Date: Mon, 8 Apr 2013 18:41:23 +0000 Subject: Vendor import of llvm trunk r178860: http://llvm.org/svn/llvm-project/llvm/trunk@178860 --- docs/tutorial/LangImpl5.html | 1772 ------------------------------------------ 1 file changed, 1772 deletions(-) delete mode 100644 docs/tutorial/LangImpl5.html (limited to 'docs/tutorial/LangImpl5.html') diff --git a/docs/tutorial/LangImpl5.html b/docs/tutorial/LangImpl5.html deleted file mode 100644 index 9a9fd8c..0000000 --- a/docs/tutorial/LangImpl5.html +++ /dev/null @@ -1,1772 +0,0 @@ - - - - - Kaleidoscope: Extending the Language: Control Flow - - - - - - - -

Kaleidoscope: Extending the Language: Control Flow

- -

Up to Tutorial Index
Chapter 5 -
1. Chapter 5 Introduction
2. If/Then/Else -
  1. Lexer Extensions
  2. AST Extensions
  3. Parser Extensions
  4. LLVM IR
  5. Code Generation
  -
3. 'for' Loop Expression -
  1. Lexer Extensions
  2. AST Extensions
  3. Parser Extensions
  4. LLVM IR
  5. Code Generation
  -
4. Full Code Listing
-
Chapter 6: Extending the Language: -User-defined Operators

- -

Written by Chris Lattner

- - -

Chapter 5 Introduction

- - -

- -

Welcome to Chapter 5 of the "Implementing a language -with LLVM" tutorial. Parts 1-4 described the implementation of the simple -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 -power. In this episode of "build that compiler", we'll extend Kaleidoscope to -have an if/then/else expression plus a simple 'for' loop.

- -

- - -

If/Then/Else

- - -

- -

-Extending Kaleidoscope to support if/then/else is quite straightforward. It -basically requires adding 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.

- -

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: -

- -

-def fib(x)
-  if x < 3 then
-    1
-  else
-    fib(x-1)+fib(x-2);
-

- -

In Kaleidoscope, every construct is an expression: there are no statements. -As such, the if/then/else expression needs to return a value like any other. -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.

- -

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 -the condition is false, the second subexpression is evaluated and returned. -Since Kaleidoscope allows side-effects, this behavior is important to nail down. -

- -

Now that we know what we "want", lets break this down into its constituent -pieces.

- - -

Lexer Extensions for If/Then/Else

- - - -

- -

The lexer extensions are straightforward. First we add new enum values -for the relevant tokens:

- -

-  // control
-  tok_if = -6, tok_then = -7, tok_else = -8,
-

- -

Once we have that, we recognize the new keywords in the lexer. This is pretty simple -stuff:

- -

-    ...
-    if (IdentifierStr == "def") return tok_def;
-    if (IdentifierStr == "extern") return tok_extern;
-    if (IdentifierStr == "if") return tok_if;
-    if (IdentifierStr == "then") return tok_then;
-    if (IdentifierStr == "else") return tok_else;
-    return tok_identifier;
-

- -

- - -

AST Extensions for If/Then/Else

- - -

- -

To represent the new expression we add a new AST node for it:

- -

-/// IfExprAST - Expression class for if/then/else.
-class IfExprAST : public ExprAST {
-  ExprAST *Cond, *Then, *Else;
-public:
-  IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
-    : Cond(cond), Then(then), Else(_else) {}
-  virtual Value *Codegen();
-};
-

- -

The AST node just has pointers to the various subexpressions.

- -

- - -

Parser Extensions for If/Then/Else

- - -

- -

Now that we have the relevant tokens coming from the lexer and we have the -AST node to build, our parsing logic is relatively straightforward. First we -define a new parsing function:

- -

-/// ifexpr ::= 'if' expression 'then' expression 'else' expression
-static ExprAST *ParseIfExpr() {
-  getNextToken();  // eat the if.
-  
-  // condition.
-  ExprAST *Cond = ParseExpression();
-  if (!Cond) return 0;
-  
-  if (CurTok != tok_then)
-    return Error("expected then");
-  getNextToken();  // eat the then
-  
-  ExprAST *Then = ParseExpression();
-  if (Then == 0) return 0;
-  
-  if (CurTok != tok_else)
-    return Error("expected else");
-  
-  getNextToken();
-  
-  ExprAST *Else = ParseExpression();
-  if (!Else) return 0;
-  
-  return new IfExprAST(Cond, Then, Else);
-}
-

- -

Next we hook it up as a primary expression:

- -

-static ExprAST *ParsePrimary() {
-  switch (CurTok) {
-  default: return Error("unknown token when expecting an expression");
-  case tok_identifier: return ParseIdentifierExpr();
-  case tok_number:     return ParseNumberExpr();
-  case '(':            return ParseParenExpr();
-  case tok_if:         return ParseIfExpr();
-  }
-}
-

- -

- - -

LLVM IR for If/Then/Else

- - -

- -

Now that we have it parsing and building the AST, the final piece is adding -LLVM code generation support. This is the most interesting part of the -if/then/else example, because this is where it starts to introduce new concepts. -All of the code above has been thoroughly described in previous chapters. -

- -

To motivate the code we want to produce, lets take a look at a simple -example. Consider:

- -

-extern foo();
-extern bar();
-def baz(x) if x then foo() else bar();
-

- -

If you disable optimizations, the code you'll (soon) get from Kaleidoscope -looks like this:

- -

-declare double @foo()
-
-declare double @bar()
-
-define double @baz(double %x) {
-entry:
-  %ifcond = fcmp one double %x, 0.000000e+00
-  br i1 %ifcond, label %then, label %else
-
-then:		; preds = %entry
-  %calltmp = call double @foo()
-  br label %ifcont
-
-else:		; preds = %entry
-  %calltmp1 = call double @bar()
-  br label %ifcont
-
-ifcont:		; preds = %else, %then
-  %iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ]
-  ret double %iftmp
-}
-

- -

To visualize the control flow graph, you can use a nifty feature of the LLVM -'opt' tool. If you put this LLVM IR -into "t.ll" and run "llvm-as < t.ll | opt -analyze -view-cfg", a window will pop up and you'll -see this graph:

- -

- -

Another way to get this is to call "F->viewCFG()" or -"F->viewCFGOnly()" (where F is a "Function*") either by -inserting actual calls into the code and recompiling or by calling these in the -debugger. LLVM has many nice features for visualizing various graphs.

- -

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 "fcmp one" -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.

- -

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?

- -

The answer to this question involves an important SSA operation: the -Phi -operation. If you're not familiar with SSA, the wikipedia -article is a good introduction and there are various other introductions to -it available on your favorite search engine. The short version is that -"execution" of the Phi operation requires "remembering" which block control came -from. The Phi operation takes on the value corresponding to the input control -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".

- -

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 -not 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 for your average imperative -programming language that might need Phi nodes:

- -

Code that involves user variables: x = 1; x = x + 1;
Values that are implicit in the structure of your AST, such as the Phi node -in this case.

- -

In Chapter 7 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 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.

- -

Okay, enough of the motivation and overview, lets generate code!

- -

- - -

Code Generation for If/Then/Else

- - -

- -

In order to generate code for this, we implement the Codegen method -for IfExprAST:

- -

-Value *IfExprAST::Codegen() {
-  Value *CondV = Cond->Codegen();
-  if (CondV == 0) return 0;
-  
-  // Convert condition to a bool by comparing equal to 0.0.
-  CondV = Builder.CreateFCmpONE(CondV, 
-                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
-                                "ifcond");
-

- -

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.

- -

-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-  
-  // Create blocks for the then and else cases.  Insert the 'then' block at the
-  // end of the function.
-  BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
-  BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
-  BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
-
-  Builder.CreateCondBr(CondV, ThenBB, ElseBB);
-

- -

This code creates the basic blocks that are related to the if/then/else -statement, and correspond directly to the blocks in the example above. The -first line gets the current Function object that is being built. It -gets this by asking the builder for the current BasicBlock, and asking that -block for its "parent" (the function it is currently embedded into).

- -

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 into the end of the specified function. The -other two blocks are created, but aren't yet inserted into the function.

- -

Once the blocks are created, we can emit the conditional branch that chooses -between them. Note that creating new blocks does not implicitly affect the -IRBuilder, so it is still inserting into the block that the condition -went into. Also note that it is creating a branch to the "then" block and the -"else" block, even though the "else" block isn't inserted into the function yet. -This is all ok: it is the standard way that LLVM supports forward -references.

- -

-  // Emit then value.
-  Builder.SetInsertPoint(ThenBB);
-  
-  Value *ThenV = Then->Codegen();
-  if (ThenV == 0) return 0;
-  
-  Builder.CreateBr(MergeBB);
-  // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
-  ThenBB = Builder.GetInsertBlock();
-

- -

After the conditional branch is inserted, we move the builder to start -inserting into the "then" block. Strictly speaking, this call moves the -insertion point to be at the end of the specified block. However, since the -"then" block is empty, it also starts out by inserting at the beginning of the -block. :)

- -

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 -to the merge block. One interesting (and very important) aspect of the LLVM IR -is that it requires all basic blocks -to be "terminated" with a control flow -instruction such as return or branch. This means that all control flow, -including fall throughs must be made explicit in the LLVM IR. If you -violate this rule, the verifier will emit an error.

- -

The final line here is quite subtle, but is very important. The basic issue -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 -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 -arbitrarily change the notion of the current block, we are required to get an -up-to-date value for code that will set up the Phi node.

- -

-  // Emit else block.
-  TheFunction->getBasicBlockList().push_back(ElseBB);
-  Builder.SetInsertPoint(ElseBB);
-  
-  Value *ElseV = Else->Codegen();
-  if (ElseV == 0) return 0;
-  
-  Builder.CreateBr(MergeBB);
-  // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
-  ElseBB = Builder.GetInsertBlock();
-

- -

Code generation for the 'else' block is basically identical to codegen for -the 'then' block. The only significant difference is the first line, which adds -the 'else' block to the function. Recall previously that the 'else' block was -created, but not added to the function. Now that the 'then' and 'else' blocks -are emitted, we can finish up with the merge code:

- -

-  // Emit merge block.
-  TheFunction->getBasicBlockList().push_back(MergeBB);
-  Builder.SetInsertPoint(MergeBB);
-  PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), 2,
-                                  "iftmp");
-  
-  PN->addIncoming(ThenV, ThenBB);
-  PN->addIncoming(ElseV, ElseBB);
-  return PN;
-}
-

- -

The first two lines here are now familiar: the first adds the "merge" block -to the Function object (it was previously floating, like the else block above). -The second block changes the insertion point so that newly created code will go -into the "merge" block. Once that is done, we need to create the PHI node and -set up the block/value pairs for the PHI.

- -

Finally, the CodeGen function returns the phi node as the value computed by -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.

- -

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...

- -

- - -

'for' Loop Expression

- - -

- -

Now that we know how to add basic control flow constructs to the language, -we have the tools to add more powerful things. Lets add something more -aggressive, a 'for' expression:

- -

- extern putchard(char)
- def printstar(n)
-   for i = 1, i < n, 1.0 in
-     putchard(42);  # ascii 42 = '*'
-     
- # print 100 '*' characters
- printstar(100);
-

- -

This expression defines a new variable ("i" in this case) which iterates from -a starting value, while the condition ("i < n" in this case) is true, -incrementing by an optional step value ("1.0" in this case). If the step value -is omitted, it defaults to 1.0. While the loop is true, it executes its -body expression. Because we don't have anything better to return, we'll just -define the loop as always returning 0.0. In the future when we have mutable -variables, it will get more useful.

- -

As before, lets talk about the changes that we need to Kaleidoscope to -support this.

- - -

Lexer Extensions for the 'for' Loop

- - -

- -

The lexer extensions are the same sort of thing as for if/then/else:

- -

-  ... in enum Token ...
-  // control
-  tok_if = -6, tok_then = -7, tok_else = -8,
-  tok_for = -9, tok_in = -10
-
-  ... in gettok ...
-  if (IdentifierStr == "def") return tok_def;
-  if (IdentifierStr == "extern") return tok_extern;
-  if (IdentifierStr == "if") return tok_if;
-  if (IdentifierStr == "then") return tok_then;
-  if (IdentifierStr == "else") return tok_else;
-  if (IdentifierStr == "for") return tok_for;
-  if (IdentifierStr == "in") return tok_in;
-  return tok_identifier;
-

- -

- - -

AST Extensions for the 'for' Loop

- - -

- -

The AST node is just as simple. It basically boils down to capturing -the variable name and the constituent expressions in the node.

- -

-/// ForExprAST - Expression class for for/in.
-class ForExprAST : public ExprAST {
-  std::string VarName;
-  ExprAST *Start, *End, *Step, *Body;
-public:
-  ForExprAST(const std::string &varname, ExprAST *start, ExprAST *end,
-             ExprAST *step, ExprAST *body)
-    : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
-  virtual Value *Codegen();
-};
-

- -

- - -

Parser Extensions for the 'for' Loop

- - -

- -

The parser code is also fairly standard. The only interesting thing here is -handling of the optional step value. The parser code handles it by checking to -see if the second comma is present. If not, it sets the step value to null in -the AST node:

- -

-/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
-static ExprAST *ParseForExpr() {
-  getNextToken();  // eat the for.
-
-  if (CurTok != tok_identifier)
-    return Error("expected identifier after for");
-  
-  std::string IdName = IdentifierStr;
-  getNextToken();  // eat identifier.
-  
-  if (CurTok != '=')
-    return Error("expected '=' after for");
-  getNextToken();  // eat '='.
-  
-  
-  ExprAST *Start = ParseExpression();
-  if (Start == 0) return 0;
-  if (CurTok != ',')
-    return Error("expected ',' after for start value");
-  getNextToken();
-  
-  ExprAST *End = ParseExpression();
-  if (End == 0) return 0;
-  
-  // The step value is optional.
-  ExprAST *Step = 0;
-  if (CurTok == ',') {
-    getNextToken();
-    Step = ParseExpression();
-    if (Step == 0) return 0;
-  }
-  
-  if (CurTok != tok_in)
-    return Error("expected 'in' after for");
-  getNextToken();  // eat 'in'.
-  
-  ExprAST *Body = ParseExpression();
-  if (Body == 0) return 0;
-
-  return new ForExprAST(IdName, Start, End, Step, Body);
-}
-

- -

- - -

LLVM IR for the 'for' Loop

- - -

- -

Now we get to the good part: the LLVM IR we want to generate for this thing. -With the simple example above, we get this LLVM IR (note that this dump is -generated with optimizations disabled for clarity): -

- -

-declare double @putchard(double)
-
-define double @printstar(double %n) {
-entry:
-  ; initial value = 1.0 (inlined into phi)
-  br label %loop
-
-loop:		; preds = %loop, %entry
-  %i = phi double [ 1.000000e+00, %entry ], [ %nextvar, %loop ]
-  ; body
-  %calltmp = call double @putchard(double 4.200000e+01)
-  ; increment
-  %nextvar = fadd double %i, 1.000000e+00
-
-  ; termination test
-  %cmptmp = fcmp ult double %i, %n
-  %booltmp = uitofp i1 %cmptmp to double
-  %loopcond = fcmp one double %booltmp, 0.000000e+00
-  br i1 %loopcond, label %loop, label %afterloop
-
-afterloop:		; preds = %loop
-  ; loop always returns 0.0
-  ret double 0.000000e+00
-}
-

- -

This loop contains all the same constructs we saw before: a phi node, several -expressions, and some basic blocks. Lets see how this fits together.

- -

- - -

Code Generation for the 'for' Loop

- - -

- -

The first part of Codegen is very simple: we just output the start expression -for the loop value:

- -

-Value *ForExprAST::Codegen() {
-  // Emit the start code first, without 'variable' in scope.
-  Value *StartVal = Start->Codegen();
-  if (StartVal == 0) return 0;
-

- -

With this out of the way, the next step is to set up the LLVM basic block -for the start of the loop body. In the case above, the whole loop body is one -block, but remember that the body code itself could consist of multiple blocks -(e.g. if it contains an if/then/else or a for/in expression).

- -

-  // Make the new basic block for the loop header, inserting after current
-  // block.
-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-  BasicBlock *PreheaderBB = Builder.GetInsertBlock();
-  BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
-  
-  // Insert an explicit fall through from the current block to the LoopBB.
-  Builder.CreateBr(LoopBB);
-

- -

This code is similar to what we saw for if/then/else. Because we will need -it to create the Phi node, we remember the block that falls through into the -loop. Once we have that, we create the actual block that starts the loop and -create an unconditional branch for the fall-through between the two blocks.

- -

-  // Start insertion in LoopBB.
-  Builder.SetInsertPoint(LoopBB);
-  
-  // Start the PHI node with an entry for Start.
-  PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), 2, VarName.c_str());
-  Variable->addIncoming(StartVal, PreheaderBB);
-

- -

Now that the "preheader" for the loop is set up, we switch to emitting code -for the loop body. To begin with, we move the insertion point and create the -PHI node for the loop induction variable. Since we already know the incoming -value for the starting value, we add it to the Phi node. Note that the Phi will -eventually get a second value for the backedge, but we can't set it up yet -(because it doesn't exist!).

- -

-  // Within the loop, the variable is defined equal to the PHI node.  If it
-  // shadows an existing variable, we have to restore it, so save it now.
-  Value *OldVal = NamedValues[VarName];
-  NamedValues[VarName] = Variable;
-  
-  // Emit the body of the loop.  This, like any other expr, can change the
-  // current BB.  Note that we ignore the value computed by the body, but don't
-  // allow an error.
-  if (Body->Codegen() == 0)
-    return 0;
-

- -

Now the code starts to get more interesting. Our 'for' loop introduces a new -variable to the symbol table. This means that our symbol table can now contain -either function arguments or loop variables. To handle this, before we codegen -the body of the loop, we add the loop variable as the current value for its -name. Note that it is possible that there is a variable of the same name in the -outer scope. It would be easy to make this an error (emit an error and return -null if there is already an entry for VarName) but we choose to allow shadowing -of variables. In order to handle this correctly, we remember the Value that -we are potentially shadowing in OldVal (which will be null if there is -no shadowed variable).

- -

Once the loop variable is set into the symbol table, the code recursively -codegen's the body. This allows the body to use the loop variable: any -references to it will naturally find it in the symbol table.

- -

-  // Emit the step value.
-  Value *StepVal;
-  if (Step) {
-    StepVal = Step->Codegen();
-    if (StepVal == 0) return 0;
-  } else {
-    // If not specified, use 1.0.
-    StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
-  }
-  
-  Value *NextVar = Builder.CreateFAdd(Variable, StepVal, "nextvar");
-

- -

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. 'NextVar' -will be the value of the loop variable on the next iteration of the loop.

- -

-  // Compute the end condition.
-  Value *EndCond = End->Codegen();
-  if (EndCond == 0) return EndCond;
-  
-  // Convert condition to a bool by comparing equal to 0.0.
-  EndCond = Builder.CreateFCmpONE(EndCond, 
-                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
-                                  "loopcond");
-

- -

Finally, we evaluate the exit value of the loop, to determine whether the -loop should exit. This mirrors the condition evaluation for the if/then/else -statement.

- -

-  // Create the "after loop" block and insert it.
-  BasicBlock *LoopEndBB = Builder.GetInsertBlock();
-  BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
-  
-  // Insert the conditional branch into the end of LoopEndBB.
-  Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
-  
-  // Any new code will be inserted in AfterBB.
-  Builder.SetInsertPoint(AfterBB);
-

- -

With the code for the body of the loop complete, we just need to finish up -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.

- -

-  // Add a new entry to the PHI node for the backedge.
-  Variable->addIncoming(NextVar, LoopEndBB);
-  
-  // Restore the unshadowed variable.
-  if (OldVal)
-    NamedValues[VarName] = OldVal;
-  else
-    NamedValues.erase(VarName);
-  
-  // for expr always returns 0.0.
-  return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
-}
-

- -

The final code handles various cleanups: now that we have the "NextVar" -value, we can add the incoming value to the loop PHI node. After that, we -remove the loop variable from the symbol table, so that it isn't in scope after -the for loop. Finally, code generation of the for loop always returns 0.0, so -that is what we return from ForExprAST::Codegen.

- -

With this, we conclude the "adding control flow to Kaleidoscope" chapter of -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 -user-defined operators to our poor innocent -language.

- -

- - -

Full Code Listing

- - -

- -

-Here is the complete code listing for our running example, enhanced with the -if/then/else and for expressions.. To build this example, use: -

- -

-# Compile
-clang++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
-# Run
-./toy
-

- -

Here is the code:

- -

-#include "llvm/DerivedTypes.h"
-#include "llvm/ExecutionEngine/ExecutionEngine.h"
-#include "llvm/ExecutionEngine/JIT.h"
-#include "llvm/IRBuilder.h"
-#include "llvm/LLVMContext.h"
-#include "llvm/Module.h"
-#include "llvm/PassManager.h"
-#include "llvm/Analysis/Verifier.h"
-#include "llvm/Analysis/Passes.h"
-#include "llvm/DataLayout.h"
-#include "llvm/Transforms/Scalar.h"
-#include "llvm/Support/TargetSelect.h"
-#include <cstdio>
-#include <string>
-#include <map>
-#include <vector>
-using namespace llvm;
-
-//===----------------------------------------------------------------------===//
-// Lexer
-//===----------------------------------------------------------------------===//
-
-// 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,
-  
-  // control
-  tok_if = -6, tok_then = -7, tok_else = -8,
-  tok_for = -9, tok_in = -10
-};
-
-static std::string IdentifierStr;  // Filled in if tok_identifier
-static double NumVal;              // Filled in if tok_number
-
-/// gettok - Return the next token from standard input.
-static int gettok() {
-  static int LastChar = ' ';
-
-  // Skip any whitespace.
-  while (isspace(LastChar))
-    LastChar = getchar();
-
-  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;
-    if (IdentifierStr == "if") return tok_if;
-    if (IdentifierStr == "then") return tok_then;
-    if (IdentifierStr == "else") return tok_else;
-    if (IdentifierStr == "for") return tok_for;
-    if (IdentifierStr == "in") return tok_in;
-    return tok_identifier;
-  }
-
-  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;
-  }
-
-  if (LastChar == '#') {
-    // Comment until end of line.
-    do LastChar = getchar();
-    while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
-    
-    if (LastChar != EOF)
-      return gettok();
-  }
-  
-  // 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;
-}
-
-//===----------------------------------------------------------------------===//
-// Abstract Syntax Tree (aka Parse Tree)
-//===----------------------------------------------------------------------===//
-
-/// ExprAST - Base class for all expression nodes.
-class ExprAST {
-public:
-  virtual ~ExprAST() {}
-  virtual Value *Codegen() = 0;
-};
-
-/// NumberExprAST - Expression class for numeric literals like "1.0".
-class NumberExprAST : public ExprAST {
-  double Val;
-public:
-  NumberExprAST(double val) : Val(val) {}
-  virtual Value *Codegen();
-};
-
-/// VariableExprAST - Expression class for referencing a variable, like "a".
-class VariableExprAST : public ExprAST {
-  std::string Name;
-public:
-  VariableExprAST(const std::string &name) : Name(name) {}
-  virtual Value *Codegen();
-};
-
-/// BinaryExprAST - Expression class for a binary operator.
-class BinaryExprAST : public ExprAST {
-  char Op;
-  ExprAST *LHS, *RHS;
-public:
-  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
-    : Op(op), LHS(lhs), RHS(rhs) {}
-  virtual Value *Codegen();
-};
-
-/// CallExprAST - Expression class for function calls.
-class CallExprAST : public ExprAST {
-  std::string Callee;
-  std::vector<ExprAST*> Args;
-public:
-  CallExprAST(const std::string &callee, std::vector<ExprAST*> &args)
-    : Callee(callee), Args(args) {}
-  virtual Value *Codegen();
-};
-
-/// IfExprAST - Expression class for if/then/else.
-class IfExprAST : public ExprAST {
-  ExprAST *Cond, *Then, *Else;
-public:
-  IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
-  : Cond(cond), Then(then), Else(_else) {}
-  virtual Value *Codegen();
-};
-
-/// ForExprAST - Expression class for for/in.
-class ForExprAST : public ExprAST {
-  std::string VarName;
-  ExprAST *Start, *End, *Step, *Body;
-public:
-  ForExprAST(const std::string &varname, ExprAST *start, ExprAST *end,
-             ExprAST *step, ExprAST *body)
-    : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
-  virtual Value *Codegen();
-};
-
-/// PrototypeAST - This class represents the "prototype" for a function,
-/// which captures its name, and its argument names (thus implicitly the number
-/// of arguments the function takes).
-class PrototypeAST {
-  std::string Name;
-  std::vector<std::string> Args;
-public:
-  PrototypeAST(const std::string &name, const std::vector<std::string> &args)
-    : Name(name), Args(args) {}
-  
-  Function *Codegen();
-};
-
-/// FunctionAST - This class represents a function definition itself.
-class FunctionAST {
-  PrototypeAST *Proto;
-  ExprAST *Body;
-public:
-  FunctionAST(PrototypeAST *proto, ExprAST *body)
-    : Proto(proto), Body(body) {}
-  
-  Function *Codegen();
-};
-
-//===----------------------------------------------------------------------===//
-// Parser
-//===----------------------------------------------------------------------===//
-
-/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
-/// token the parser is looking at.  getNextToken reads another token from the
-/// lexer and updates CurTok with its results.
-static int CurTok;
-static int getNextToken() {
-  return CurTok = gettok();
-}
-
-/// BinopPrecedence - This holds the precedence for each binary operator that is
-/// defined.
-static std::map<char, int> BinopPrecedence;
-
-/// GetTokPrecedence - Get the precedence of the pending binary operator token.
-static int GetTokPrecedence() {
-  if (!isascii(CurTok))
-    return -1;
-  
-  // Make sure it's a declared binop.
-  int TokPrec = BinopPrecedence[CurTok];
-  if (TokPrec <= 0) return -1;
-  return TokPrec;
-}
-
-/// Error* - These are little helper functions for error handling.
-ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
-PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
-FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
-
-static ExprAST *ParseExpression();
-
-/// identifierexpr
-///   ::= identifier
-///   ::= identifier '(' expression* ')'
-static ExprAST *ParseIdentifierExpr() {
-  std::string IdName = IdentifierStr;
-  
-  getNextToken();  // eat identifier.
-  
-  if (CurTok != '(') // Simple variable ref.
-    return new VariableExprAST(IdName);
-  
-  // Call.
-  getNextToken();  // eat (
-  std::vector<ExprAST*> Args;
-  if (CurTok != ')') {
-    while (1) {
-      ExprAST *Arg = ParseExpression();
-      if (!Arg) return 0;
-      Args.push_back(Arg);
-
-      if (CurTok == ')') break;
-
-      if (CurTok != ',')
-        return Error("Expected ')' or ',' in argument list");
-      getNextToken();
-    }
-  }
-
-  // Eat the ')'.
-  getNextToken();
-  
-  return new CallExprAST(IdName, Args);
-}
-
-/// numberexpr ::= number
-static ExprAST *ParseNumberExpr() {
-  ExprAST *Result = new NumberExprAST(NumVal);
-  getNextToken(); // consume the number
-  return Result;
-}
-
-/// parenexpr ::= '(' expression ')'
-static ExprAST *ParseParenExpr() {
-  getNextToken();  // eat (.
-  ExprAST *V = ParseExpression();
-  if (!V) return 0;
-  
-  if (CurTok != ')')
-    return Error("expected ')'");
-  getNextToken();  // eat ).
-  return V;
-}
-
-/// ifexpr ::= 'if' expression 'then' expression 'else' expression
-static ExprAST *ParseIfExpr() {
-  getNextToken();  // eat the if.
-  
-  // condition.
-  ExprAST *Cond = ParseExpression();
-  if (!Cond) return 0;
-  
-  if (CurTok != tok_then)
-    return Error("expected then");
-  getNextToken();  // eat the then
-  
-  ExprAST *Then = ParseExpression();
-  if (Then == 0) return 0;
-  
-  if (CurTok != tok_else)
-    return Error("expected else");
-  
-  getNextToken();
-  
-  ExprAST *Else = ParseExpression();
-  if (!Else) return 0;
-  
-  return new IfExprAST(Cond, Then, Else);
-}
-
-/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
-static ExprAST *ParseForExpr() {
-  getNextToken();  // eat the for.
-
-  if (CurTok != tok_identifier)
-    return Error("expected identifier after for");
-  
-  std::string IdName = IdentifierStr;
-  getNextToken();  // eat identifier.
-  
-  if (CurTok != '=')
-    return Error("expected '=' after for");
-  getNextToken();  // eat '='.
-  
-  
-  ExprAST *Start = ParseExpression();
-  if (Start == 0) return 0;
-  if (CurTok != ',')
-    return Error("expected ',' after for start value");
-  getNextToken();
-  
-  ExprAST *End = ParseExpression();
-  if (End == 0) return 0;
-  
-  // The step value is optional.
-  ExprAST *Step = 0;
-  if (CurTok == ',') {
-    getNextToken();
-    Step = ParseExpression();
-    if (Step == 0) return 0;
-  }
-  
-  if (CurTok != tok_in)
-    return Error("expected 'in' after for");
-  getNextToken();  // eat 'in'.
-  
-  ExprAST *Body = ParseExpression();
-  if (Body == 0) return 0;
-
-  return new ForExprAST(IdName, Start, End, Step, Body);
-}
-
-/// primary
-///   ::= identifierexpr
-///   ::= numberexpr
-///   ::= parenexpr
-///   ::= ifexpr
-///   ::= forexpr
-static ExprAST *ParsePrimary() {
-  switch (CurTok) {
-  default: return Error("unknown token when expecting an expression");
-  case tok_identifier: return ParseIdentifierExpr();
-  case tok_number:     return ParseNumberExpr();
-  case '(':            return ParseParenExpr();
-  case tok_if:         return ParseIfExpr();
-  case tok_for:        return ParseForExpr();
-  }
-}
-
-/// binoprhs
-///   ::= ('+' primary)*
-static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
-  // If this is a binop, find its precedence.
-  while (1) {
-    int TokPrec = GetTokPrecedence();
-    
-    // If this is a binop that binds at least as tightly as the current binop,
-    // consume it, otherwise we are done.
-    if (TokPrec < ExprPrec)
-      return LHS;
-    
-    // Okay, we know this is a binop.
-    int BinOp = CurTok;
-    getNextToken();  // eat binop
-    
-    // Parse the primary expression after the binary operator.
-    ExprAST *RHS = ParsePrimary();
-    if (!RHS) return 0;
-    
-    // If BinOp binds less tightly with RHS than the operator after RHS, let
-    // the pending operator take RHS as its LHS.
-    int NextPrec = GetTokPrecedence();
-    if (TokPrec < NextPrec) {
-      RHS = ParseBinOpRHS(TokPrec+1, RHS);
-      if (RHS == 0) return 0;
-    }
-    
-    // Merge LHS/RHS.
-    LHS = new BinaryExprAST(BinOp, LHS, RHS);
-  }
-}
-
-/// expression
-///   ::= primary binoprhs
-///
-static ExprAST *ParseExpression() {
-  ExprAST *LHS = ParsePrimary();
-  if (!LHS) return 0;
-  
-  return ParseBinOpRHS(0, LHS);
-}
-
-/// prototype
-///   ::= id '(' id* ')'
-static PrototypeAST *ParsePrototype() {
-  if (CurTok != tok_identifier)
-    return ErrorP("Expected function name in prototype");
-
-  std::string FnName = IdentifierStr;
-  getNextToken();
-  
-  if (CurTok != '(')
-    return ErrorP("Expected '(' in prototype");
-  
-  std::vector<std::string> ArgNames;
-  while (getNextToken() == tok_identifier)
-    ArgNames.push_back(IdentifierStr);
-  if (CurTok != ')')
-    return ErrorP("Expected ')' in prototype");
-  
-  // success.
-  getNextToken();  // eat ')'.
-  
-  return new PrototypeAST(FnName, ArgNames);
-}
-
-/// definition ::= 'def' prototype expression
-static FunctionAST *ParseDefinition() {
-  getNextToken();  // eat def.
-  PrototypeAST *Proto = ParsePrototype();
-  if (Proto == 0) return 0;
-
-  if (ExprAST *E = ParseExpression())
-    return new FunctionAST(Proto, E);
-  return 0;
-}
-
-/// toplevelexpr ::= expression
-static FunctionAST *ParseTopLevelExpr() {
-  if (ExprAST *E = ParseExpression()) {
-    // Make an anonymous proto.
-    PrototypeAST *Proto = new PrototypeAST("", std::vector<std::string>());
-    return new FunctionAST(Proto, E);
-  }
-  return 0;
-}
-
-/// external ::= 'extern' prototype
-static PrototypeAST *ParseExtern() {
-  getNextToken();  // eat extern.
-  return ParsePrototype();
-}
-
-//===----------------------------------------------------------------------===//
-// Code Generation
-//===----------------------------------------------------------------------===//
-
-static Module *TheModule;
-static IRBuilder<> Builder(getGlobalContext());
-static std::map<std::string, Value*> NamedValues;
-static FunctionPassManager *TheFPM;
-
-Value *ErrorV(const char *Str) { Error(Str); return 0; }
-
-Value *NumberExprAST::Codegen() {
-  return ConstantFP::get(getGlobalContext(), APFloat(Val));
-}
-
-Value *VariableExprAST::Codegen() {
-  // Look this variable up in the function.
-  Value *V = NamedValues[Name];
-  return V ? V : ErrorV("Unknown variable name");
-}
-
-Value *BinaryExprAST::Codegen() {
-  Value *L = LHS->Codegen();
-  Value *R = RHS->Codegen();
-  if (L == 0 || R == 0) return 0;
-  
-  switch (Op) {
-  case '+': return Builder.CreateFAdd(L, R, "addtmp");
-  case '-': return Builder.CreateFSub(L, R, "subtmp");
-  case '*': return Builder.CreateFMul(L, R, "multmp");
-  case '<':
-    L = Builder.CreateFCmpULT(L, R, "cmptmp");
-    // Convert bool 0/1 to double 0.0 or 1.0
-    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
-                                "booltmp");
-  default: return ErrorV("invalid binary operator");
-  }
-}
-
-Value *CallExprAST::Codegen() {
-  // Look up the name in the global module table.
-  Function *CalleeF = TheModule->getFunction(Callee);
-  if (CalleeF == 0)
-    return ErrorV("Unknown function referenced");
-  
-  // If argument mismatch error.
-  if (CalleeF->arg_size() != Args.size())
-    return ErrorV("Incorrect # arguments passed");
-
-  std::vector<Value*> ArgsV;
-  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
-    ArgsV.push_back(Args[i]->Codegen());
-    if (ArgsV.back() == 0) return 0;
-  }
-  
-  return Builder.CreateCall(CalleeF, ArgsV, "calltmp");
-}
-
-Value *IfExprAST::Codegen() {
-  Value *CondV = Cond->Codegen();
-  if (CondV == 0) return 0;
-  
-  // Convert condition to a bool by comparing equal to 0.0.
-  CondV = Builder.CreateFCmpONE(CondV, 
-                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
-                                "ifcond");
-  
-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-  
-  // Create blocks for the then and else cases.  Insert the 'then' block at the
-  // end of the function.
-  BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
-  BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
-  BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
-  
-  Builder.CreateCondBr(CondV, ThenBB, ElseBB);
-  
-  // Emit then value.
-  Builder.SetInsertPoint(ThenBB);
-  
-  Value *ThenV = Then->Codegen();
-  if (ThenV == 0) return 0;
-  
-  Builder.CreateBr(MergeBB);
-  // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
-  ThenBB = Builder.GetInsertBlock();
-  
-  // Emit else block.
-  TheFunction->getBasicBlockList().push_back(ElseBB);
-  Builder.SetInsertPoint(ElseBB);
-  
-  Value *ElseV = Else->Codegen();
-  if (ElseV == 0) return 0;
-  
-  Builder.CreateBr(MergeBB);
-  // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
-  ElseBB = Builder.GetInsertBlock();
-  
-  // Emit merge block.
-  TheFunction->getBasicBlockList().push_back(MergeBB);
-  Builder.SetInsertPoint(MergeBB);
-  PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), 2,
-                                  "iftmp");
-  
-  PN->addIncoming(ThenV, ThenBB);
-  PN->addIncoming(ElseV, ElseBB);
-  return PN;
-}
-
-Value *ForExprAST::Codegen() {
-  // Output this as:
-  //   ...
-  //   start = startexpr
-  //   goto loop
-  // loop: 
-  //   variable = phi [start, loopheader], [nextvariable, loopend]
-  //   ...
-  //   bodyexpr
-  //   ...
-  // loopend:
-  //   step = stepexpr
-  //   nextvariable = variable + step
-  //   endcond = endexpr
-  //   br endcond, loop, endloop
-  // outloop:
-  
-  // Emit the start code first, without 'variable' in scope.
-  Value *StartVal = Start->Codegen();
-  if (StartVal == 0) return 0;
-  
-  // Make the new basic block for the loop header, inserting after current
-  // block.
-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-  BasicBlock *PreheaderBB = Builder.GetInsertBlock();
-  BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
-  
-  // Insert an explicit fall through from the current block to the LoopBB.
-  Builder.CreateBr(LoopBB);
-
-  // Start insertion in LoopBB.
-  Builder.SetInsertPoint(LoopBB);
-  
-  // Start the PHI node with an entry for Start.
-  PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), 2, VarName.c_str());
-  Variable->addIncoming(StartVal, PreheaderBB);
-  
-  // Within the loop, the variable is defined equal to the PHI node.  If it
-  // shadows an existing variable, we have to restore it, so save it now.
-  Value *OldVal = NamedValues[VarName];
-  NamedValues[VarName] = Variable;
-  
-  // Emit the body of the loop.  This, like any other expr, can change the
-  // current BB.  Note that we ignore the value computed by the body, but don't
-  // allow an error.
-  if (Body->Codegen() == 0)
-    return 0;
-  
-  // Emit the step value.
-  Value *StepVal;
-  if (Step) {
-    StepVal = Step->Codegen();
-    if (StepVal == 0) return 0;
-  } else {
-    // If not specified, use 1.0.
-    StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
-  }
-  
-  Value *NextVar = Builder.CreateFAdd(Variable, StepVal, "nextvar");
-
-  // Compute the end condition.
-  Value *EndCond = End->Codegen();
-  if (EndCond == 0) return EndCond;
-  
-  // Convert condition to a bool by comparing equal to 0.0.
-  EndCond = Builder.CreateFCmpONE(EndCond, 
-                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
-                                  "loopcond");
-  
-  // Create the "after loop" block and insert it.
-  BasicBlock *LoopEndBB = Builder.GetInsertBlock();
-  BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
-  
-  // Insert the conditional branch into the end of LoopEndBB.
-  Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
-  
-  // Any new code will be inserted in AfterBB.
-  Builder.SetInsertPoint(AfterBB);
-  
-  // Add a new entry to the PHI node for the backedge.
-  Variable->addIncoming(NextVar, LoopEndBB);
-  
-  // Restore the unshadowed variable.
-  if (OldVal)
-    NamedValues[VarName] = OldVal;
-  else
-    NamedValues.erase(VarName);
-
-  
-  // for expr always returns 0.0.
-  return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
-}
-
-Function *PrototypeAST::Codegen() {
-  // Make the function type:  double(double,double) etc.
-  std::vector<Type*> Doubles(Args.size(),
-                             Type::getDoubleTy(getGlobalContext()));
-  FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
-                                       Doubles, false);
-  
-  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.
-  if (F->getName() != Name) {
-    // Delete the one we just made and get the existing one.
-    F->eraseFromParent();
-    F = TheModule->getFunction(Name);
-    
-    // If F already has a body, reject this.
-    if (!F->empty()) {
-      ErrorF("redefinition of function");
-      return 0;
-    }
-    
-    // If F took a different number of args, reject.
-    if (F->arg_size() != Args.size()) {
-      ErrorF("redefinition of function with different # args");
-      return 0;
-    }
-  }
-  
-  // Set names for all arguments.
-  unsigned Idx = 0;
-  for (Function::arg_iterator AI = F->arg_begin(); Idx != Args.size();
-       ++AI, ++Idx) {
-    AI->setName(Args[Idx]);
-    
-    // Add arguments to variable symbol table.
-    NamedValues[Args[Idx]] = AI;
-  }
-  
-  return F;
-}
-
-Function *FunctionAST::Codegen() {
-  NamedValues.clear();
-  
-  Function *TheFunction = Proto->Codegen();
-  if (TheFunction == 0)
-    return 0;
-  
-  // Create a new basic block to start insertion into.
-  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
-  Builder.SetInsertPoint(BB);
-  
-  if (Value *RetVal = Body->Codegen()) {
-    // Finish off the function.
-    Builder.CreateRet(RetVal);
-
-    // Validate the generated code, checking for consistency.
-    verifyFunction(*TheFunction);
-
-    // Optimize the function.
-    TheFPM->run(*TheFunction);
-    
-    return TheFunction;
-  }
-  
-  // Error reading body, remove function.
-  TheFunction->eraseFromParent();
-  return 0;
-}
-
-//===----------------------------------------------------------------------===//
-// Top-Level parsing and JIT Driver
-//===----------------------------------------------------------------------===//
-
-static ExecutionEngine *TheExecutionEngine;
-
-static void HandleDefinition() {
-  if (FunctionAST *F = ParseDefinition()) {
-    if (Function *LF = F->Codegen()) {
-      fprintf(stderr, "Read function definition:");
-      LF->dump();
-    }
-  } else {
-    // Skip token for error recovery.
-    getNextToken();
-  }
-}
-
-static void HandleExtern() {
-  if (PrototypeAST *P = ParseExtern()) {
-    if (Function *F = P->Codegen()) {
-      fprintf(stderr, "Read extern: ");
-      F->dump();
-    }
-  } else {
-    // Skip token for error recovery.
-    getNextToken();
-  }
-}
-
-static void HandleTopLevelExpression() {
-  // Evaluate a top-level expression into an anonymous function.
-  if (FunctionAST *F = ParseTopLevelExpr()) {
-    if (Function *LF = F->Codegen()) {
-      // JIT the function, returning a function pointer.
-      void *FPtr = TheExecutionEngine->getPointerToFunction(LF);
-      
-      // Cast it to the right type (takes no arguments, returns a double) so we
-      // can call it as a native function.
-      double (*FP)() = (double (*)())(intptr_t)FPtr;
-      fprintf(stderr, "Evaluated to %f\n", FP());
-    }
-  } else {
-    // Skip token for error recovery.
-    getNextToken();
-  }
-}
-
-/// top ::= definition | external | expression | ';'
-static void MainLoop() {
-  while (1) {
-    fprintf(stderr, "ready> ");
-    switch (CurTok) {
-    case tok_eof:    return;
-    case ';':        getNextToken(); break;  // ignore top-level semicolons.
-    case tok_def:    HandleDefinition(); break;
-    case tok_extern: HandleExtern(); break;
-    default:         HandleTopLevelExpression(); break;
-    }
-  }
-}
-
-//===----------------------------------------------------------------------===//
-// "Library" functions that can be "extern'd" from user code.
-//===----------------------------------------------------------------------===//
-
-/// putchard - putchar that takes a double and returns 0.
-extern "C" 
-double putchard(double X) {
-  putchar((char)X);
-  return 0;
-}
-
-//===----------------------------------------------------------------------===//
-// Main driver code.
-//===----------------------------------------------------------------------===//
-
-int main() {
-  InitializeNativeTarget();
-  LLVMContext &Context = getGlobalContext();
-
-  // Install standard binary operators.
-  // 1 is lowest precedence.
-  BinopPrecedence['<'] = 10;
-  BinopPrecedence['+'] = 20;
-  BinopPrecedence['-'] = 20;
-  BinopPrecedence['*'] = 40;  // highest.
-
-  // Prime the first token.
-  fprintf(stderr, "ready> ");
-  getNextToken();
-
-  // Make the module, which holds all the code.
-  TheModule = new Module("my cool jit", Context);
-
-  // Create the JIT.  This takes ownership of the module.
-  std::string ErrStr;
-  TheExecutionEngine = EngineBuilder(TheModule).setErrorStr(&ErrStr).create();
-  if (!TheExecutionEngine) {
-    fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str());
-    exit(1);
-  }
-
-  FunctionPassManager OurFPM(TheModule);
-
-  // Set up the optimizer pipeline.  Start with registering info about how the
-  // target lays out data structures.
-  OurFPM.add(new DataLayout(*TheExecutionEngine->getDataLayout()));
-  // Provide basic AliasAnalysis support for GVN.
-  OurFPM.add(createBasicAliasAnalysisPass());
-  // Do simple "peephole" optimizations and bit-twiddling optzns.
-  OurFPM.add(createInstructionCombiningPass());
-  // Reassociate expressions.
-  OurFPM.add(createReassociatePass());
-  // Eliminate Common SubExpressions.
-  OurFPM.add(createGVNPass());
-  // Simplify the control flow graph (deleting unreachable blocks, etc).
-  OurFPM.add(createCFGSimplificationPass());
-
-  OurFPM.doInitialization();
-
-  // Set the global so the code gen can use this.
-  TheFPM = &OurFPM;
-
-  // Run the main "interpreter loop" now.
-  MainLoop();
-
-  TheFPM = 0;
-
-  // Print out all of the generated code.
-  TheModule->dump();
-
-  return 0;
-}
-

- -Next: Extending the language: user-defined operators -

- - -

- - Chris Lattner
- The LLVM Compiler Infrastructure
- Last modified: $Date: 2012-10-08 18:39:34 +0200 (Mon, 08 Oct 2012) $ -

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