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2 Kaleidoscope: Tutorial Introduction and the Lexer
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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
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.
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.
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:
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.
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
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
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
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
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.
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.
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
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.
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>`_
117 # Compute the x'th fibonacci number.
124 # This expression will compute the 40th number.
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:
136 extern atan2(arg1 arg2);
138 atan2(sin(.4), cos(42))
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.
144 Lets dive into the implementation of this language!
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:
159 // The lexer returns tokens [0-255] if it is an unknown character, otherwise one
160 // of these for known things.
165 tok_def = -2, tok_extern = -3,
168 tok_identifier = -4, tok_number = -5,
171 static std::string IdentifierStr; // Filled in if tok_identifier
172 static double NumVal; // Filled in if tok_number
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 :).
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:
188 /// gettok - Return the next token from standard input.
189 static int gettok() {
190 static int LastChar = ' ';
192 // Skip any whitespace.
193 while (isspace(LastChar))
194 LastChar = getchar();
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.
202 The next thing ``gettok`` needs to do is recognize identifiers and
203 specific keywords like "def". Kaleidoscope does this with this simple
208 if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
209 IdentifierStr = LastChar;
210 while (isalnum((LastChar = getchar())))
211 IdentifierStr += LastChar;
213 if (IdentifierStr == "def") return tok_def;
214 if (IdentifierStr == "extern") return tok_extern;
215 return tok_identifier;
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:
224 if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
228 LastChar = getchar();
229 } while (isdigit(LastChar) || LastChar == '.');
231 NumVal = strtod(NumStr.c_str(), 0);
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:
244 if (LastChar == '#') {
245 // Comment until end of line.
246 do LastChar = getchar();
247 while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
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:
260 // Check for end of file. Don't eat the EOF.
264 // Otherwise, just return the character as its ascii value.
265 int ThisChar = LastChar;
266 LastChar = getchar();
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.
277 `Next: Implementing a Parser and AST <LangImpl2.html>`_