aboutsummaryrefslogtreecommitdiffstats
path: root/docs/main/tutorial/LangImpl1.html
diff options
context:
space:
mode:
Diffstat (limited to 'docs/main/tutorial/LangImpl1.html')
-rw-r--r--docs/main/tutorial/LangImpl1.html348
1 files changed, 348 insertions, 0 deletions
diff --git a/docs/main/tutorial/LangImpl1.html b/docs/main/tutorial/LangImpl1.html
new file mode 100644
index 0000000..66843db
--- /dev/null
+++ b/docs/main/tutorial/LangImpl1.html
@@ -0,0 +1,348 @@
+<!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="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Tutorial Introduction and the Lexer</div>
+
+<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>
+
+<!-- *********************************************************************** -->
+<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. 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>
+
+<!-- *********************************************************************** -->
+<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 (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 &lt; 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>
+
+<!-- *********************************************************************** -->
+<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 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 &amp;&amp; LastChar != '\n' &amp;&amp; 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>