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...
