. Note that we don't check for conflicting
@ -421,8 +420,8 @@ Function *FunctionAST::Codegen() {
</pre>
</div>
<p>Code generation for function definitions starts out simply enough: first we
codegen the prototype (Proto) and verify that it is ok. We also clear out the
<p>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
<tt>NamedValues</tt> 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.</p>
@ -445,7 +444,7 @@ the end of the new basic block. Basic blocks in LLVM are an important part
of functions that define the <a
href="http://en.wikipedia.org/wiki/Control_flow_graph">Control Flow Graph</a>.
Since we don't have any control flow, our functions will only contain one
block so far. We'll fix this in <a href="LangImpl5.html">Chapter 5</a> :).</p>
block at this point. We'll fix this in <a href="LangImpl5.html">Chapter 5</a> :).</p>
<div class="doc_code">
<pre>
@ -465,7 +464,7 @@ 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 <a
href="../LangRef.html#i_ret">ret instruction</a>, which completes the function.
Once the function is built, we call the <tt>verifyFunction</tt> function, which
Once the function is built, we call <tt>verifyFunction</tt>, 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
@ -481,13 +480,13 @@ and validated, we return it.</p>
</div>
<p>The only piece left here is handling of the error case. For simplicity, we
simply handle this by deleting the function we produced with the
handle this by merely deleting the function we produced with the
<tt>eraseFromParent</tt> 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.</p>
<p>This code does have a bug though. Since the <tt>PrototypeAST::Codegen</tt>
can return a previously defined forward declaration, this can actually delete
<p>This code does have a bug, though. Since the <tt>PrototypeAST::Codegen</tt>
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:</p>
@ -571,7 +570,7 @@ entry:
<p>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 make recursion actually be useful :).</p>
flow to actually make recursion useful :).</p>
<div class="doc_code">
<pre>
@ -636,7 +635,7 @@ entry:
generated. Here you can see the big picture with all the functions referencing
each other.</p>
<p>This wraps up this chapter of the Kaleidoscope tutorial. Up next we'll
<p>This wraps up the third chapter of the Kaleidoscope tutorial. Up next, we'll
describe how to <a href="LangImpl4.html">add JIT codegen and optimizer
support</a> to this so we can actually start running code!</p>

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@ -42,8 +42,8 @@ Flow</li>
with LLVM</a>" 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. This shows how to get nice efficient code for your
language.</p>
compiler support. These additions will demonstrate how to get nice, efficient code
for the Kaleidoscope language.</p>
</div>
@ -72,14 +72,13 @@ entry:
</pre>
</div>
<p>This code is a very very literal transcription of the AST built by parsing
our code, and as such, lacks optimizations like constant folding (we'd like to
get "<tt>add x, 3.0</tt>" in the example above) as well as other more important
optimizations. Constant folding in particular is a very common and very
<p>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 "<tt>add x, 3.0</tt>" 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.</p>
<p>With LLVM, you don't need to. Since all calls to build LLVM IR go through
<p>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.
@ -93,9 +92,9 @@ static LLVMFoldingBuilder Builder;
</div>
<p>All we did was switch from <tt>LLVMBuilder</tt> to
<tt>LLVMFoldingBuilder</tt>. Though we change no other code, now all of our
instructions are implicitly constant folded without us having to do anything
about it. For example, our example above now compiles to:</p>
<tt>LLVMFoldingBuilder</tt>. 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:</p>
<div class="doc_code">
<pre>
@ -153,7 +152,7 @@ range of optimizations that you can use, in the form of "passes".</p>
<div class="doc_text">
<p>LLVM provides many optimization passes which do many different sorts of
<p>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
@ -165,7 +164,7 @@ 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 the get run, see the <a href="../WritingAnLLVMPass.html">How
on passes and how they are run, see the <a href="../WritingAnLLVMPass.html">How
to Write a Pass</a> document and the <a href="../Passes.html">List of LLVM
Passes</a>.</p>
@ -207,13 +206,13 @@ add a set of optimizations to run. The code looks like this:</p>
</pre>
</div>
<p>This code defines two objects, a <tt>ExistingModuleProvider</tt> and a
<p>This code defines two objects, an <tt>ExistingModuleProvider</tt> and a
<tt>FunctionPassManager</tt>. The former is basically a wrapper around our
<tt>Module</tt> that the PassManager requires. It provides certain flexibility
that we're not going to take advantage of here, so I won't dive into what it is
all about.</p>
that we're not going to take advantage of here, so I won't dive into any details
about it.</p>
<p>The meat of the matter is the definition of "<tt>OurFPM</tt>". It
<p>The meat of the matter here, is the definition of "<tt>OurFPM</tt>". It
requires a pointer to the <tt>Module</tt> (through the <tt>ModuleProvider</tt>)
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
@ -223,7 +222,7 @@ which we will get to in the next section.</p>
<p>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 that
a wide variety of code. I won't delve into what they do but, believe me,
they are a good starting place :).</p>
<p>Once the PassManager is set up, we need to make use of it. We do this by
@ -247,7 +246,7 @@ running it after our newly created function is constructed (in
</pre>
</div>
<p>As you can see, this is pretty straight-forward. The
<p>As you can see, this is pretty straightforward. The
<tt>FunctionPassManager</tt> optimizes and updates the LLVM Function* in place,
improving (hopefully) its body. With this in place, we can try our test above
again:</p>
@ -271,7 +270,7 @@ add instruction from every execution of this function.</p>
<p>LLVM provides a wide variety of optimizations that can be used in certain
circumstances. Some <a href="../Passes.html">documentation about the various
passes</a> is available, but it isn't very complete. Another good source of
ideas is to look at the passes that <tt>llvm-gcc</tt> or
ideas can come from looking at the passes that <tt>llvm-gcc</tt> or
<tt>llvm-ld</tt> run to get started. The "<tt>opt</tt>" tool allows you to
experiment with passes from the command line, so you can see if they do
anything.</p>
@ -324,7 +323,7 @@ for you if one is available for your platform, otherwise it will fall back to
the interpreter.</p>
<p>Once the <tt>ExecutionEngine</tt> is created, the JIT is ready to be used.
There are a variety of APIs that are useful, but the most simple one is the
There are a variety of APIs that are useful, but the simplest one is the
"<tt>getPointerToFunction(F)</tt>" method. 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
@ -353,7 +352,7 @@ static void HandleTopLevelExpression() {
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.
As such, there is no difference between JIT compiled code and native machine
This means, there is no difference between JIT compiled code and native machine
code that is statically linked into your application.</p>
<p>With just these two changes, lets see how Kaleidoscope works now!</p>
@ -372,7 +371,7 @@ entry:
<p>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 it. This demonstrates very basic
for each top level expression that is typed in. This demonstrates very basic
functionality, but can we do more?</p>
<div class="doc_code">
@ -397,19 +396,19 @@ entry:
</pre>
</div>
<p>This illustrates that we can now call user code, but it is a bit subtle what
is going on here. Note that we only invoke the JIT on the anonymous functions
that <em>calls testfunc</em>, but we never invoked it on <em>testfunc
itself</em>.</p>
<p>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 <em>call testfunc</em>, but we never invoked it on <em>testfunc
</em>itself.</p>
<p>What actually happened here is that the anonymous function is
<p>What actually happened here is that the anonymous function was
JIT'd when requested. When the Kaleidoscope app calls through the function
pointer that is returned, the anonymous function starts executing. It ends up
making the call to the "testfunc" function, and ends up in a stub that invokes
the JIT, lazily, on testfunc. Once the JIT finishes lazily compiling testfunc,
it returns and the code re-executes the call.</p>
<p>In summary, the JIT will lazily JIT code on the fly as it is needed. The
<p>In summary, the JIT will lazily JIT code, on the fly, as it is needed. 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

52
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@ -55,7 +55,7 @@ User-defined Operators</li>
<p>Welcome to Chapter 5 of the "<a href="index.html">Implementing a language
with LLVM</a>" tutorial. Parts 1-4 described the implementation of the simple
Kaleidoscope language and included support for generating LLVM IR, following by
Kaleidoscope language and included support for generating LLVM IR, followed by
optimizations and a JIT compiler. Unfortunately, as presented, Kaleidoscope is
mostly useless: it has no control flow other than call and return. This means
that you can't have conditional branches in the code, significantly limiting its
@ -71,13 +71,13 @@ have an if/then/else expression plus a simple 'for' loop.</p>
<div class="doc_text">
<p>
Extending Kaleidoscope to support if/then/else is quite straight-forward. It
Extending Kaleidoscope to support if/then/else is quite straightforward. It
basically requires adding lexer support for this "new" concept to the lexer,
parser, AST, and LLVM code emitter. This example is nice, because it shows how
easy it is to "grow" a language over time, incrementally extending it as new
ideas are discovered.</p>
<p>Before we get going on "how" we do this extension, lets talk about what we
<p>Before we get going on "how" we add this extension, lets talk about "what" we
want. The basic idea is that we want to be able to write this sort of thing:
</p>
@ -97,7 +97,7 @@ Since we're using a mostly functional form, we'll have it evaluate its
conditional, then return the 'then' or 'else' value based on how the condition
was resolved. This is very similar to the C "?:" expression.</p>
<p>The semantics of the if/then/else expression is that it first evaluates the
<p>The semantics of the if/then/else expression is that it evaluates the
condition to a boolean equality value: 0.0 is considered to be false and
everything else is considered to be true.
If the condition is true, the first subexpression is evaluated and returned, if
@ -105,7 +105,7 @@ the condition is false, the second subexpression is evaluated and returned.
Since Kaleidoscope allows side-effects, this behavior is important to nail down.
</p>
<p>Now that we know what we want, lets break this down into its constituent
<p>Now that we know what we "want", lets break this down into its constituent
pieces.</p>
</div>
@ -118,7 +118,7 @@ If/Then/Else</a></div>
<div class="doc_text">
<p>The lexer extensions are straight-forward. First we add new enum values
<p>The lexer extensions are straightforward. First we add new enum values
for the relevant tokens:</p>
<div class="doc_code">
@ -128,7 +128,7 @@ for the relevant tokens:</p>
</pre>
</div>
<p>Once we have that, we recognize the new keywords in the lexer, pretty simple
<p>Once we have that, we recognize the new keywords in the lexer. This is pretty simple
stuff:</p>
<div class="doc_code">
@ -179,7 +179,7 @@ If/Then/Else</a></div>
<div class="doc_text">
<p>Now that we have the relevant tokens coming from the lexer and we have the
AST node to build, our parsing logic is relatively straight-forward. First we
AST node to build, our parsing logic is relatively straightforward. First we
define a new parsing function:</p>
<div class="doc_code">
@ -296,14 +296,14 @@ height="315"></center>
inserting actual calls into the code and recompiling or by calling these in the
debugger. LLVM has many nice features for visualizing various graphs.</p>
<p>Coming back to the generated code, it is fairly simple: the entry block
<p>Getting back to the generated code, it is fairly simple: the entry block
evaluates the conditional expression ("x" in our case here) and compares the
result to 0.0 with the "<tt><a href="../LangRef.html#i_fcmp">fcmp</a> one</tt>"
instruction ('one' is "Ordered and Not Equal"). Based on the result of this
expression, the code jumps to either the "then" or "else" blocks, which contain
the expressions for the true/false cases.</p>
<p>Once the then/else blocks is finished executing, they both branch back to the
<p>Once the then/else blocks are finished executing, they both branch back to the
'ifcont' block to execute the code that happens after the if/then/else. In this
case the only thing left to do is to return to the caller of the function. The
question then becomes: how does the code know which expression to return?</p>
@ -320,25 +320,25 @@ block. In this case, if control comes in from the "then" block, it gets the
value of "calltmp". If control comes from the "else" block, it gets the value
of "calltmp1".</p>
<p>At this point, you are probably starting to think "oh no! this means my
<p>At this point, you are probably starting to think "Oh no! This means my
simple and elegant front-end will have to start generating SSA form in order to
use LLVM!". Fortunately, this is not the case, and we strongly advise
<em>not</em> implementing an SSA construction algorithm in your front-end
unless there is an amazingly good reason to do so. In practice, there are two
sorts of values that float around in code written in your average imperative
sorts of values that float around in code written for your average imperative
programming language that might need Phi nodes:</p>
<ol>
<li>Code that involves user variables: <tt>x = 1; x = x + 1; </tt></li>
<li>Values that are implicit in the structure of your AST, such as the phi node
<li>Values that are implicit in the structure of your AST, such as the Phi node
in this case.</li>
</ol>
<p>In <a href="LangImpl7.html">Chapter 7</a> of this tutorial ("mutable
variables"), we'll talk about #1
in depth. For now, just believe me that you don't need SSA construction to
handle them. For #2, you have the choice of using the techniques that we will
describe for #1, or you can insert Phi nodes directly if convenient. In this
handle this case. For #2, you have the choice of using the techniques that we will
describe for #1, or you can insert Phi nodes directly, if convenient. In this
case, it is really really easy to generate the Phi node, so we choose to do it
directly.</p>
@ -369,7 +369,7 @@ Value *IfExprAST::Codegen() {
</pre>
</div>
<p>This code is straight-forward and similar to what we saw before. We emit the
<p>This code is straightforward and similar to what we saw before. We emit the
expression for the condition, then compare that value to zero to get a truth
value as a 1-bit (bool) value.</p>
@ -395,7 +395,7 @@ block for its "parent" (the function it is currently embedded into).</p>
<p>Once it has that, it creates three blocks. Note that it passes "TheFunction"
into the constructor for the "then" block. This causes the constructor to
automatically insert the new block onto the end of the specified function. The
automatically insert the new block into the end of the specified function. The
other two blocks are created, but aren't yet inserted into the function.</p>
<p>Once the blocks are created, we can emit the conditional branch that chooses
@ -427,7 +427,7 @@ insertion point to be at the end of the specified block. However, since the
block. :)</p>
<p>Once the insertion point is set, we recursively codegen the "then" expression
from the AST. To finish off the then block, we create an unconditional branch
from the AST. To finish off the "then" block, we create an unconditional branch
to the merge block. One interesting (and very important) aspect of the LLVM IR
is that it <a href="../LangRef.html#functionstructure">requires all basic blocks
to be "terminated"</a> with a <a href="../LangRef.html#terminators">control flow
@ -439,7 +439,7 @@ violate this rule, the verifier will emit an error.</p>
is that when we create the Phi node in the merge block, we need to set up the
block/value pairs that indicate how the Phi will work. Importantly, the Phi
node expects to have an entry for each predecessor of the block in the CFG. Why
then are we getting the current block when we just set it to ThenBB 5 lines
then, are we getting the current block when we just set it to ThenBB 5 lines
above? The problem is that the "Then" expression may actually itself change the
block that the Builder is emitting into if, for example, it contains a nested
"if/then/else" expression. Because calling Codegen recursively could
@ -492,7 +492,7 @@ the if/then/else expression. In our example above, this returned value will
feed into the code for the top-level function, which will create the return
instruction.</p>
<p>Overall, we now have the ability to execution conditional code in
<p>Overall, we now have the ability to execute conditional code in
Kaleidoscope. With this extension, Kaleidoscope is a fairly complete language
that can calculate a wide variety of numeric functions. Next up we'll add
another useful expression that is familiar from non-functional languages...</p>
@ -571,7 +571,7 @@ the 'for' Loop</a></div>
<div class="doc_text">
<p>The AST node is similarly simple. It basically boils down to capturing
<p>The AST node is just as simple. It basically boils down to capturing
the variable name and the constituent expressions in the node.</p>
<div class="doc_code">
@ -704,7 +704,7 @@ the 'for' Loop</a></div>
<div class="doc_text">
<p>The first part of codegen is very simple: we just output the start expression
<p>The first part of Codegen is very simple: we just output the start expression
for the loop value:</p>
<div class="doc_code">
@ -804,7 +804,7 @@ references to it will naturally find it in the symbol table.</p>
</div>
<p>Now that the body is emitted, we compute the next value of the iteration
variable by adding the step value or 1.0 if it isn't present. '<tt>NextVar</tt>'
variable by adding the step value, or 1.0 if it isn't present. '<tt>NextVar</tt>'
will be the value of the loop variable on the next iteration of the loop.</p>
<div class="doc_code">
@ -839,8 +839,7 @@ statement.</p>
</div>
<p>With the code for the body of the loop complete, we just need to finish up
the control flow for it. This remembers the end block (for the phi node), then
creates the block for the loop exit ("afterloop"). Based on the value of the
the control flow for it. This code remembers the end block (for the phi node), then creates the block for the loop exit ("afterloop"). Based on the value of the
exit condition, it creates a conditional branch that chooses between executing
the loop again and exiting the loop. Any future code is emitted in the
"afterloop" block, so it sets the insertion position to it.</p>
@ -869,8 +868,7 @@ the for loop. Finally, code generation of the for loop always returns 0.0, so
that is what we return from <tt>ForExprAST::Codegen</tt>.</p>
<p>With this, we conclude the "adding control flow to Kaleidoscope" chapter of
the tutorial. We added two control flow constructs, and used them to motivate
a couple of aspects of the LLVM IR that are important for front-end implementors
the tutorial. In this chapter we added two control flow constructs, and used them to motivate a couple of aspects of the LLVM IR that are important for front-end implementors
to know. In the next chapter of our saga, we will get a bit crazier and add
<a href="LangImpl6.html">user-defined operators</a> to our poor innocent
language.</p>