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349 lines
14 KiB
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<head>
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<title>Kaleidoscope: Tutorial Introduction and the Lexer</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: Tutorial Introduction and the Lexer</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 1
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<ol>
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<li><a href="#intro">Tutorial Introduction</a></li>
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<li><a href="#language">The Basic Language</a></li>
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<li><a href="#lexer">The Lexer</a></li>
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</ol>
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</li>
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<li><a href="LangImpl2.html">Chapter 2</a>: Implementing a Parser and AST</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">Tutorial Introduction</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
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runs through the implementation of a simple language, showing how fun and
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easy it can be. This tutorial will get you up and started as well as help to
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build a framework you can extend to other languages. The code in this tutorial
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can also be used as a playground to hack on other LLVM specific things.
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</p>
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<p>
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The goal of this tutorial is to progressively unveil our language, describing
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how it is built up over time. This will let us cover a fairly broad range of
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language design and LLVM-specific usage issues, showing and explaining the code
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for it all along the way, without overwhelming you with tons of details up
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front.</p>
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<p>It is useful to point out ahead of time that this tutorial is really about
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teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
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modern and sane software engineering principles. In practice, this means that
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we'll take a number of shortcuts to simplify the exposition. For example, the
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code leaks memory, uses global variables all over the place, doesn't use nice
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design patterns like <a
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href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
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is very simple. If you dig in and use the code as a basis for future projects,
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fixing these deficiencies shouldn't be hard.</p>
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<p>I've tried to put this tutorial together in a way that makes chapters easy to
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skip over if you are already familiar with or are uninterested in the various
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pieces. The structure of the tutorial is:
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</p>
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<ul>
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<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
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language, and the definition of its Lexer</b> - This shows where we are going
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and the basic functionality that we want it to do. In order to make this
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tutorial maximally understandable and hackable, we choose to implement
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everything in C++ instead of using lexer and parser generators. LLVM obviously
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works just fine with such tools, feel free to use one if you prefer.</li>
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<li><b><a href="LangImpl2.html">Chapter #2</a>: Implementing a Parser and
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AST</b> - With the lexer in place, we can talk about parsing techniques and
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basic AST construction. This tutorial describes recursive descent parsing and
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operator precedence parsing. Nothing in Chapters 1 or 2 is LLVM-specific,
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the code doesn't even link in LLVM at this point. :)</li>
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<li><b><a href="LangImpl3.html">Chapter #3</a>: Code generation to LLVM IR</b> -
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With the AST ready, we can show off how easy generation of LLVM IR really
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is.</li>
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<li><b><a href="LangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
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Support</b> - Because a lot of people are interested in using LLVM as a JIT,
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we'll dive right into it and show you the 3 lines it takes to add JIT support.
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LLVM is also useful in many other ways, but this is one simple and "sexy" way
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to shows off its power. :)</li>
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<li><b><a href="LangImpl5.html">Chapter #5</a>: Extending the Language: Control
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Flow</b> - With the language up and running, we show how to extend it with
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control flow operations (if/then/else and a 'for' loop). This gives us a chance
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to talk about simple SSA construction and control flow.</li>
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<li><b><a href="LangImpl6.html">Chapter #6</a>: Extending the Language:
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User-defined Operators</b> - This is a silly but fun chapter that talks about
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extending the language to let the user program define their own arbitrary
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unary and binary operators (with assignable precedence!). This lets us build a
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significant piece of the "language" as library routines.</li>
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<li><b><a href="LangImpl7.html">Chapter #7</a>: Extending the Language: Mutable
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Variables</b> - This chapter talks about adding user-defined local variables
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along with an assignment operator. The interesting part about this is how
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easy and trivial it is to construct SSA form in LLVM: no, LLVM does <em>not</em>
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require your front-end to construct SSA form!</li>
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<li><b><a href="LangImpl8.html">Chapter #8</a>: Conclusion and other useful LLVM
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tidbits</b> - This chapter wraps up the series by talking about potential
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ways to extend the language, but also includes a bunch of pointers to info about
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"special topics" like adding garbage collection support, exceptions, debugging,
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support for "spaghetti stacks", and a bunch of other tips and tricks.</li>
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</ul>
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<p>By the end of the tutorial, we'll have written a bit less than 700 lines of
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non-comment, non-blank, lines of code. With this small amount of code, we'll
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have built up a very reasonable compiler for a non-trivial language including
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a hand-written lexer, parser, AST, as well as code generation support with a JIT
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compiler. While other systems may have interesting "hello world" tutorials,
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I think the breadth of this tutorial is a great testament to the strengths of
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LLVM and why you should consider it if you're interested in language or compiler
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design.</p>
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<p>A note about this tutorial: we expect you to extend the language and play
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with it on your own. Take the code and go crazy hacking away at it, compilers
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don't need to be scary creatures - it can be a lot of fun to play with
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languages!</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="language">The Basic Language</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This tutorial will be illustrated with a toy language that we'll call
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"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived
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from "meaning beautiful, form, and view").
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Kaleidoscope is a procedural language that allows you to define functions, use
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conditionals, math, etc. Over the course of the tutorial, we'll extend
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Kaleidoscope to support the if/then/else construct, a for loop, user defined
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operators, JIT compilation with a simple command line interface, etc.</p>
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<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
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64-bit floating point type (aka 'double' in C parlance). As such, all values
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are implicitly double precision and the language doesn't require type
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declarations. This gives the language a very nice and simple syntax. For
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example, the following simple example computes <a
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href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
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<div class="doc_code">
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<pre>
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# Compute the x'th fibonacci number.
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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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# This expression will compute the 40th number.
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fib(40)
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</pre>
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</div>
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<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
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JIT makes this completely trivial). This means that you can use the 'extern'
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keyword to define a function before you use it (this is also useful for mutually
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recursive functions). For example:</p>
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<div class="doc_code">
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<pre>
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extern sin(arg);
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extern cos(arg);
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extern atan2(arg1 arg2);
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atan2(sin(.4), cos(42))
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</pre>
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</div>
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<p>A more interesting example is included in Chapter 6 where we write a little
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Kaleidoscope application that <a href="LangImpl6.html#example">displays
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a Mandelbrot Set</a> at various levels of magnification.</p>
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<p>Lets dive into the implementation of this language!</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="lexer">The Lexer</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>When it comes to implementing a language, the first thing needed is
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the ability to process a text file and recognize what it says. The traditional
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way to do this is to use a "<a
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href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
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to break the input up into "tokens". Each token returned by the lexer includes
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a token code and potentially some metadata (e.g. the numeric value of a number).
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First, we define the possibilities:
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</p>
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<div class="doc_code">
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<pre>
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// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
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// of these for known things.
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enum Token {
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tok_eof = -1,
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// commands
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tok_def = -2, tok_extern = -3,
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// primary
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tok_identifier = -4, tok_number = -5,
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};
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static std::string IdentifierStr; // Filled in if tok_identifier
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static double NumVal; // Filled in if tok_number
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</pre>
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</div>
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<p>Each token returned by our lexer will either be one of the Token enum values
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or it will be an 'unknown' character like '+', which is returned as its ASCII
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value. If the current token is an identifier, the <tt>IdentifierStr</tt>
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global variable holds the name of the identifier. If the current token is a
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numeric literal (like 1.0), <tt>NumVal</tt> holds its value. Note that we use
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global variables for simplicity, this is not the best choice for a real language
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implementation :).
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</p>
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<p>The actual implementation of the lexer is a single function named
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<tt>gettok</tt>. The <tt>gettok</tt> function is called to return the next token
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from standard input. Its definition starts as:</p>
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<div class="doc_code">
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<pre>
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/// gettok - Return the next token from standard input.
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static int gettok() {
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static int LastChar = ' ';
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// Skip any whitespace.
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while (isspace(LastChar))
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LastChar = getchar();
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</pre>
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</div>
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<p>
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<tt>gettok</tt> works by calling the C <tt>getchar()</tt> function to read
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characters one at a time from standard input. It eats them as it recognizes
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them and stores the last character read, but not processed, in LastChar. The
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first thing that it has to do is ignore whitespace between tokens. This is
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accomplished with the loop above.</p>
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<p>The next thing <tt>gettok</tt> needs to do is recognize identifiers and
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specific keywords like "def". Kaleidoscope does this with this simple loop:</p>
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<div class="doc_code">
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<pre>
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if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
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IdentifierStr = LastChar;
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while (isalnum((LastChar = getchar())))
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IdentifierStr += LastChar;
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if (IdentifierStr == "def") return tok_def;
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if (IdentifierStr == "extern") return tok_extern;
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return tok_identifier;
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}
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</pre>
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</div>
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<p>Note that this code sets the '<tt>IdentifierStr</tt>' global whenever it
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lexes an identifier. Also, since language keywords are matched by the same
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loop, we handle them here inline. Numeric values are similar:</p>
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<div class="doc_code">
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<pre>
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if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
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std::string NumStr;
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do {
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NumStr += LastChar;
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LastChar = getchar();
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} while (isdigit(LastChar) || LastChar == '.');
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NumVal = strtod(NumStr.c_str(), 0);
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return tok_number;
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}
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</pre>
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</div>
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<p>This is all pretty straight-forward code for processing input. When reading
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a numeric value from input, we use the C <tt>strtod</tt> function to convert it
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to a numeric value that we store in <tt>NumVal</tt>. Note that this isn't doing
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sufficient error checking: it will incorrectly read "1.23.45.67" and handle it as
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if you typed in "1.23". Feel free to extend it :). Next we handle comments:
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</p>
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<div class="doc_code">
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<pre>
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if (LastChar == '#') {
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// Comment until end of line.
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do LastChar = getchar();
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while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
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if (LastChar != EOF)
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return gettok();
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}
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</pre>
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</div>
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<p>We handle comments by skipping to the end of the line and then return the
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next token. Finally, if the input doesn't match one of the above cases, it is
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either an operator character like '+' or the end of the file. These are handled
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with this code:</p>
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<div class="doc_code">
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<pre>
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// Check for end of file. Don't eat the EOF.
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if (LastChar == EOF)
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return tok_eof;
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// Otherwise, just return the character as its ascii value.
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int ThisChar = LastChar;
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LastChar = getchar();
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return ThisChar;
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}
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</pre>
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</div>
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<p>With this, we have the complete lexer for the basic Kaleidoscope language
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(the <a href="LangImpl2.html#code">full code listing</a> for the Lexer is
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available in the <a href="LangImpl2.html">next chapter</a> of the tutorial).
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Next we'll <a href="LangImpl2.html">build a simple parser that uses this to
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build an Abstract Syntax Tree</a>. When we have that, we'll include a driver
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so that you can use the lexer and parser together.
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</p>
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<a href="LangImpl2.html">Next: Implementing a Parser and AST</a>
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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: $Date$
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</address>
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