<div class="doc_title">Kaleidoscope: Code generation to LLVM IR</div>
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 3
+ <ol>
+ <li><a href="#intro">Chapter 3 Introduction</a></li>
+ <li><a href="#basics">Code Generation Setup</a></li>
+ <li><a href="#exprs">Expression Code Generation</a></li>
+ <li><a href="#funcs">Function Code Generation</a></li>
+ <li><a href="#driver">Driver Changes and Closing Thoughts</a></li>
+ <li><a href="#code">Full Code Listing</a></li>
+ </ol>
+</li>
+<li><a href="LangImpl4.html">Chapter 4</a>: Adding JIT and Optimizer
+Support</li>
+</ul>
+
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>
<!-- *********************************************************************** -->
-<div class="doc_section"><a name="intro">Part 3 Introduction</a></div>
+<div class="doc_section"><a name="intro">Chapter 3 Introduction</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
-<p>Welcome to part 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.
+<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</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.
+parser than it is to generate LLVM IR code. :)
</p>
+<p><b>Please note</b>: the code in this chapter and later require LLVM 2.2 or
+LLVM SVN to work. LLVM 2.1 and before will not work with it.</p>
+
</div>
<!-- *********************************************************************** -->
-<div class="doc_section"><a name="basics">Code Generation setup</a></div>
+<div class="doc_section"><a name="basics">Code Generation Setup</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
-In order to generate LLVM IR, we want some simple setup to get started. First,
-we define virtual codegen methods in each AST class:</p>
+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:</p>
<div class="doc_code">
<pre>
class ExprAST {
public:
virtual ~ExprAST() {}
- virtual Value *Codegen() = 0;
+ <b>virtual Value *Codegen() = 0;</b>
};
/// NumberExprAST - Expression class for numeric literals like "1.0".
double Val;
public:
explicit NumberExprAST(double val) : Val(val) {}
- virtual Value *Codegen();
+ <b>virtual Value *Codegen();</b>
};
...
</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>
+<p>Note that instead of adding virtual methods to the ExprAST class hierarchy,
+it could also make sense to use a <a
+href="http://en.wikipedia.org/wiki/Visitor_pattern">visitor pattern</a> or some
+other way to model 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>
Value *ErrorV(const char *Str) { Error(Str); return 0; }
static Module *TheModule;
-static LLVMBuilder Builder;
+static IRBuilder Builder;
static std::map<std::string, Value*> NamedValues;
</pre>
</div>
uses to contain code.</p>
<p>The <tt>Builder</tt> object is a helper object that makes it easy to generate
-LLVM instructions. The <tt>Builder</tt> keeps track of the current place to
-insert instructions and has methods to create new instructions.</p>
+LLVM instructions. Instances of the <a
+href="http://llvm.org/doxygen/IRBuilder_8h-source.html"><tt>IRBuilder</tt></a>
+class keep track of the current place to insert instructions and has methods to
+create new instructions.</p>
<p>The <tt>NamedValues</tt> map keeps track of which values are defined in the
-current scope and what their LLVM representation is. 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.</p>
+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.</p>
<p>
With these basics in place, we can start talking about how to generate code for
<div class="doc_text">
-<p>Generating LLVM code for expression nodes is very straight-forward: less
-than 45 lines of commented code for all four of our expression nodes. First,
+<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>
<div class="doc_code">
constants of <em>A</em>rbitrary <em>P</em>recision). This code basically just
creates and returns a <tt>ConstantFP</tt>. 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(..).</p>
+uses "the foo::get(..)" idiom instead of "new foo(..)" or "foo::create(..)".</p>
<div class="doc_code">
<pre>
</pre>
</div>
-<p>References to variables is 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
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.</p>
+unknown variable is being referenced) and returns the value for it. In future
+chapters, we'll add support for <a href="LangImpl5.html#for">loop induction
+variables</a> in the symbol table, and for <a
+href="LangImpl7.html#localvars">local variables</a>.</p>
<div class="doc_code">
<pre>
case '-': return Builder.CreateSub(L, R, "subtmp");
case '*': return Builder.CreateMul(L, R, "multmp");
case '<':
- L = Builder.CreateFCmpULT(L, R, "multmp");
+ L = Builder.CreateFCmpULT(L, R, "cmptmp");
// Convert bool 0/1 to double 0.0 or 1.0
return Builder.CreateUIToFP(L, Type::DoubleTy, "booltmp");
default: return ErrorV("invalid binary operator");
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
-specificy 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.
+IRBuilder 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
-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 to
-have very strict type properties: 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 of the add matches the operands. Because all values
-in Kaleidoscope are doubles, this makes for very simple code for add, sub and
-mul.</p>
+for the generated instruction.</p>
+
+<p>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.</p>
+
+<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> 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
-href="../LangRef.html#i_sitofp">sitofp instruction</a>, the Kaleidoscope '<'
+href="../LangRef.html#i_sitofp">sitofp instruction</a>, the Kaleidoscope '<'
operator would return 0.0 and -1.0, depending on the input value.</p>
<div class="doc_code">
</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
<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
<div class="doc_text">
-<p>Code generation for prototypes and functions has to handle a number of
-details, which make their code less beautiful and elegant than expression code
-generation, but they illustrate some important points. First, lets talk about
-code generation for prototypes: this is used both for function bodies as well
-as external function declarations. The code starts with:</p>
+<p>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:</p>
<div class="doc_code">
<pre>
std::vector<const Type*> Doubles(Args.size(), Type::DoubleTy);
FunctionType *FT = FunctionType::get(Type::DoubleTy, Doubles, false);
- Function *F = new Function(FT, Function::ExternalLinkage, Name, TheModule);
+ Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
</pre>
</div>
-<p>This code packs a lot of power into a few lines. The first step is to create
+<p>This code packs a lot of power into a few lines. Note first that this
+function returns a "Function*" instead of a "Value*". 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.</p>
+
+<p>The call to <tt>FunctionType::get</tt> creates
the <tt>FunctionType</tt> 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 <tt>FunctionType::get</tt>
+a vector of "N" LLVM double types. It then uses the <tt>FunctionType::get</tt>
method to create a function type that takes "N" doubles as arguments, returns
one double as a result, and that is not vararg (the false parameter indicates
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
-module to insert into. "<a href="LangRef.html#linkage">external linkage</a>"
+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
user specified: since "<tt>TheModule</tt>" is specified, this name is registered
<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:
-first, we want to allow 'extern'ing a function more than once, so long as the
+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 definining a body for it. This is useful
</pre>
</div>
-<p>In order to verify the logic above, we first check to see if the preexisting
+<p>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, this means its a forward
+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
</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
argument names here (e.g. "extern foo(a b a)"). Doing so would be very
-straight-forward.</p>
+straight-forward with the mechanics we have already used above.</p>
<div class="doc_code">
<pre>
</pre>
</div>
-<p>Code generation for function definitions starts out simply enough: first we
-codegen the prototype 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.</p>
+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>
<div class="doc_code">
<pre>
// Create a new basic block to start insertion into.
- BasicBlock *BB = new BasicBlock("entry", TheFunction);
+ BasicBlock *BB = BasicBlock::Create("entry", TheFunction);
Builder.SetInsertPoint(BB);
if (Value *RetVal = Body->Codegen()) {
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 future installment :).</p>
+block at this point. We'll fix this in <a href="LangImpl5.html">Chapter 5</a> :).</p>
<div class="doc_code">
<pre>
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
</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>
<div class="doc_code">
<pre>
ready> <b>4+5</b>;
-ready> Read top-level expression:
+Read top-level expression:
define double @""() {
entry:
%addtmp = add double 4.000000e+00, 5.000000e+00
</div>
<p>Note how the parser turns the top-level expression into anonymous functions
-for us. This will be handy when we add JIT support in the next chapter. Also
-note that the code is very literally transcribed, no optimizations are being
-performed. We will add optimizations explicitly in the next chapter.</p>
+for us. This will be handy when we add <a href="LangImpl4.html#jit">JIT
+support</a> in the next chapter. Also note that the code is very literally
+transcribed, no optimizations are being performed. We will
+<a href="LangImpl4.html#trivialconstfold">add optimizations</a> explicitly in
+the next chapter.</p>
<div class="doc_code">
<pre>
ready> <b>def foo(a b) a*a + 2*a*b + b*b;</b>
-ready> Read function definition:
+Read function definition:
define double @foo(double %a, double %b) {
entry:
%multmp = mul double %a, %a
<div class="doc_code">
<pre>
ready> <b>def bar(a) foo(a, 4.0) + bar(31337);</b>
-ready> Read function definition:
+Read function definition:
define double @bar(double %a) {
entry:
%calltmp = call double @foo( double %a, double 4.000000e+00 )
<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>
ready> <b>extern cos(x);</b>
-ready> Read extern:
+Read extern:
declare double @cos(double)
ready> <b>cos(1.234);</b>
-ready> Read top-level expression:
+Read top-level expression:
define double @""() {
entry:
%calltmp = call double @cos( double 1.234000e+00 )
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>
<div class="doc_code">
<pre>
# Compile
- g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core` -o toy
+ g++ -g -O3 toy.cpp `llvm-config --cppflags --ldflags --libs core` -o toy
# Run
./toy
</pre>
#include "llvm/DerivedTypes.h"
#include "llvm/Module.h"
#include "llvm/Analysis/Verifier.h"
-#include "llvm/Support/LLVMBuilder.h"
+#include "llvm/Support/IRBuilder.h"
#include <cstdio>
#include <string>
#include <map>
if (LastChar == '#') {
// Comment until end of line.
do LastChar = getchar();
- while (LastChar != EOF && LastChar != '\n' & LastChar != '\r');
+ while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
if (LastChar != EOF)
return gettok();
static ExprAST *ParseExpression();
/// identifierexpr
-/// ::= identifer
-/// ::= identifer '(' expression* ')'
+/// ::= identifier
+/// ::= identifier '(' expression* ')'
static ExprAST *ParseIdentifierExpr() {
std::string IdName = IdentifierStr;
- getNextToken(); // eat identifer.
+ getNextToken(); // eat identifier.
if (CurTok != '(') // Simple variable ref.
return new VariableExprAST(IdName);
// Call.
getNextToken(); // eat (
std::vector<ExprAST*> Args;
- while (1) {
- ExprAST *Arg = ParseExpression();
- if (!Arg) return 0;
- Args.push_back(Arg);
+ if (CurTok != ')') {
+ while (1) {
+ ExprAST *Arg = ParseExpression();
+ if (!Arg) return 0;
+ Args.push_back(Arg);
- if (CurTok == ')') break;
+ if (CurTok == ')') break;
- if (CurTok != ',')
- return Error("Expected ')'");
- getNextToken();
+ if (CurTok != ',')
+ return Error("Expected ')' or ',' in argument list");
+ getNextToken();
+ }
}
// Eat the ')'.
//===----------------------------------------------------------------------===//
static Module *TheModule;
-static LLVMBuilder Builder;
+static IRBuilder Builder;
static std::map<std::string, Value*> NamedValues;
Value *ErrorV(const char *Str) { Error(Str); return 0; }
case '-': return Builder.CreateSub(L, R, "subtmp");
case '*': return Builder.CreateMul(L, R, "multmp");
case '<':
- L = Builder.CreateFCmpULT(L, R, "multmp");
+ L = Builder.CreateFCmpULT(L, R, "cmptmp");
// Convert bool 0/1 to double 0.0 or 1.0
return Builder.CreateUIToFP(L, Type::DoubleTy, "booltmp");
default: return ErrorV("invalid binary operator");
std::vector<const Type*> Doubles(Args.size(), Type::DoubleTy);
FunctionType *FT = FunctionType::get(Type::DoubleTy, Doubles, false);
- Function *F = new Function(FT, Function::ExternalLinkage, Name, TheModule);
+ Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
// If F conflicted, there was already something named 'Name'. If it has a
// body, don't allow redefinition or reextern.
return 0;
// Create a new basic block to start insertion into.
- BasicBlock *BB = new BasicBlock("entry", TheFunction);
+ BasicBlock *BB = BasicBlock::Create("entry", TheFunction);
Builder.SetInsertPoint(BB);
if (Value *RetVal = Body->Codegen()) {
}
</pre>
</div>
+<a href="LangImpl4.html">Next: Adding JIT and Optimizer Support</a>
</div>
<!-- *********************************************************************** -->
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