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<!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">
<meta name="author" content="Max Shawabkeh">
<link rel="stylesheet"
href="http://www.llvm.org/docs/llvm.css"
type="text/css">
</head>
<body>
<div class="doc_title">Kaleidoscope: Tutorial Introduction and the Lexer</div>
<ul>
<li>
<a href="http://www.llvm.org/docs/tutorial/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="PythonLangImpl2.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>
and <a href="http://max99x.com">Max Shawabkeh</a>
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="intro">Tutorial Introduction</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<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. If you dig in and
use the code as a basis for future projects, fixing its deficiencies shouldn't
be hard.</p>
<p>We'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 Python 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="PythonLangImpl2.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 import the LLVM modules at this point. :)</li>
<li><b><a href="PythonLangImpl3.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="PythonLangImpl4.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="PythonLangImpl5.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="PythonLangImpl6.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="PythonLangImpl7.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="PythonLangImpl8.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 540 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>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="language">The Basic Language</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<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. 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(0.4), cos(42))
</pre>
</div>
<p>A more interesting example is included in Chapter 6 where we write a little
Kaleidoscope application that <a href="PythonLangImpl6.html#example">displays
a Mandelbrot Set</a> at various levels of magnification.</p>
<p>Lets dive into the implementation of this language!</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="lexer">The Lexer</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<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 type 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 yields one of these types for each token.
class EOFToken(object):
pass
class DefToken(object):
pass
class ExternToken(object):
pass
class IdentifierToken(object):
def __init__(self, name): self.name = name
class NumberToken(object):
def __init__(self, value): self.value = value
class CharacterToken(object):
def __init__(self, char): self.char = char
def __eq__(self, other):
return isinstance(other, CharacterToken) and self.char == other.char
def __ne__(self, other): return not self == other
</pre>
</div>
<p>Each token yielded by our lexer will be of one of the above types. For simple
tokens that are always the same, like the "def" keyword, the lexer will yield
<tt>DefToken()</tt>. Identifiers, numbers and characters, on the other
hand, have extra data, so when the lexer encounteres the number 123.45, it will
emit it as <tt>NumberToken(123.45)</tt>. An identifier <tt>foo</tt> will be
emitted as <tt>IdentifierToken('foo')</tt>. And finally, an unknown character
like '+' will be returned as <tt>CharacterToken('+')</tt>. You may notice that
we overload the equality and inequality operators for the characters; this will
later simplify character comparisons in the parser code.</p>
<p>The actual implementation of the lexer is a single function called
<tt>Tokenize</tt>, which takes a string and
<a href="http://docs.python.org/reference/simple_stmts.html#the-yield-statement">yields</a>
tokens. For simplicity, we will use
<a href="http://docs.python.org/library/re.html">regular
expressions</a> to parse out the tokens. This is terribly inefficient, but
perfectly sufficient for our needs.</p>
<p>First, we define the regular expressions for our tokens. Numbers and strings
of digits, optionally followed by a period and another string of digits.
Identifiers (and keywords) are alphanumeric string starting with a letter and
comments are anything between a hash (<tt>#</tt>) and the end of the line.
<div class="doc_code">
<pre>
import re
...
# Regular expressions that tokens and comments of our language.
REGEX_NUMBER = re.compile('[0-9]+(?:\.[0-9]+)?')
REGEX_IDENTIFIER = re.compile('[a-zA-Z][a-zA-Z0-9]*')
REGEX_COMMENT = re.compile('#.*')
</pre>
</div>
<p>
Next, let's start defining the <tt>Tokenize</tt> function itself. The first
thing we need to do is set up a loop that scans the string, while ignoring
whitespace between tokens:</p>
<div class="doc_code">
<pre>
def Tokenize(string):
while string:
# Skip whitespace.
if string[0].isspace():
string = string[1:]
continue
...
</pre>
</div>
<p>Next we want to find out what the next token is. For this we run the regexes
we defined above on the remainder of the string. To simplify the rest of the
code, we run all three regexes each time. As mentioned above, inefficiencies are
ignored for the purpose of this tutorial:<p>
<div class="doc_code">
<pre>
# Run regexes.
comment_match = REGEX_COMMENT.match(string)
number_match = REGEX_NUMBER.match(string)
identifier_match = REGEX_IDENTIFIER.match(string)
</pre>
</div>
<p>Now se check if any of the regexes matched. For comments, we simply
ignore the captured match:</p>
<div class="doc_code">
<pre>
# Check if any of the regexes matched and yield the appropriate result.
if comment_match:
comment = comment_match.group(0)
string = string[len(comment):]
</pre>
</div>
<p>For numbers, we yield the captured match, converted to a float and tagged
with the appropriate token type:</p>
<div class="doc_code">
<pre>
elif number_match:
number = number_match.group(0)
yield NumberToken(float(number))
string = string[len(number):]
</pre>
</div>
<p>The identifier case is a little more complex. We have to check for keywords
to decide whether we have captured an identifier or a keyword:</p>
<div class="doc_code">
<pre>
elif identifier_match:
identifier = identifier_match.group(0)
# Check if we matched a keyword.
if identifier == 'def':
yield DefToken()
elif identifier == 'extern':
yield ExternToken()
else:
yield IdentifierToken(identifier)
string = string[len(identifier):]
</pre>
</div>
<p>Finally, if we haven't recognized a comment, a number of an identifier, we
yield the current character as an "unknown character" token. This is used, for
example, for operators like <tt>+</tt> or <tt>*</tt>:</p>
<div class="doc_code">
<pre>
else:
# Yield the unknown character.
yield CharacterToken(string[0])
string = string[1:]
</pre>
</div>
<p>Once we're done with the
loop, we return a final end-of-file token:</p>
<div class="doc_code">
<pre>
yield EOFToken()
</pre>
</div>
<p>With this, we have the complete lexer for the basic Kaleidoscope language
(the <a href="PythonLangImpl2.html#code">full code listing</a> for the Lexer is
available in the <a href="PythonLangImpl2.html">next chapter</a> of the
tutorial). Next we'll <a href="PythonLangImpl2.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="PythonLangImpl2.html">Next: Implementing a Parser and AST</a>
</div>
<!-- *********************************************************************** -->
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