mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-25 12:12:47 +01:00
349 lines
14 KiB
HTML
349 lines
14 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
|
|
<html>
|
|
<head>
|
|
<title>Kaleidoscope: Tutorial Introduction and the Lexer</title>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<meta name="author" content="Chris Lattner">
|
|
<link rel="stylesheet" href="../_static/llvm.css" type="text/css">
|
|
</head>
|
|
|
|
<body>
|
|
|
|
<h1>Kaleidoscope: Tutorial Introduction and the Lexer</h1>
|
|
|
|
<ul>
|
|
<li><a href="index.html">Up to Tutorial Index</a></li>
|
|
<li>Chapter 1
|
|
<ol>
|
|
<li><a href="#intro">Tutorial Introduction</a></li>
|
|
<li><a href="#language">The Basic Language</a></li>
|
|
<li><a href="#lexer">The Lexer</a></li>
|
|
</ol>
|
|
</li>
|
|
<li><a href="LangImpl2.html">Chapter 2</a>: Implementing a Parser and AST</li>
|
|
</ul>
|
|
|
|
<div class="doc_author">
|
|
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<h2><a name="intro">Tutorial Introduction</a></h2>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div>
|
|
|
|
<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
|
|
runs through the implementation of a simple language, showing how fun and
|
|
easy it can be. This tutorial will get you up and started as well as help to
|
|
build a framework you can extend to other languages. The code in this tutorial
|
|
can also be used as a playground to hack on other LLVM specific things.
|
|
</p>
|
|
|
|
<p>
|
|
The goal of this tutorial is to progressively unveil our language, describing
|
|
how it is built up over time. This will let us cover a fairly broad range of
|
|
language design and LLVM-specific usage issues, showing and explaining the code
|
|
for it all along the way, without overwhelming you with tons of details up
|
|
front.</p>
|
|
|
|
<p>It is useful to point out ahead of time that this tutorial is really about
|
|
teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
|
|
modern and sane software engineering principles. In practice, this means that
|
|
we'll take a number of shortcuts to simplify the exposition. For example, the
|
|
code leaks memory, uses global variables all over the place, doesn't use nice
|
|
design patterns like <a
|
|
href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
|
|
is very simple. If you dig in and use the code as a basis for future projects,
|
|
fixing these deficiencies shouldn't be hard.</p>
|
|
|
|
<p>I've tried to put this tutorial together in a way that makes chapters easy to
|
|
skip over if you are already familiar with or are uninterested in the various
|
|
pieces. The structure of the tutorial is:
|
|
</p>
|
|
|
|
<ul>
|
|
<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
|
|
language, and the definition of its Lexer</b> - This shows where we are going
|
|
and the basic functionality that we want it to do. In order to make this
|
|
tutorial maximally understandable and hackable, we choose to implement
|
|
everything in C++ instead of using lexer and parser generators. LLVM obviously
|
|
works just fine with such tools, feel free to use one if you prefer.</li>
|
|
<li><b><a href="LangImpl2.html">Chapter #2</a>: Implementing a Parser and
|
|
AST</b> - With the lexer in place, we can talk about parsing techniques and
|
|
basic AST construction. This tutorial describes recursive descent parsing and
|
|
operator precedence parsing. Nothing in Chapters 1 or 2 is LLVM-specific,
|
|
the code doesn't even link in LLVM at this point. :)</li>
|
|
<li><b><a href="LangImpl3.html">Chapter #3</a>: Code generation to LLVM IR</b> -
|
|
With the AST ready, we can show off how easy generation of LLVM IR really
|
|
is.</li>
|
|
<li><b><a href="LangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
|
|
Support</b> - Because a lot of people are interested in using LLVM as a JIT,
|
|
we'll dive right into it and show you the 3 lines it takes to add JIT support.
|
|
LLVM is also useful in many other ways, but this is one simple and "sexy" way
|
|
to shows off its power. :)</li>
|
|
<li><b><a href="LangImpl5.html">Chapter #5</a>: Extending the Language: Control
|
|
Flow</b> - With the language up and running, we show how to extend it with
|
|
control flow operations (if/then/else and a 'for' loop). This gives us a chance
|
|
to talk about simple SSA construction and control flow.</li>
|
|
<li><b><a href="LangImpl6.html">Chapter #6</a>: Extending the Language:
|
|
User-defined Operators</b> - This is a silly but fun chapter that talks about
|
|
extending the language to let the user program define their own arbitrary
|
|
unary and binary operators (with assignable precedence!). This lets us build a
|
|
significant piece of the "language" as library routines.</li>
|
|
<li><b><a href="LangImpl7.html">Chapter #7</a>: Extending the Language: Mutable
|
|
Variables</b> - This chapter talks about adding user-defined local variables
|
|
along with an assignment operator. The interesting part about this is how
|
|
easy and trivial it is to construct SSA form in LLVM: no, LLVM does <em>not</em>
|
|
require your front-end to construct SSA form!</li>
|
|
<li><b><a href="LangImpl8.html">Chapter #8</a>: Conclusion and other useful LLVM
|
|
tidbits</b> - This chapter wraps up the series by talking about potential
|
|
ways to extend the language, but also includes a bunch of pointers to info about
|
|
"special topics" like adding garbage collection support, exceptions, debugging,
|
|
support for "spaghetti stacks", and a bunch of other tips and tricks.</li>
|
|
|
|
</ul>
|
|
|
|
<p>By the end of the tutorial, we'll have written a bit less than 700 lines of
|
|
non-comment, non-blank, lines of code. With this small amount of code, we'll
|
|
have built up a very reasonable compiler for a non-trivial language including
|
|
a hand-written lexer, parser, AST, as well as code generation support with a JIT
|
|
compiler. While other systems may have interesting "hello world" tutorials,
|
|
I think the breadth of this tutorial is a great testament to the strengths of
|
|
LLVM and why you should consider it if you're interested in language or compiler
|
|
design.</p>
|
|
|
|
<p>A note about this tutorial: we expect you to extend the language and play
|
|
with it on your own. Take the code and go crazy hacking away at it, compilers
|
|
don't need to be scary creatures - it can be a lot of fun to play with
|
|
languages!</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<h2><a name="language">The Basic Language</a></h2>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div>
|
|
|
|
<p>This tutorial will be illustrated with a toy language that we'll call
|
|
"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived
|
|
from "meaning beautiful, form, and view").
|
|
Kaleidoscope is a procedural language that allows you to define functions, use
|
|
conditionals, math, etc. Over the course of the tutorial, we'll extend
|
|
Kaleidoscope to support the if/then/else construct, a for loop, user defined
|
|
operators, JIT compilation with a simple command line interface, etc.</p>
|
|
|
|
<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
|
|
64-bit floating point type (aka 'double' in C parlance). As such, all values
|
|
are implicitly double precision and the language doesn't require type
|
|
declarations. This gives the language a very nice and simple syntax. For
|
|
example, the following simple example computes <a
|
|
href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
# Compute the x'th fibonacci number.
|
|
def fib(x)
|
|
if x < 3 then
|
|
1
|
|
else
|
|
fib(x-1)+fib(x-2)
|
|
|
|
# This expression will compute the 40th number.
|
|
fib(40)
|
|
</pre>
|
|
</div>
|
|
|
|
<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
|
|
JIT makes this completely trivial). This means that you can use the 'extern'
|
|
keyword to define a function before you use it (this is also useful for mutually
|
|
recursive functions). For example:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
extern sin(arg);
|
|
extern cos(arg);
|
|
extern atan2(arg1 arg2);
|
|
|
|
atan2(sin(.4), cos(42))
|
|
</pre>
|
|
</div>
|
|
|
|
<p>A more interesting example is included in Chapter 6 where we write a little
|
|
Kaleidoscope application that <a href="LangImpl6.html#example">displays
|
|
a Mandelbrot Set</a> at various levels of magnification.</p>
|
|
|
|
<p>Lets dive into the implementation of this language!</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<h2><a name="lexer">The Lexer</a></h2>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div>
|
|
|
|
<p>When it comes to implementing a language, the first thing needed is
|
|
the ability to process a text file and recognize what it says. The traditional
|
|
way to do this is to use a "<a
|
|
href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
|
|
to break the input up into "tokens". Each token returned by the lexer includes
|
|
a token code and potentially some metadata (e.g. the numeric value of a number).
|
|
First, we define the possibilities:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// 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,
|
|
};
|
|
|
|
static std::string IdentifierStr; // Filled in if tok_identifier
|
|
static double NumVal; // Filled in if tok_number
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Each token returned by our lexer will either be one of the Token enum values
|
|
or it will be an 'unknown' character like '+', which is returned as its ASCII
|
|
value. If the current token is an identifier, the <tt>IdentifierStr</tt>
|
|
global variable holds the name of the identifier. If the current token is a
|
|
numeric literal (like 1.0), <tt>NumVal</tt> holds its value. Note that we use
|
|
global variables for simplicity, this is not the best choice for a real language
|
|
implementation :).
|
|
</p>
|
|
|
|
<p>The actual implementation of the lexer is a single function named
|
|
<tt>gettok</tt>. The <tt>gettok</tt> function is called to return the next token
|
|
from standard input. Its definition starts as:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
/// gettok - Return the next token from standard input.
|
|
static int gettok() {
|
|
static int LastChar = ' ';
|
|
|
|
// Skip any whitespace.
|
|
while (isspace(LastChar))
|
|
LastChar = getchar();
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>gettok</tt> works by calling the C <tt>getchar()</tt> function to read
|
|
characters one at a time from standard input. It eats them as it recognizes
|
|
them and stores the last character read, but not processed, in LastChar. The
|
|
first thing that it has to do is ignore whitespace between tokens. This is
|
|
accomplished with the loop above.</p>
|
|
|
|
<p>The next thing <tt>gettok</tt> needs to do is recognize identifiers and
|
|
specific keywords like "def". Kaleidoscope does this with this simple loop:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
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;
|
|
return tok_identifier;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Note that this code sets the '<tt>IdentifierStr</tt>' global whenever it
|
|
lexes an identifier. Also, since language keywords are matched by the same
|
|
loop, we handle them here inline. Numeric values are similar:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
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;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>This is all pretty straight-forward code for processing input. When reading
|
|
a numeric value from input, we use the C <tt>strtod</tt> function to convert it
|
|
to a numeric value that we store in <tt>NumVal</tt>. Note that this isn't doing
|
|
sufficient error checking: it will incorrectly read "1.23.45.67" and handle it as
|
|
if you typed in "1.23". Feel free to extend it :). Next we handle comments:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
if (LastChar == '#') {
|
|
// Comment until end of line.
|
|
do LastChar = getchar();
|
|
while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
|
|
|
|
if (LastChar != EOF)
|
|
return gettok();
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>We handle comments by skipping to the end of the line and then return the
|
|
next token. Finally, if the input doesn't match one of the above cases, it is
|
|
either an operator character like '+' or the end of the file. These are handled
|
|
with this code:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// 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;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>With this, we have the complete lexer for the basic Kaleidoscope language
|
|
(the <a href="LangImpl2.html#code">full code listing</a> for the Lexer is
|
|
available in the <a href="LangImpl2.html">next chapter</a> of the tutorial).
|
|
Next we'll <a href="LangImpl2.html">build a simple parser that uses this to
|
|
build an Abstract Syntax Tree</a>. When we have that, we'll include a driver
|
|
so that you can use the lexer and parser together.
|
|
</p>
|
|
|
|
<a href="LangImpl2.html">Next: Implementing a Parser and AST</a>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
|
|
|
|
<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>
|
|
</body>
|
|
</html>
|