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add the code for expression code that we'll add, though most of the

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llvm-svn: 43547
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Chris Lattner 2007-10-31 06:47:39 +00:00
parent e94d86d349
commit b2b616f4d5
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docs/tutorial/LangImpl5.html
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@ -473,7 +473,271 @@ another useful expression that is familiar from non-functional languages...</p>
<div class="doc_text">
<p>...</p>
<p>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:</p>
<div class="doc_code">
<pre>
# print 100 '*' (ascii 42) characters
extern putchard(char)
for x = 1, x &lt; 100, 1.0 in putchard(42);
</pre>
</div>
<p>This expression defines a new variable ("x" in this case) which iterates from
a starting value, while the condition ("x &lt; 100" 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.</p>
<p>As before, lets talk about the changes that we need to Kaleidoscope to
support this.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forlexer">Lexer Extensions for
the 'for' Loop</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>The lexer extensions are the same sort of thing as for if/then/else:</p>
<div class="doc_code">
<pre>
... in enum Token ...
// control
tok_if = -6, tok_then = -7, tok_else = -8,
<b> tok_for = -9, tok_in = -10</b>
... 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;
<b>if (IdentifierStr == "for") return tok_for;
if (IdentifierStr == "in") return tok_in;</b>
return tok_identifier;
</pre>
</div>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forast">AST Extensions for
the 'for' Loop</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>The AST node is similarly simple. It basically boils down to capturing
the variable name and the consituent expressions in the node.</p>
<div class="doc_code">
<pre>
/// ForExprAST - Expression class for for/in.
class ForExprAST : public ExprAST {
std::string VarName;
ExprAST *Start, *End, *Step, *Body;
public:
ForExprAST(const std::string &amp;varname, ExprAST *start, ExprAST *end,
ExprAST *step, ExprAST *body)
: VarName(varname), Start(start), End(end), Step(step), Body(body) {}
virtual Value *Codegen();
};
</pre>
</div>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forparser">Parser Extensions for
the 'for' Loop</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>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:</p>
<div class="doc_code">
<pre>
/// forexpr ::= 'for' identifer '=' 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 identifer.
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);
}
</pre>
</div>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forir">LLVM IR for
the 'for' Loop</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>Now we get to the good part: the LLVM IR we want to generate for this thing.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="forcodegen">Code Generation for
the 'for' Loop</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<div class="doc_code">
<pre>
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-&gt;Codegen();
if (StartVal == 0) return 0;
// Make the new basic block for the loop header, inserting after current
// block.
Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
BasicBlock *PreheaderBB = Builder.GetInsertBlock();
BasicBlock *LoopBB = new BasicBlock("loop", TheFunction);
// Insert an explicit fall through from the current block to the LoopBB.
// Start insertion in LoopBB.
Builder.CreateBr(LoopBB);
Builder.SetInsertPoint(LoopBB);
// Start the PHI node with an entry for Start.
PHINode *Variable = Builder.CreatePHI(Type::DoubleTy, VarName.c_str());
Variable-&gt;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-&gt;Codegen() == 0)
return 0;
// Emit the step value.
Value *StepVal;
if (Step) {
StepVal = Step-&gt;Codegen();
if (StepVal == 0) return 0;
} else {
// If not specified, use 1.0.
StepVal = ConstantFP::get(Type::DoubleTy, APFloat(1.0));
}
Value *NextVar = Builder.CreateAdd(Variable, StepVal, "nextvar");
// When evaluating the end condition, the value of the variable is the
// incremented value.
NamedValues[VarName] = Variable;
// Compute the end condition.
Value *EndCond = End-&gt;Codegen();
if (EndCond == 0) return EndCond;
// Convert condition to a bool by comparing equal to 0.0.
EndCond = Builder.CreateFCmpONE(EndCond,
ConstantFP::get(Type::DoubleTy, APFloat(0.0)),
"loopcond");
// Create the "after loop" block and insert it.
BasicBlock *LoopEndBB = Builder.GetInsertBlock();
BasicBlock *AfterBB = new BasicBlock("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-&gt;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::DoubleTy);
}
</pre>
</div>
</div>