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merge with the LLVM_original
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Thu, 12 Dec 2013 14:37:49 +0900
parents 95c75e76d11b
children 60c9769439b8
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2:4bc3e1cd2659 31:d22a1cf4041c
1 =================================================
2 Kaleidoscope: Tutorial Introduction and the Lexer
3 =================================================
4
5 .. contents::
6 :local:
7
8 Tutorial Introduction
9 =====================
10
11 Welcome to the "Implementing a language with LLVM" tutorial. This
12 tutorial runs through the implementation of a simple language, showing
13 how fun and easy it can be. This tutorial will get you up and started as
14 well as help to build a framework you can extend to other languages. The
15 code in this tutorial can also be used as a playground to hack on other
16 LLVM specific things.
17
18 The goal of this tutorial is to progressively unveil our language,
19 describing how it is built up over time. This will let us cover a fairly
20 broad range of language design and LLVM-specific usage issues, showing
21 and explaining the code for it all along the way, without overwhelming
22 you with tons of details up front.
23
24 It is useful to point out ahead of time that this tutorial is really
25 about teaching compiler techniques and LLVM specifically, *not* about
26 teaching modern and sane software engineering principles. In practice,
27 this means that we'll take a number of shortcuts to simplify the
28 exposition. For example, the code leaks memory, uses global variables
29 all over the place, doesn't use nice design patterns like
30 `visitors <http://en.wikipedia.org/wiki/Visitor_pattern>`_, etc... but
31 it is very simple. If you dig in and use the code as a basis for future
32 projects, fixing these deficiencies shouldn't be hard.
33
34 I've tried to put this tutorial together in a way that makes chapters
35 easy to skip over if you are already familiar with or are uninterested
36 in the various pieces. The structure of the tutorial is:
37
38 - `Chapter #1 <#language>`_: Introduction to the Kaleidoscope
39 language, and the definition of its Lexer - This shows where we are
40 going and the basic functionality that we want it to do. In order to
41 make this tutorial maximally understandable and hackable, we choose
42 to implement everything in C++ instead of using lexer and parser
43 generators. LLVM obviously works just fine with such tools, feel free
44 to use one if you prefer.
45 - `Chapter #2 <LangImpl2.html>`_: Implementing a Parser and AST -
46 With the lexer in place, we can talk about parsing techniques and
47 basic AST construction. This tutorial describes recursive descent
48 parsing and operator precedence parsing. Nothing in Chapters 1 or 2
49 is LLVM-specific, the code doesn't even link in LLVM at this point.
50 :)
51 - `Chapter #3 <LangImpl3.html>`_: Code generation to LLVM IR - With
52 the AST ready, we can show off how easy generation of LLVM IR really
53 is.
54 - `Chapter #4 <LangImpl4.html>`_: Adding JIT and Optimizer Support
55 - Because a lot of people are interested in using LLVM as a JIT,
56 we'll dive right into it and show you the 3 lines it takes to add JIT
57 support. LLVM is also useful in many other ways, but this is one
58 simple and "sexy" way to show off its power. :)
59 - `Chapter #5 <LangImpl5.html>`_: Extending the Language: Control
60 Flow - With the language up and running, we show how to extend it
61 with control flow operations (if/then/else and a 'for' loop). This
62 gives us a chance to talk about simple SSA construction and control
63 flow.
64 - `Chapter #6 <LangImpl6.html>`_: Extending the Language:
65 User-defined Operators - This is a silly but fun chapter that talks
66 about extending the language to let the user program define their own
67 arbitrary unary and binary operators (with assignable precedence!).
68 This lets us build a significant piece of the "language" as library
69 routines.
70 - `Chapter #7 <LangImpl7.html>`_: Extending the Language: Mutable
71 Variables - This chapter talks about adding user-defined local
72 variables along with an assignment operator. The interesting part
73 about this is how easy and trivial it is to construct SSA form in
74 LLVM: no, LLVM does *not* require your front-end to construct SSA
75 form!
76 - `Chapter #8 <LangImpl8.html>`_: Conclusion and other useful LLVM
77 tidbits - This chapter wraps up the series by talking about
78 potential ways to extend the language, but also includes a bunch of
79 pointers to info about "special topics" like adding garbage
80 collection support, exceptions, debugging, support for "spaghetti
81 stacks", and a bunch of other tips and tricks.
82
83 By the end of the tutorial, we'll have written a bit less than 700 lines
84 of non-comment, non-blank, lines of code. With this small amount of
85 code, we'll have built up a very reasonable compiler for a non-trivial
86 language including a hand-written lexer, parser, AST, as well as code
87 generation support with a JIT compiler. While other systems may have
88 interesting "hello world" tutorials, I think the breadth of this
89 tutorial is a great testament to the strengths of LLVM and why you
90 should consider it if you're interested in language or compiler design.
91
92 A note about this tutorial: we expect you to extend the language and
93 play with it on your own. Take the code and go crazy hacking away at it,
94 compilers don't need to be scary creatures - it can be a lot of fun to
95 play with languages!
96
97 The Basic Language
98 ==================
99
100 This tutorial will be illustrated with a toy language that we'll call
101 "`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived
102 from "meaning beautiful, form, and view"). Kaleidoscope is a procedural
103 language that allows you to define functions, use conditionals, math,
104 etc. Over the course of the tutorial, we'll extend Kaleidoscope to
105 support the if/then/else construct, a for loop, user defined operators,
106 JIT compilation with a simple command line interface, etc.
107
108 Because we want to keep things simple, the only datatype in Kaleidoscope
109 is a 64-bit floating point type (aka 'double' in C parlance). As such,
110 all values are implicitly double precision and the language doesn't
111 require type declarations. This gives the language a very nice and
112 simple syntax. For example, the following simple example computes
113 `Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_
114
115 ::
116
117 # Compute the x'th fibonacci number.
118 def fib(x)
119 if x < 3 then
120 1
121 else
122 fib(x-1)+fib(x-2)
123
124 # This expression will compute the 40th number.
125 fib(40)
126
127 We also allow Kaleidoscope to call into standard library functions (the
128 LLVM JIT makes this completely trivial). This means that you can use the
129 'extern' keyword to define a function before you use it (this is also
130 useful for mutually recursive functions). For example:
131
132 ::
133
134 extern sin(arg);
135 extern cos(arg);
136 extern atan2(arg1 arg2);
137
138 atan2(sin(.4), cos(42))
139
140 A more interesting example is included in Chapter 6 where we write a
141 little Kaleidoscope application that `displays a Mandelbrot
142 Set <LangImpl6.html#example>`_ at various levels of magnification.
143
144 Lets dive into the implementation of this language!
145
146 The Lexer
147 =========
148
149 When it comes to implementing a language, the first thing needed is the
150 ability to process a text file and recognize what it says. The
151 traditional way to do this is to use a
152 "`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka
153 'scanner') to break the input up into "tokens". Each token returned by
154 the lexer includes a token code and potentially some metadata (e.g. the
155 numeric value of a number). First, we define the possibilities:
156
157 .. code-block:: c++
158
159 // The lexer returns tokens [0-255] if it is an unknown character, otherwise one
160 // of these for known things.
161 enum Token {
162 tok_eof = -1,
163
164 // commands
165 tok_def = -2, tok_extern = -3,
166
167 // primary
168 tok_identifier = -4, tok_number = -5,
169 };
170
171 static std::string IdentifierStr; // Filled in if tok_identifier
172 static double NumVal; // Filled in if tok_number
173
174 Each token returned by our lexer will either be one of the Token enum
175 values or it will be an 'unknown' character like '+', which is returned
176 as its ASCII value. If the current token is an identifier, the
177 ``IdentifierStr`` global variable holds the name of the identifier. If
178 the current token is a numeric literal (like 1.0), ``NumVal`` holds its
179 value. Note that we use global variables for simplicity, this is not the
180 best choice for a real language implementation :).
181
182 The actual implementation of the lexer is a single function named
183 ``gettok``. The ``gettok`` function is called to return the next token
184 from standard input. Its definition starts as:
185
186 .. code-block:: c++
187
188 /// gettok - Return the next token from standard input.
189 static int gettok() {
190 static int LastChar = ' ';
191
192 // Skip any whitespace.
193 while (isspace(LastChar))
194 LastChar = getchar();
195
196 ``gettok`` works by calling the C ``getchar()`` function to read
197 characters one at a time from standard input. It eats them as it
198 recognizes them and stores the last character read, but not processed,
199 in LastChar. The first thing that it has to do is ignore whitespace
200 between tokens. This is accomplished with the loop above.
201
202 The next thing ``gettok`` needs to do is recognize identifiers and
203 specific keywords like "def". Kaleidoscope does this with this simple
204 loop:
205
206 .. code-block:: c++
207
208 if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
209 IdentifierStr = LastChar;
210 while (isalnum((LastChar = getchar())))
211 IdentifierStr += LastChar;
212
213 if (IdentifierStr == "def") return tok_def;
214 if (IdentifierStr == "extern") return tok_extern;
215 return tok_identifier;
216 }
217
218 Note that this code sets the '``IdentifierStr``' global whenever it
219 lexes an identifier. Also, since language keywords are matched by the
220 same loop, we handle them here inline. Numeric values are similar:
221
222 .. code-block:: c++
223
224 if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
225 std::string NumStr;
226 do {
227 NumStr += LastChar;
228 LastChar = getchar();
229 } while (isdigit(LastChar) || LastChar == '.');
230
231 NumVal = strtod(NumStr.c_str(), 0);
232 return tok_number;
233 }
234
235 This is all pretty straight-forward code for processing input. When
236 reading a numeric value from input, we use the C ``strtod`` function to
237 convert it to a numeric value that we store in ``NumVal``. Note that
238 this isn't doing sufficient error checking: it will incorrectly read
239 "1.23.45.67" and handle it as if you typed in "1.23". Feel free to
240 extend it :). Next we handle comments:
241
242 .. code-block:: c++
243
244 if (LastChar == '#') {
245 // Comment until end of line.
246 do LastChar = getchar();
247 while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
248
249 if (LastChar != EOF)
250 return gettok();
251 }
252
253 We handle comments by skipping to the end of the line and then return
254 the next token. Finally, if the input doesn't match one of the above
255 cases, it is either an operator character like '+' or the end of the
256 file. These are handled with this code:
257
258 .. code-block:: c++
259
260 // Check for end of file. Don't eat the EOF.
261 if (LastChar == EOF)
262 return tok_eof;
263
264 // Otherwise, just return the character as its ascii value.
265 int ThisChar = LastChar;
266 LastChar = getchar();
267 return ThisChar;
268 }
269
270 With this, we have the complete lexer for the basic Kaleidoscope
271 language (the `full code listing <LangImpl2.html#code>`_ for the Lexer
272 is available in the `next chapter <LangImpl2.html>`_ of the tutorial).
273 Next we'll `build a simple parser that uses this to build an Abstract
274 Syntax Tree <LangImpl2.html>`_. When we have that, we'll include a
275 driver so that you can use the lexer and parser together.
276
277 `Next: Implementing a Parser and AST <LangImpl2.html>`_
278