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 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 <LangImpl02.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 <LangImpl03.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 <LangImpl04.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 <LangImpl05.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 <LangImpl06.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 <LangImpl07.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 <LangImpl08.html>`_: Compiling to Object Files - This 77 chapter explains how to take LLVM IR and compile it down to object 78 files. 79 - `Chapter #9 <LangImpl09.html>`_: Extending the Language: Debug 80 Information - Having built a decent little programming language with 81 control flow, functions and mutable variables, we consider what it 82 takes to add debug information to standalone executables. This debug 83 information will allow you to set breakpoints in Kaleidoscope 84 functions, print out argument variables, and call functions - all 85 from within the debugger! 86 - `Chapter #10 <LangImpl10.html>`_: Conclusion and other useful LLVM 87 tidbits - This chapter wraps up the series by talking about 88 potential ways to extend the language, but also includes a bunch of 89 pointers to info about "special topics" like adding garbage 90 collection support, exceptions, debugging, support for "spaghetti 91 stacks", and a bunch of other tips and tricks. 92 93 By the end of the tutorial, we'll have written a bit less than 1000 lines 94 of non-comment, non-blank, lines of code. With this small amount of 95 code, we'll have built up a very reasonable compiler for a non-trivial 96 language including a hand-written lexer, parser, AST, as well as code 97 generation support with a JIT compiler. While other systems may have 98 interesting "hello world" tutorials, I think the breadth of this 99 tutorial is a great testament to the strengths of LLVM and why you 100 should consider it if you're interested in language or compiler design. 101 102 A note about this tutorial: we expect you to extend the language and 103 play with it on your own. Take the code and go crazy hacking away at it, 104 compilers don't need to be scary creatures - it can be a lot of fun to 105 play with languages! 106 107 The Basic Language 108 ================== 109 110 This tutorial will be illustrated with a toy language that we'll call 111 "`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived 112 from "meaning beautiful, form, and view"). Kaleidoscope is a procedural 113 language that allows you to define functions, use conditionals, math, 114 etc. Over the course of the tutorial, we'll extend Kaleidoscope to 115 support the if/then/else construct, a for loop, user defined operators, 116 JIT compilation with a simple command line interface, etc. 117 118 Because we want to keep things simple, the only datatype in Kaleidoscope 119 is a 64-bit floating point type (aka 'double' in C parlance). As such, 120 all values are implicitly double precision and the language doesn't 121 require type declarations. This gives the language a very nice and 122 simple syntax. For example, the following simple example computes 123 `Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_ 124 125 :: 126 127 # Compute the x'th fibonacci number. 128 def fib(x) 129 if x < 3 then 130 1 131 else 132 fib(x-1)+fib(x-2) 133 134 # This expression will compute the 40th number. 135 fib(40) 136 137 We also allow Kaleidoscope to call into standard library functions (the 138 LLVM JIT makes this completely trivial). This means that you can use the 139 'extern' keyword to define a function before you use it (this is also 140 useful for mutually recursive functions). For example: 141 142 :: 143 144 extern sin(arg); 145 extern cos(arg); 146 extern atan2(arg1 arg2); 147 148 atan2(sin(.4), cos(42)) 149 150 A more interesting example is included in Chapter 6 where we write a 151 little Kaleidoscope application that `displays a Mandelbrot 152 Set <LangImpl06.html#kicking-the-tires>`_ at various levels of magnification. 153 154 Lets dive into the implementation of this language! 155 156 The Lexer 157 ========= 158 159 When it comes to implementing a language, the first thing needed is the 160 ability to process a text file and recognize what it says. The 161 traditional way to do this is to use a 162 "`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka 163 'scanner') to break the input up into "tokens". Each token returned by 164 the lexer includes a token code and potentially some metadata (e.g. the 165 numeric value of a number). First, we define the possibilities: 166 167 .. code-block:: c++ 168 169 // The lexer returns tokens [0-255] if it is an unknown character, otherwise one 170 // of these for known things. 171 enum Token { 172 tok_eof = -1, 173 174 // commands 175 tok_def = -2, 176 tok_extern = -3, 177 178 // primary 179 tok_identifier = -4, 180 tok_number = -5, 181 }; 182 183 static std::string IdentifierStr; // Filled in if tok_identifier 184 static double NumVal; // Filled in if tok_number 185 186 Each token returned by our lexer will either be one of the Token enum 187 values or it will be an 'unknown' character like '+', which is returned 188 as its ASCII value. If the current token is an identifier, the 189 ``IdentifierStr`` global variable holds the name of the identifier. If 190 the current token is a numeric literal (like 1.0), ``NumVal`` holds its 191 value. Note that we use global variables for simplicity, this is not the 192 best choice for a real language implementation :). 193 194 The actual implementation of the lexer is a single function named 195 ``gettok``. The ``gettok`` function is called to return the next token 196 from standard input. Its definition starts as: 197 198 .. code-block:: c++ 199 200 /// gettok - Return the next token from standard input. 201 static int gettok() { 202 static int LastChar = ' '; 203 204 // Skip any whitespace. 205 while (isspace(LastChar)) 206 LastChar = getchar(); 207 208 ``gettok`` works by calling the C ``getchar()`` function to read 209 characters one at a time from standard input. It eats them as it 210 recognizes them and stores the last character read, but not processed, 211 in LastChar. The first thing that it has to do is ignore whitespace 212 between tokens. This is accomplished with the loop above. 213 214 The next thing ``gettok`` needs to do is recognize identifiers and 215 specific keywords like "def". Kaleidoscope does this with this simple 216 loop: 217 218 .. code-block:: c++ 219 220 if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]* 221 IdentifierStr = LastChar; 222 while (isalnum((LastChar = getchar()))) 223 IdentifierStr += LastChar; 224 225 if (IdentifierStr == "def") 226 return tok_def; 227 if (IdentifierStr == "extern") 228 return tok_extern; 229 return tok_identifier; 230 } 231 232 Note that this code sets the '``IdentifierStr``' global whenever it 233 lexes an identifier. Also, since language keywords are matched by the 234 same loop, we handle them here inline. Numeric values are similar: 235 236 .. code-block:: c++ 237 238 if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+ 239 std::string NumStr; 240 do { 241 NumStr += LastChar; 242 LastChar = getchar(); 243 } while (isdigit(LastChar) || LastChar == '.'); 244 245 NumVal = strtod(NumStr.c_str(), 0); 246 return tok_number; 247 } 248 249 This is all pretty straight-forward code for processing input. When 250 reading a numeric value from input, we use the C ``strtod`` function to 251 convert it to a numeric value that we store in ``NumVal``. Note that 252 this isn't doing sufficient error checking: it will incorrectly read 253 "1.23.45.67" and handle it as if you typed in "1.23". Feel free to 254 extend it :). Next we handle comments: 255 256 .. code-block:: c++ 257 258 if (LastChar == '#') { 259 // Comment until end of line. 260 do 261 LastChar = getchar(); 262 while (LastChar != EOF && LastChar != '\n' && LastChar != '\r'); 263 264 if (LastChar != EOF) 265 return gettok(); 266 } 267 268 We handle comments by skipping to the end of the line and then return 269 the next token. Finally, if the input doesn't match one of the above 270 cases, it is either an operator character like '+' or the end of the 271 file. These are handled with this code: 272 273 .. code-block:: c++ 274 275 // Check for end of file. Don't eat the EOF. 276 if (LastChar == EOF) 277 return tok_eof; 278 279 // Otherwise, just return the character as its ascii value. 280 int ThisChar = LastChar; 281 LastChar = getchar(); 282 return ThisChar; 283 } 284 285 With this, we have the complete lexer for the basic Kaleidoscope 286 language (the `full code listing <LangImpl02.html#full-code-listing>`_ for the Lexer 287 is available in the `next chapter <LangImpl02.html>`_ of the tutorial). 288 Next we'll `build a simple parser that uses this to build an Abstract 289 Syntax Tree <LangImpl02.html>`_. When we have that, we'll include a 290 driver so that you can use the lexer and parser together. 291 292 `Next: Implementing a Parser and AST <LangImpl02.html>`_ 293 294
