228 lines
7.9 KiB
HTML
228 lines
7.9 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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<title>Kaleidoscope: Extending the Language: Operator Overloading</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: Operator Overloading</div>
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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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<div class="doc_section"><a name="intro">Part 6 Introduction</a></div>
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<div class="doc_text">
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<p>Welcome to Part 6 of the "<a href="index.html">Implementing a language with
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LLVM</a>" tutorial. At this point in our tutorial, we now have a fully
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functional language that is fairly minimal, but also useful. One big problem
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with it though is that it doesn't have many useful operators (like division,
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logical negation, or even any comparisons other than less-than.</p>
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<p>This chapter of the tutorial takes a wild digression into adding operator
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overloading to the simple and beautiful Kaleidoscope language, giving us a
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simple and ugly language in some ways, but also a powerful one at the same time.
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One of the great things about creating your own language is that you get to
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decide what is good or bad. In this tutorial we'll assume that it is okay and
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use this as a way to show some interesting parsing techniques.</p>
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</div>
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<div class="doc_section"><a name="idea">Operator Overloading: the Idea</a></div>
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<div class="doc_text">
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<p>
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The operator overloading that we will add to Kaleidoscope is more general than
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languages like C++. In C++, you are only allowed to redefine existing
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operators: you can't programatically change the grammar, introduce new
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operators, change precedence levels, etc. In this chapter, we will add this
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capability to Kaleidoscope, which will allow us to round out the set of
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operators that are supported, culminating in a more interesting example app.</p>
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<p>The point of going into operator overloading in a tutorial like this is to
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show the power and flexibility of using a hand-written parser. The parser we
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are using so far is using recursive descent for most parts of the grammar, and
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operator precedence parsing for the expressions. See <a
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href="LangImpl2.html">Chapter 2</a> for details. Without using operator
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precedence parsing, it would be very difficult to allow the programmer to
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introduce new operators into the grammar: the grammar is dynamically extensible
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as the JIT runs.</p>
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<p>The two specific features we'll add are programmable unary operators (right
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now, Kaleidoscope has no unary operators at all) as well as binary operators.
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An example of this is:</p>
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<div class="doc_code">
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<pre>
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# Logical unary not.
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def unary!(v)
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if v then
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0
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else
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1;
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# Define > with the same precedence as <.
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def binary> 10 (LHS RHS)
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!(LHS < RHS); # alternatively, could just use "RHS < LHS"
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# Binary "logical or", (note that it does not "short circuit")
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def binary| 5 (LHS RHS)
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if LHS then
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1
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else if RHS then
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1
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else
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0;
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# Define = with slightly lower precedence than relationals.
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def binary= 9 (LHS RHS)
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!(LHS < RHS | LHS > RHS);
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</pre>
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</div>
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<p>Many languages aspire to being able to implement their standard runtime
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library in the language itself. In Kaleidoscope, we can implement significant
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parts of the language in the library!</p>
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<p>We will break down implementation of these features into two parts:
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implementing support for overloading of binary operators and adding unary
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operators.</p>
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</div>
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<div class="doc_section"><a name="binary">Overloading Binary Operators</a></div>
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<div class="doc_text">
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<p>Adding support for overloaded binary operators is pretty simple with our
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current framework. We'll first add support for the unary/binary keywords:</p>
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<div class="doc_code">
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<pre>
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enum Token {
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...
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<b>// operators
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tok_binary = -11, tok_unary = -12</b>
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};
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...
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static int gettok() {
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...
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if (IdentifierStr == "for") return tok_for;
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if (IdentifierStr == "in") return tok_in;
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<b>if (IdentifierStr == "binary") return tok_binary;
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if (IdentifierStr == "unary") return tok_unary;</b>
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return tok_identifier;
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</pre>
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</div>
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<p>This just adds lexer support for the unary and binary keywords, like we
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did in <a href="LangImpl5.html#iflexer">previous chapters</a>. One nice thing
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about our current AST is that we represent binary operators fully generally
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with their ASCII code as the opcode. For our extended operators, we'll use the
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same representation, so we don't need any new AST or parser support.</p>
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<p>On the other hand, we have to be able to represent the definitions of these
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new operators, in the "def binary| 5" part of the function definition. In the
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grammar so far, the "name" for the function definition is parsed as the
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"prototype" production and into the <tt>PrototypeAST</tt> AST node. To
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represent our new user-defined operators as prototypes, we have to extend
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the <tt>PrototypeAST</tt> AST node like this:</p>
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<div class="doc_code">
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<pre>
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/// PrototypeAST - This class represents the "prototype" for a function,
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/// which captures its argument names as well as if it is an operator.
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class PrototypeAST {
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std::string Name;
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std::vector<std::string> Args;
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<b>bool isOperator;
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unsigned Precedence; // Precedence if a binary op.</b>
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public:
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PrototypeAST(const std::string &name, const std::vector<std::string> &args,
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<b>bool isoperator = false, unsigned prec = 0</b>)
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: Name(name), Args(args), <b>isOperator(isoperator), Precedence(prec)</b> {}
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<b>bool isUnaryOp() const { return isOperator && Args.size() == 1; }
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bool isBinaryOp() const { return isOperator && Args.size() == 2; }
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char getOperatorName() const {
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assert(isUnaryOp() || isBinaryOp());
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return Name[Name.size()-1];
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}
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unsigned getBinaryPrecedence() const { return Precedence; }</b>
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Function *Codegen();
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};
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</pre>
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</div>
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<p>Basically, in addition to knowing a name for the prototype, we now keep track
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of whether it was an operator, and if it was, what precedence level the operator
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is at. The precedence is only used for binary operators.</p>
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<p>...</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="code">Full Code Listing</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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Here is the complete code listing for our running example, enhanced with the
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if/then/else and for expressions.. To build this example, use:
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</p>
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<div class="doc_code">
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<pre>
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# Compile
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g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
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# Run
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./toy
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</pre>
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</div>
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<p>Here is the code:</p>
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<div class="doc_code">
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<pre>
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</pre>
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</div>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
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</address>
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</body>
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</html>
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