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1778 lines
57 KiB
HTML
1778 lines
57 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>Kaleidoscope: Extending the Language: Control Flow</title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="author" content="Chris Lattner">
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<link rel="stylesheet" href="../llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">Kaleidoscope: Extending the Language: Control Flow</div>
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<ul>
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<li><a href="index.html">Up to Tutorial Index</a></li>
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<li>Chapter 5
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<ol>
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<li><a href="#intro">Chapter 5 Introduction</a></li>
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<li><a href="#ifthen">If/Then/Else</a>
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<ol>
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<li><a href="#iflexer">Lexer Extensions</a></li>
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<li><a href="#ifast">AST Extensions</a></li>
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<li><a href="#ifparser">Parser Extensions</a></li>
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<li><a href="#ifir">LLVM IR</a></li>
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<li><a href="#ifcodegen">Code Generation</a></li>
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</ol>
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</li>
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<li><a href="#for">'for' Loop Expression</a>
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<ol>
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<li><a href="#forlexer">Lexer Extensions</a></li>
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<li><a href="#forast">AST Extensions</a></li>
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<li><a href="#forparser">Parser Extensions</a></li>
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<li><a href="#forir">LLVM IR</a></li>
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<li><a href="#forcodegen">Code Generation</a></li>
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</ol>
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</li>
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<li><a href="#code">Full Code Listing</a></li>
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</ol>
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</li>
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<li><a href="LangImpl6.html">Chapter 6</a>: Extending the Language:
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User-defined Operators</li>
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</ul>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="intro">Chapter 5 Introduction</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Welcome to Chapter 5 of the "<a href="index.html">Implementing a language
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with LLVM</a>" tutorial. Parts 1-4 described the implementation of the simple
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Kaleidoscope language and included support for generating LLVM IR, followed by
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optimizations and a JIT compiler. Unfortunately, as presented, Kaleidoscope is
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mostly useless: it has no control flow other than call and return. This means
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that you can't have conditional branches in the code, significantly limiting its
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power. In this episode of "build that compiler", we'll extend Kaleidoscope to
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have an if/then/else expression plus a simple 'for' loop.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="ifthen">If/Then/Else</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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Extending Kaleidoscope to support if/then/else is quite straightforward. It
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basically requires adding lexer support for this "new" concept to the lexer,
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parser, AST, and LLVM code emitter. This example is nice, because it shows how
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easy it is to "grow" a language over time, incrementally extending it as new
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ideas are discovered.</p>
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<p>Before we get going on "how" we add this extension, lets talk about "what" we
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want. The basic idea is that we want to be able to write this sort of thing:
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</p>
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<div class="doc_code">
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<pre>
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def fib(x)
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if x < 3 then
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1
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else
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fib(x-1)+fib(x-2);
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</pre>
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</div>
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<p>In Kaleidoscope, every construct is an expression: there are no statements.
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As such, the if/then/else expression needs to return a value like any other.
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Since we're using a mostly functional form, we'll have it evaluate its
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conditional, then return the 'then' or 'else' value based on how the condition
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was resolved. This is very similar to the C "?:" expression.</p>
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<p>The semantics of the if/then/else expression is that it evaluates the
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condition to a boolean equality value: 0.0 is considered to be false and
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everything else is considered to be true.
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If the condition is true, the first subexpression is evaluated and returned, if
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the condition is false, the second subexpression is evaluated and returned.
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Since Kaleidoscope allows side-effects, this behavior is important to nail down.
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</p>
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<p>Now that we know what we "want", lets break this down into its constituent
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pieces.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsubsection"><a name="iflexer">Lexer Extensions for
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If/Then/Else</a></div>
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<!-- ======================================================================= -->
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<div class="doc_text">
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<p>The lexer extensions are straightforward. First we add new enum values
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for the relevant tokens:</p>
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<div class="doc_code">
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<pre>
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// control
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tok_if = -6, tok_then = -7, tok_else = -8,
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</pre>
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</div>
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<p>Once we have that, we recognize the new keywords in the lexer. This is pretty simple
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stuff:</p>
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<div class="doc_code">
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<pre>
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...
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if (IdentifierStr == "def") return tok_def;
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if (IdentifierStr == "extern") return tok_extern;
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<b>if (IdentifierStr == "if") return tok_if;
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if (IdentifierStr == "then") return tok_then;
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if (IdentifierStr == "else") return tok_else;</b>
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return tok_identifier;
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</pre>
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</div>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsubsection"><a name="ifast">AST Extensions for
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If/Then/Else</a></div>
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<!-- ======================================================================= -->
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<div class="doc_text">
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<p>To represent the new expression we add a new AST node for it:</p>
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<div class="doc_code">
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<pre>
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/// IfExprAST - Expression class for if/then/else.
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class IfExprAST : public ExprAST {
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ExprAST *Cond, *Then, *Else;
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public:
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IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
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: Cond(cond), Then(then), Else(_else) {}
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virtual Value *Codegen();
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};
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</pre>
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</div>
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<p>The AST node just has pointers to the various subexpressions.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsubsection"><a name="ifparser">Parser Extensions for
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If/Then/Else</a></div>
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<!-- ======================================================================= -->
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<div class="doc_text">
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<p>Now that we have the relevant tokens coming from the lexer and we have the
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AST node to build, our parsing logic is relatively straightforward. First we
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define a new parsing function:</p>
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<div class="doc_code">
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<pre>
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/// ifexpr ::= 'if' expression 'then' expression 'else' expression
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static ExprAST *ParseIfExpr() {
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getNextToken(); // eat the if.
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// condition.
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ExprAST *Cond = ParseExpression();
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if (!Cond) return 0;
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if (CurTok != tok_then)
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return Error("expected then");
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getNextToken(); // eat the then
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ExprAST *Then = ParseExpression();
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if (Then == 0) return 0;
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if (CurTok != tok_else)
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return Error("expected else");
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getNextToken();
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ExprAST *Else = ParseExpression();
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if (!Else) return 0;
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return new IfExprAST(Cond, Then, Else);
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}
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</pre>
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</div>
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<p>Next we hook it up as a primary expression:</p>
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<div class="doc_code">
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<pre>
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static ExprAST *ParsePrimary() {
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switch (CurTok) {
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default: return Error("unknown token when expecting an expression");
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case tok_identifier: return ParseIdentifierExpr();
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case tok_number: return ParseNumberExpr();
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case '(': return ParseParenExpr();
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<b>case tok_if: return ParseIfExpr();</b>
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}
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}
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</pre>
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</div>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsubsection"><a name="ifir">LLVM IR for If/Then/Else</a></div>
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<!-- ======================================================================= -->
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<div class="doc_text">
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<p>Now that we have it parsing and building the AST, the final piece is adding
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LLVM code generation support. This is the most interesting part of the
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if/then/else example, because this is where it starts to introduce new concepts.
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All of the code above has been thoroughly described in previous chapters.
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</p>
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<p>To motivate the code we want to produce, lets take a look at a simple
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example. Consider:</p>
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<div class="doc_code">
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<pre>
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extern foo();
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extern bar();
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def baz(x) if x then foo() else bar();
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</pre>
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</div>
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<p>If you disable optimizations, the code you'll (soon) get from Kaleidoscope
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looks like this:</p>
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<div class="doc_code">
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<pre>
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declare double @foo()
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declare double @bar()
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define double @baz(double %x) {
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entry:
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%ifcond = fcmp one double %x, 0.000000e+00
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br i1 %ifcond, label %then, label %else
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then: ; preds = %entry
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%calltmp = call double @foo()
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br label %ifcont
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else: ; preds = %entry
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%calltmp1 = call double @bar()
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br label %ifcont
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ifcont: ; preds = %else, %then
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%iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ]
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ret double %iftmp
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}
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</pre>
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</div>
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<p>To visualize the control flow graph, you can use a nifty feature of the LLVM
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'<a href="http://llvm.org/cmds/opt.html">opt</a>' tool. If you put this LLVM IR
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into "t.ll" and run "<tt>llvm-as < t.ll | opt -analyze -view-cfg</tt>", <a
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href="../ProgrammersManual.html#ViewGraph">a window will pop up</a> and you'll
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see this graph:</p>
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<div style="text-align: center"><img src="LangImpl5-cfg.png" alt="Example CFG" width="423"
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height="315"></div>
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<p>Another way to get this is to call "<tt>F->viewCFG()</tt>" or
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"<tt>F->viewCFGOnly()</tt>" (where F is a "<tt>Function*</tt>") either by
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inserting actual calls into the code and recompiling or by calling these in the
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debugger. LLVM has many nice features for visualizing various graphs.</p>
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<p>Getting back to the generated code, it is fairly simple: the entry block
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evaluates the conditional expression ("x" in our case here) and compares the
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result to 0.0 with the "<tt><a href="../LangRef.html#i_fcmp">fcmp</a> one</tt>"
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instruction ('one' is "Ordered and Not Equal"). Based on the result of this
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expression, the code jumps to either the "then" or "else" blocks, which contain
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the expressions for the true/false cases.</p>
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<p>Once the then/else blocks are finished executing, they both branch back to the
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'ifcont' block to execute the code that happens after the if/then/else. In this
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case the only thing left to do is to return to the caller of the function. The
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question then becomes: how does the code know which expression to return?</p>
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<p>The answer to this question involves an important SSA operation: the
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<a href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Phi
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operation</a>. If you're not familiar with SSA, <a
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href="http://en.wikipedia.org/wiki/Static_single_assignment_form">the wikipedia
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article</a> is a good introduction and there are various other introductions to
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it available on your favorite search engine. The short version is that
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"execution" of the Phi operation requires "remembering" which block control came
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from. The Phi operation takes on the value corresponding to the input control
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block. In this case, if control comes in from the "then" block, it gets the
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value of "calltmp". If control comes from the "else" block, it gets the value
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of "calltmp1".</p>
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<p>At this point, you are probably starting to think "Oh no! This means my
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simple and elegant front-end will have to start generating SSA form in order to
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use LLVM!". Fortunately, this is not the case, and we strongly advise
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<em>not</em> implementing an SSA construction algorithm in your front-end
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unless there is an amazingly good reason to do so. In practice, there are two
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sorts of values that float around in code written for your average imperative
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programming language that might need Phi nodes:</p>
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<ol>
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<li>Code that involves user variables: <tt>x = 1; x = x + 1; </tt></li>
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<li>Values that are implicit in the structure of your AST, such as the Phi node
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in this case.</li>
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</ol>
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<p>In <a href="LangImpl7.html">Chapter 7</a> of this tutorial ("mutable
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variables"), we'll talk about #1
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in depth. For now, just believe me that you don't need SSA construction to
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handle this case. For #2, you have the choice of using the techniques that we will
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describe for #1, or you can insert Phi nodes directly, if convenient. In this
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case, it is really really easy to generate the Phi node, so we choose to do it
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directly.</p>
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<p>Okay, enough of the motivation and overview, lets generate code!</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsubsection"><a name="ifcodegen">Code Generation for
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If/Then/Else</a></div>
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<!-- ======================================================================= -->
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<div class="doc_text">
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<p>In order to generate code for this, we implement the <tt>Codegen</tt> method
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for <tt>IfExprAST</tt>:</p>
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<div class="doc_code">
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<pre>
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Value *IfExprAST::Codegen() {
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Value *CondV = Cond->Codegen();
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if (CondV == 0) return 0;
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// Convert condition to a bool by comparing equal to 0.0.
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CondV = Builder.CreateFCmpONE(CondV,
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ConstantFP::get(getGlobalContext(), APFloat(0.0)),
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"ifcond");
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</pre>
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</div>
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<p>This code is straightforward and similar to what we saw before. We emit the
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expression for the condition, then compare that value to zero to get a truth
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value as a 1-bit (bool) value.</p>
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<div class="doc_code">
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<pre>
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Function *TheFunction = Builder.GetInsertBlock()->getParent();
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// Create blocks for the then and else cases. Insert the 'then' block at the
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// end of the function.
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BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
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BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
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BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
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Builder.CreateCondBr(CondV, ThenBB, ElseBB);
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</pre>
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</div>
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<p>This code creates the basic blocks that are related to the if/then/else
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statement, and correspond directly to the blocks in the example above. The
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first line gets the current Function object that is being built. It
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gets this by asking the builder for the current BasicBlock, and asking that
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block for its "parent" (the function it is currently embedded into).</p>
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<p>Once it has that, it creates three blocks. Note that it passes "TheFunction"
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into the constructor for the "then" block. This causes the constructor to
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automatically insert the new block into the end of the specified function. The
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other two blocks are created, but aren't yet inserted into the function.</p>
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<p>Once the blocks are created, we can emit the conditional branch that chooses
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between them. Note that creating new blocks does not implicitly affect the
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IRBuilder, so it is still inserting into the block that the condition
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went into. Also note that it is creating a branch to the "then" block and the
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"else" block, even though the "else" block isn't inserted into the function yet.
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This is all ok: it is the standard way that LLVM supports forward
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references.</p>
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<div class="doc_code">
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<pre>
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// Emit then value.
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Builder.SetInsertPoint(ThenBB);
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Value *ThenV = Then->Codegen();
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if (ThenV == 0) return 0;
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Builder.CreateBr(MergeBB);
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// Codegen of 'Then' can change the current block, update ThenBB for the PHI.
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ThenBB = Builder.GetInsertBlock();
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</pre>
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</div>
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<p>After the conditional branch is inserted, we move the builder to start
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inserting into the "then" block. Strictly speaking, this call moves the
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insertion point to be at the end of the specified block. However, since the
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"then" block is empty, it also starts out by inserting at the beginning of the
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block. :)</p>
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<p>Once the insertion point is set, we recursively codegen the "then" expression
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from the AST. To finish off the "then" block, we create an unconditional branch
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to the merge block. One interesting (and very important) aspect of the LLVM IR
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|
is that it <a href="../LangRef.html#functionstructure">requires all basic blocks
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to be "terminated"</a> with a <a href="../LangRef.html#terminators">control flow
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instruction</a> such as return or branch. This means that all control flow,
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<em>including fall throughs</em> must be made explicit in the LLVM IR. If you
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violate this rule, the verifier will emit an error.</p>
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<p>The final line here is quite subtle, but is very important. The basic issue
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is that when we create the Phi node in the merge block, we need to set up the
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block/value pairs that indicate how the Phi will work. Importantly, the Phi
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node expects to have an entry for each predecessor of the block in the CFG. Why
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|
then, are we getting the current block when we just set it to ThenBB 5 lines
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above? The problem is that the "Then" expression may actually itself change the
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block that the Builder is emitting into if, for example, it contains a nested
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|
"if/then/else" expression. Because calling Codegen recursively could
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arbitrarily change the notion of the current block, we are required to get an
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up-to-date value for code that will set up the Phi node.</p>
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<div class="doc_code">
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<pre>
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// Emit else block.
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TheFunction->getBasicBlockList().push_back(ElseBB);
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Builder.SetInsertPoint(ElseBB);
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Value *ElseV = Else->Codegen();
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if (ElseV == 0) return 0;
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Builder.CreateBr(MergeBB);
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// Codegen of 'Else' can change the current block, update ElseBB for the PHI.
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ElseBB = Builder.GetInsertBlock();
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</pre>
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</div>
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<p>Code generation for the 'else' block is basically identical to codegen for
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the 'then' block. The only significant difference is the first line, which adds
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the 'else' block to the function. Recall previously that the 'else' block was
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created, but not added to the function. Now that the 'then' and 'else' blocks
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are emitted, we can finish up with the merge code:</p>
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<div class="doc_code">
|
|
<pre>
|
|
// Emit merge block.
|
|
TheFunction->getBasicBlockList().push_back(MergeBB);
|
|
Builder.SetInsertPoint(MergeBB);
|
|
PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()),
|
|
"iftmp");
|
|
|
|
PN->addIncoming(ThenV, ThenBB);
|
|
PN->addIncoming(ElseV, ElseBB);
|
|
return PN;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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.</p>
|
|
|
|
<p>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.</p>
|
|
|
|
<p>Overall, we now have the ability to execute conditional code in
|
|
Kaleidoscope. With this extension, Kaleidoscope is a fairly complete language
|
|
that can calculate a wide variety of numeric functions. Next up we'll add
|
|
another useful expression that is familiar from non-functional languages...</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="for">'for' Loop Expression</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<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>
|
|
extern putchard(char)
|
|
def printstar(n)
|
|
for i = 1, i < n, 1.0 in
|
|
putchard(42); # ascii 42 = '*'
|
|
|
|
# print 100 '*' characters
|
|
printstar(100);
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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.</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 just as simple. It basically boils down to capturing
|
|
the variable name and the constituent 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 &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' 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);
|
|
}
|
|
</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.
|
|
With the simple example above, we get this LLVM IR (note that this dump is
|
|
generated with optimizations disabled for clarity):
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
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
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection"><a name="forcodegen">Code Generation for
|
|
the 'for' Loop</a></div>
|
|
<!-- ======================================================================= -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The first part of Codegen is very simple: we just output the start expression
|
|
for the loop value:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
Value *ForExprAST::Codegen() {
|
|
// Emit the start code first, without 'variable' in scope.
|
|
Value *StartVal = Start->Codegen();
|
|
if (StartVal == 0) return 0;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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).</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// 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);
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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.</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// Start insertion in LoopBB.
|
|
Builder.SetInsertPoint(LoopBB);
|
|
|
|
// Start the PHI node with an entry for Start.
|
|
PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), VarName.c_str());
|
|
Variable->addIncoming(StartVal, PreheaderBB);
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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!).</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// 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;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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 <tt>OldVal</tt> (which will be null if there is
|
|
no shadowed variable).</p>
|
|
|
|
<p>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.</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// 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");
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Now that the body is emitted, we compute the next value of the iteration
|
|
variable by adding the step value, or 1.0 if it isn't present. '<tt>NextVar</tt>'
|
|
will be the value of the loop variable on the next iteration of the loop.</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// 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");
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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.</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// 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);
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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.</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// 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()));
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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 <tt>ForExprAST::Codegen</tt>.</p>
|
|
|
|
<p>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
|
|
<a href="LangImpl6.html">user-defined operators</a> to our poor innocent
|
|
language.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="code">Full Code Listing</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
Here is the complete code listing for our running example, enhanced with the
|
|
if/then/else and for expressions.. To build this example, use:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
# Compile
|
|
g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
|
|
# Run
|
|
./toy
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Here is the code:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
#include "llvm/DerivedTypes.h"
|
|
#include "llvm/ExecutionEngine/ExecutionEngine.h"
|
|
#include "llvm/ExecutionEngine/JIT.h"
|
|
#include "llvm/LLVMContext.h"
|
|
#include "llvm/Module.h"
|
|
#include "llvm/PassManager.h"
|
|
#include "llvm/Analysis/Verifier.h"
|
|
#include "llvm/Target/TargetData.h"
|
|
#include "llvm/Target/TargetSelect.h"
|
|
#include "llvm/Transforms/Scalar.h"
|
|
#include "llvm/Support/IRBuilder.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.begin(), ArgsV.end(), "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()),
|
|
"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()), 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<const 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 TargetData(*TheExecutionEngine->getTargetData()));
|
|
// 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;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<a href="LangImpl6.html">Next: Extending the language: user-defined operators</a>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<address>
|
|
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src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
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|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
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|
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