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>`_ 278 279
