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 <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>`_: Extending the Language: Debug 77 Information - Having built a decent little programming language with 78 control flow, functions and mutable variables, we consider what it 79 takes to add debug information to standalone executables. This debug 80 information will allow you to set breakpoints in Kaleidoscope 81 functions, print out argument variables, and call functions - all 82 from within the debugger! 83 - `Chapter #9 <LangImpl8.html>`_: Conclusion and other useful LLVM 84 tidbits - This chapter wraps up the series by talking about 85 potential ways to extend the language, but also includes a bunch of 86 pointers to info about "special topics" like adding garbage 87 collection support, exceptions, debugging, support for "spaghetti 88 stacks", and a bunch of other tips and tricks. 89 90 By the end of the tutorial, we'll have written a bit less than 1000 lines 91 of non-comment, non-blank, lines of code. With this small amount of 92 code, we'll have built up a very reasonable compiler for a non-trivial 93 language including a hand-written lexer, parser, AST, as well as code 94 generation support with a JIT compiler. While other systems may have 95 interesting "hello world" tutorials, I think the breadth of this 96 tutorial is a great testament to the strengths of LLVM and why you 97 should consider it if you're interested in language or compiler design. 98 99 A note about this tutorial: we expect you to extend the language and 100 play with it on your own. Take the code and go crazy hacking away at it, 101 compilers don't need to be scary creatures - it can be a lot of fun to 102 play with languages! 103 104 The Basic Language 105 ================== 106 107 This tutorial will be illustrated with a toy language that we'll call 108 "`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived 109 from "meaning beautiful, form, and view"). Kaleidoscope is a procedural 110 language that allows you to define functions, use conditionals, math, 111 etc. Over the course of the tutorial, we'll extend Kaleidoscope to 112 support the if/then/else construct, a for loop, user defined operators, 113 JIT compilation with a simple command line interface, etc. 114 115 Because we want to keep things simple, the only datatype in Kaleidoscope 116 is a 64-bit floating point type (aka 'double' in C parlance). As such, 117 all values are implicitly double precision and the language doesn't 118 require type declarations. This gives the language a very nice and 119 simple syntax. For example, the following simple example computes 120 `Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_ 121 122 :: 123 124 # Compute the x'th fibonacci number. 125 def fib(x) 126 if x < 3 then 127 1 128 else 129 fib(x-1)+fib(x-2) 130 131 # This expression will compute the 40th number. 132 fib(40) 133 134 We also allow Kaleidoscope to call into standard library functions (the 135 LLVM JIT makes this completely trivial). This means that you can use the 136 'extern' keyword to define a function before you use it (this is also 137 useful for mutually recursive functions). For example: 138 139 :: 140 141 extern sin(arg); 142 extern cos(arg); 143 extern atan2(arg1 arg2); 144 145 atan2(sin(.4), cos(42)) 146 147 A more interesting example is included in Chapter 6 where we write a 148 little Kaleidoscope application that `displays a Mandelbrot 149 Set <LangImpl6.html#kicking-the-tires>`_ at various levels of magnification. 150 151 Lets dive into the implementation of this language! 152 153 The Lexer 154 ========= 155 156 When it comes to implementing a language, the first thing needed is the 157 ability to process a text file and recognize what it says. The 158 traditional way to do this is to use a 159 "`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka 160 'scanner') to break the input up into "tokens". Each token returned by 161 the lexer includes a token code and potentially some metadata (e.g. the 162 numeric value of a number). First, we define the possibilities: 163 164 .. code-block:: c++ 165 166 // The lexer returns tokens [0-255] if it is an unknown character, otherwise one 167 // of these for known things. 168 enum Token { 169 tok_eof = -1, 170 171 // commands 172 tok_def = -2, 173 tok_extern = -3, 174 175 // primary 176 tok_identifier = -4, 177 tok_number = -5, 178 }; 179 180 static std::string IdentifierStr; // Filled in if tok_identifier 181 static double NumVal; // Filled in if tok_number 182 183 Each token returned by our lexer will either be one of the Token enum 184 values or it will be an 'unknown' character like '+', which is returned 185 as its ASCII value. If the current token is an identifier, the 186 ``IdentifierStr`` global variable holds the name of the identifier. If 187 the current token is a numeric literal (like 1.0), ``NumVal`` holds its 188 value. Note that we use global variables for simplicity, this is not the 189 best choice for a real language implementation :). 190 191 The actual implementation of the lexer is a single function named 192 ``gettok``. The ``gettok`` function is called to return the next token 193 from standard input. Its definition starts as: 194 195 .. code-block:: c++ 196 197 /// gettok - Return the next token from standard input. 198 static int gettok() { 199 static int LastChar = ' '; 200 201 // Skip any whitespace. 202 while (isspace(LastChar)) 203 LastChar = getchar(); 204 205 ``gettok`` works by calling the C ``getchar()`` function to read 206 characters one at a time from standard input. It eats them as it 207 recognizes them and stores the last character read, but not processed, 208 in LastChar. The first thing that it has to do is ignore whitespace 209 between tokens. This is accomplished with the loop above. 210 211 The next thing ``gettok`` needs to do is recognize identifiers and 212 specific keywords like "def". Kaleidoscope does this with this simple 213 loop: 214 215 .. code-block:: c++ 216 217 if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]* 218 IdentifierStr = LastChar; 219 while (isalnum((LastChar = getchar()))) 220 IdentifierStr += LastChar; 221 222 if (IdentifierStr == "def") 223 return tok_def; 224 if (IdentifierStr == "extern") 225 return tok_extern; 226 return tok_identifier; 227 } 228 229 Note that this code sets the '``IdentifierStr``' global whenever it 230 lexes an identifier. Also, since language keywords are matched by the 231 same loop, we handle them here inline. Numeric values are similar: 232 233 .. code-block:: c++ 234 235 if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+ 236 std::string NumStr; 237 do { 238 NumStr += LastChar; 239 LastChar = getchar(); 240 } while (isdigit(LastChar) || LastChar == '.'); 241 242 NumVal = strtod(NumStr.c_str(), 0); 243 return tok_number; 244 } 245 246 This is all pretty straight-forward code for processing input. When 247 reading a numeric value from input, we use the C ``strtod`` function to 248 convert it to a numeric value that we store in ``NumVal``. Note that 249 this isn't doing sufficient error checking: it will incorrectly read 250 "1.23.45.67" and handle it as if you typed in "1.23". Feel free to 251 extend it :). Next we handle comments: 252 253 .. code-block:: c++ 254 255 if (LastChar == '#') { 256 // Comment until end of line. 257 do 258 LastChar = getchar(); 259 while (LastChar != EOF && LastChar != '\n' && LastChar != '\r'); 260 261 if (LastChar != EOF) 262 return gettok(); 263 } 264 265 We handle comments by skipping to the end of the line and then return 266 the next token. Finally, if the input doesn't match one of the above 267 cases, it is either an operator character like '+' or the end of the 268 file. These are handled with this code: 269 270 .. code-block:: c++ 271 272 // Check for end of file. Don't eat the EOF. 273 if (LastChar == EOF) 274 return tok_eof; 275 276 // Otherwise, just return the character as its ascii value. 277 int ThisChar = LastChar; 278 LastChar = getchar(); 279 return ThisChar; 280 } 281 282 With this, we have the complete lexer for the basic Kaleidoscope 283 language (the `full code listing <LangImpl2.html#full-code-listing>`_ for the Lexer 284 is available in the `next chapter <LangImpl2.html>`_ of the tutorial). 285 Next we'll `build a simple parser that uses this to build an Abstract 286 Syntax Tree <LangImpl2.html>`_. When we have that, we'll include a 287 driver so that you can use the lexer and parser together. 288 289 `Next: Implementing a Parser and AST <LangImpl2.html>`_ 290 291
