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Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]6 <title>Kaleidoscope: Tutorial Introduction and the Lexer</title>
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13
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]14<div class="doc_title">Kaleidoscope: Tutorial Introduction and the Lexer</div>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]15
Chris Lattner639a18d2007-11-05 19:06:59 +0000[diff] [blame]16<ul>
Chris Lattnerfbfae1b2007-11-05 20:04:56 +0000[diff] [blame]17<li><a href="index.html">Up to Tutorial Index</a></li>
Chris Lattner639a18d2007-11-05 19:06:59 +0000[diff] [blame]18<li>Chapter 1
19 <ol>
20 <li><a href="#intro">Tutorial Introduction</a></li>
21 <li><a href="#language">The Basic Language</a></li>
22 <li><a href="#lexer">The Lexer</a></li>
23 </ol>
24</li>
Chris Lattnerfbfae1b2007-11-05 20:04:56 +0000[diff] [blame]25<li><a href="LangImpl2.html">Chapter 2</a>: Implementing a Parser and AST</li>
Chris Lattner639a18d2007-11-05 19:06:59 +0000[diff] [blame]26</ul>
27
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]28<div class="doc_author">
29 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
30</div>
31
32<!-- *********************************************************************** -->
33<div class="doc_section"><a name="intro">Tutorial Introduction</a></div>
34<!-- *********************************************************************** -->
35
36<div class="doc_text">
37
38<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]39runs through the implementation of a simple language, showing how fun and
Erick Tryzelaar05acd732008-03-26 00:38:56 +0000[diff] [blame]40easy it can be. This tutorial will get you up and started as well as help to
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]41build a framework you can extend to other languages. The code in this tutorial
42can also be used as a playground to hack on other LLVM specific things.
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]43</p>
44
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]45<p>
46The goal of this tutorial is to progressively unveil our language, describing
47how it is built up over time. This will let us cover a fairly broad range of
48language design and LLVM-specific usage issues, showing and explaining the code
49for it all along the way, without overwhelming you with tons of details up
50front.</p>
51
52<p>It is useful to point out ahead of time that this tutorial is really about
53teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
54modern and sane software engineering principles. In practice, this means that
55we'll take a number of shortcuts to simplify the exposition. For example, the
56code leaks memory, uses global variables all over the place, doesn't use nice
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]57design patterns like <a
58href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
59is very simple. If you dig in and use the code as a basis for future projects,
60fixing these deficiencies shouldn't be hard.</p>
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]61
62<p>I've tried to put this tutorial together in a way that makes chapters easy to
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]63skip over if you are already familiar with or are uninterested in the various
64pieces. The structure of the tutorial is:
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]65</p>
66
67<ul>
68<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
69language, and the definition of its Lexer</b> - This shows where we are going
70and the basic functionality that we want it to do. In order to make this
71tutorial maximally understandable and hackable, we choose to implement
72everything in C++ instead of using lexer and parser generators. LLVM obviously
73works just fine with such tools, feel free to use one if you prefer.</li>
74<li><b><a href="LangImpl2.html">Chapter #2</a>: Implementing a Parser and
75AST</b> - With the lexer in place, we can talk about parsing techniques and
76basic AST construction. This tutorial describes recursive descent parsing and
77operator precedence parsing. Nothing in Chapters 1 or 2 is LLVM-specific,
78the code doesn't even link in LLVM at this point. :)</li>
79<li><b><a href="LangImpl3.html">Chapter #3</a>: Code generation to LLVM IR</b> -
80With the AST ready, we can show off how easy generation of LLVM IR really
81is.</li>
82<li><b><a href="LangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
83Support</b> - Because a lot of people are interested in using LLVM as a JIT,
84we'll dive right into it and show you the 3 lines it takes to add JIT support.
85LLVM is also useful in many other ways, but this is one simple and "sexy" way
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]86to shows off its power. :)</li>
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]87<li><b><a href="LangImpl5.html">Chapter #5</a>: Extending the Language: Control
88Flow</b> - With the language up and running, we show how to extend it with
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]89control flow operations (if/then/else and a 'for' loop). This gives us a chance
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]90to talk about simple SSA construction and control flow.</li>
91<li><b><a href="LangImpl6.html">Chapter #6</a>: Extending the Language:
92User-defined Operators</b> - This is a silly but fun chapter that talks about
93extending the language to let the user program define their own arbitrary
94unary and binary operators (with assignable precedence!). This lets us build a
95significant piece of the "language" as library routines.</li>
96<li><b><a href="LangImpl7.html">Chapter #7</a>: Extending the Language: Mutable
97Variables</b> - This chapter talks about adding user-defined local variables
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]98along with an assignment operator. The interesting part about this is how
99easy and trivial it is to construct SSA form in LLVM: no, LLVM does <em>not</em>
100require your front-end to construct SSA form!</li>
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]101<li><b><a href="LangImpl8.html">Chapter #8</a>: Conclusion and other useful LLVM
102tidbits</b> - This chapter wraps up the series by talking about potential
103ways to extend the language, but also includes a bunch of pointers to info about
104"special topics" like adding garbage collection support, exceptions, debugging,
105support for "spaghetti stacks", and a bunch of other tips and tricks.</li>
106
107</ul>
108
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]109<p>By the end of the tutorial, we'll have written a bit less than 700 lines of
110non-comment, non-blank, lines of code. With this small amount of code, we'll
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]111have built up a very reasonable compiler for a non-trivial language including
112a hand-written lexer, parser, AST, as well as code generation support with a JIT
113compiler. While other systems may have interesting "hello world" tutorials,
114I think the breadth of this tutorial is a great testament to the strengths of
115LLVM and why you should consider it if you're interested in language or compiler
116design.</p>
117
118<p>A note about this tutorial: we expect you to extend the language and play
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]119with it on your own. Take the code and go crazy hacking away at it, compilers
120don't need to be scary creatures - it can be a lot of fun to play with
121languages!</p>
Chris Lattner0bf330a2007-11-06 05:02:48 +0000[diff] [blame]122
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]123</div>
124
125<!-- *********************************************************************** -->
Chris Lattner639a18d2007-11-05 19:06:59 +0000[diff] [blame]126<div class="doc_section"><a name="language">The Basic Language</a></div>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]127<!-- *********************************************************************** -->
128
129<div class="doc_text">
130
131<p>This tutorial will be illustrated with a toy language that we'll call
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]132"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived
133from "meaning beautiful, form, and view").
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]134Kaleidoscope is a procedural language that allows you to define functions, use
135conditionals, math, etc. Over the course of the tutorial, we'll extend
Chris Lattner2f86ed72007-11-05 20:13:56 +0000[diff] [blame]136Kaleidoscope to support the if/then/else construct, a for loop, user defined
137operators, JIT compilation with a simple command line interface, etc.</p>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]138
Chris Lattner2f86ed72007-11-05 20:13:56 +0000[diff] [blame]139<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]14064-bit floating point type (aka 'double' in C parlance). As such, all values
141are implicitly double precision and the language doesn't require type
142declarations. This gives the language a very nice and simple syntax. For
Chris Lattner2f86ed72007-11-05 20:13:56 +0000[diff] [blame]143example, the following simple example computes <a
144href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]145
146<div class="doc_code">
147<pre>
148# Compute the x'th fibonacci number.
149def fib(x)
Chris Lattner3a487082007-10-22 06:34:15 +0000[diff] [blame]150 if x &lt; 3 then
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]151 1
152 else
153 fib(x-1)+fib(x-2)
154
155# This expression will compute the 40th number.
156fib(40)
157</pre>
158</div>
159
Duncan Sandsd6f131b2007-11-05 16:04:58 +0000[diff] [blame]160<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]161JIT makes this completely trivial). This means that you can use the 'extern'
162keyword to define a function before you use it (this is also useful for mutually
163recursive functions). For example:</p>
164
165<div class="doc_code">
166<pre>
167extern sin(arg);
168extern cos(arg);
169extern atan2(arg1 arg2);
170
171atan2(sin(.4), cos(42))
172</pre>
173</div>
174
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]175<p>A more interesting example is included in Chapter 6 where we write a little
176Kaleidoscope application that <a href="LangImpl6.html#example">displays
177a Mandelbrot Set</a> at various levels of magnification.</p>
178
179<p>Lets dive into the implementation of this language!</p>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]180
181</div>
182
183<!-- *********************************************************************** -->
Chris Lattner639a18d2007-11-05 19:06:59 +0000[diff] [blame]184<div class="doc_section"><a name="lexer">The Lexer</a></div>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]185<!-- *********************************************************************** -->
186
187<div class="doc_text">
188
189<p>When it comes to implementing a language, the first thing needed is
190the ability to process a text file and recognize what it says. The traditional
191way to do this is to use a "<a
192href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
193to break the input up into "tokens". Each token returned by the lexer includes
194a token code and potentially some metadata (e.g. the numeric value of a number).
195First, we define the possibilities:
196</p>
197
198<div class="doc_code">
199<pre>
200// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
201// of these for known things.
202enum Token {
203 tok_eof = -1,
204
205 // commands
206 tok_def = -2, tok_extern = -3,
207
208 // primary
209 tok_identifier = -4, tok_number = -5,
210};
211
212static std::string IdentifierStr; // Filled in if tok_identifier
213static double NumVal; // Filled in if tok_number
214</pre>
215</div>
216
217<p>Each token returned by our lexer will either be one of the Token enum values
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]218or it will be an 'unknown' character like '+', which is returned as its ASCII
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]219value. If the current token is an identifier, the <tt>IdentifierStr</tt>
220global variable holds the name of the identifier. If the current token is a
221numeric literal (like 1.0), <tt>NumVal</tt> holds its value. Note that we use
222global variables for simplicity, this is not the best choice for a real language
223implementation :).
224</p>
225
Chris Lattner2f86ed72007-11-05 20:13:56 +0000[diff] [blame]226<p>The actual implementation of the lexer is a single function named
227<tt>gettok</tt>. The <tt>gettok</tt> function is called to return the next token
228from standard input. Its definition starts as:</p>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]229
230<div class="doc_code">
231<pre>
232/// gettok - Return the next token from standard input.
233static int gettok() {
234 static int LastChar = ' ';
235
236 // Skip any whitespace.
237 while (isspace(LastChar))
238 LastChar = getchar();
239</pre>
240</div>
241
242<p>
243<tt>gettok</tt> works by calling the C <tt>getchar()</tt> function to read
244characters one at a time from standard input. It eats them as it recognizes
Chris Lattner2f86ed72007-11-05 20:13:56 +0000[diff] [blame]245them and stores the last character read, but not processed, in LastChar. The
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]246first thing that it has to do is ignore whitespace between tokens. This is
247accomplished with the loop above.</p>
248
Chris Lattner2f86ed72007-11-05 20:13:56 +0000[diff] [blame]249<p>The next thing <tt>gettok</tt> needs to do is recognize identifiers and
250specific keywords like "def". Kaleidoscope does this with this simple loop:</p>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]251
252<div class="doc_code">
253<pre>
254 if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
255 IdentifierStr = LastChar;
256 while (isalnum((LastChar = getchar())))
257 IdentifierStr += LastChar;
258
259 if (IdentifierStr == "def") return tok_def;
260 if (IdentifierStr == "extern") return tok_extern;
261 return tok_identifier;
262 }
263</pre>
264</div>
265
Chris Lattner2f86ed72007-11-05 20:13:56 +0000[diff] [blame]266<p>Note that this code sets the '<tt>IdentifierStr</tt>' global whenever it
267lexes an identifier. Also, since language keywords are matched by the same
268loop, we handle them here inline. Numeric values are similar:</p>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]269
270<div class="doc_code">
271<pre>
272 if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
273 std::string NumStr;
274 do {
275 NumStr += LastChar;
276 LastChar = getchar();
277 } while (isdigit(LastChar) || LastChar == '.');
278
279 NumVal = strtod(NumStr.c_str(), 0);
280 return tok_number;
281 }
282</pre>
283</div>
284
285<p>This is all pretty straight-forward code for processing input. When reading
286a numeric value from input, we use the C <tt>strtod</tt> function to convert it
287to a numeric value that we store in <tt>NumVal</tt>. Note that this isn't doing
Duncan Sandsd6f131b2007-11-05 16:04:58 +0000[diff] [blame]288sufficient error checking: it will incorrectly read "1.23.45.67" and handle it as
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]289if you typed in "1.23". Feel free to extend it :). Next we handle comments:
290</p>
291
292<div class="doc_code">
293<pre>
294 if (LastChar == '#') {
295 // Comment until end of line.
296 do LastChar = getchar();
Chris Lattnerb4ef0232007-12-02 22:46:01 +0000[diff] [blame]297 while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]298
299 if (LastChar != EOF)
300 return gettok();
301 }
302</pre>
303</div>
304
Chris Lattnere6819ae2007-11-06 01:39:12 +0000[diff] [blame]305<p>We handle comments by skipping to the end of the line and then return the
Chris Lattnerf0d84f12007-11-06 06:32:18 +0000[diff] [blame]306next token. Finally, if the input doesn't match one of the above cases, it is
307either an operator character like '+' or the end of the file. These are handled
308with this code:</p>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]309
310<div class="doc_code">
311<pre>
312 // Check for end of file. Don't eat the EOF.
313 if (LastChar == EOF)
314 return tok_eof;
315
316 // Otherwise, just return the character as its ascii value.
317 int ThisChar = LastChar;
318 LastChar = getchar();
319 return ThisChar;
320}
321</pre>
322</div>
323
Chris Lattner2f86ed72007-11-05 20:13:56 +0000[diff] [blame]324<p>With this, we have the complete lexer for the basic Kaleidoscope language
325(the <a href="LangImpl2.html#code">full code listing</a> for the Lexer is
326available in the <a href="LangImpl2.html">next chapter</a> of the tutorial).
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]327Next we'll <a href="LangImpl2.html">build a simple parser that uses this to
Chris Lattner3a487082007-10-22 06:34:15 +0000[diff] [blame]328build an Abstract Syntax Tree</a>. When we have that, we'll include a driver
329so that you can use the lexer and parser together.
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]330</p>
331
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]332<a href="LangImpl2.html">Next: Implementing a Parser and AST</a>
Chris Lattner5e0f4592007-10-22 04:32:37 +0000[diff] [blame]333</div>
334
335<!-- *********************************************************************** -->
336<hr>
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343 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
344 <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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