1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 4 <html> 5 <head> 6 <title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title> 7 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 8 <meta name="author" content="Chris Lattner"> 9 <link rel="stylesheet" href="../llvm.css" type="text/css"> 10 </head> 11 12 <body> 13 14 <h1>Kaleidoscope: Conclusion and other useful LLVM tidbits</h1> 15 16 <ul> 17 <li><a href="index.html">Up to Tutorial Index</a></li> 18 <li>Chapter 8 19 <ol> 20 <li><a href="#conclusion">Tutorial Conclusion</a></li> 21 <li><a href="#llvmirproperties">Properties of LLVM IR</a> 22 <ul> 23 <li><a href="#targetindep">Target Independence</a></li> 24 <li><a href="#safety">Safety Guarantees</a></li> 25 <li><a href="#langspecific">Language-Specific Optimizations</a></li> 26 </ul> 27 </li> 28 <li><a href="#tipsandtricks">Tips and Tricks</a> 29 <ul> 30 <li><a href="#offsetofsizeof">Implementing portable 31 offsetof/sizeof</a></li> 32 <li><a href="#gcstack">Garbage Collected Stack Frames</a></li> 33 </ul> 34 </li> 35 </ol> 36 </li> 37 </ul> 38 39 40 <div class="doc_author"> 41 <p>Written by <a href="mailto:sabre (a] nondot.org">Chris Lattner</a></p> 42 </div> 43 44 <!-- *********************************************************************** --> 45 <h2><a name="conclusion">Tutorial Conclusion</a></h2> 46 <!-- *********************************************************************** --> 47 48 <div> 49 50 <p>Welcome to the the final chapter of the "<a href="index.html">Implementing a 51 language with LLVM</a>" tutorial. In the course of this tutorial, we have grown 52 our little Kaleidoscope language from being a useless toy, to being a 53 semi-interesting (but probably still useless) toy. :)</p> 54 55 <p>It is interesting to see how far we've come, and how little code it has 56 taken. We built the entire lexer, parser, AST, code generator, and an 57 interactive run-loop (with a JIT!) by-hand in under 700 lines of 58 (non-comment/non-blank) code.</p> 59 60 <p>Our little language supports a couple of interesting features: it supports 61 user defined binary and unary operators, it uses JIT compilation for immediate 62 evaluation, and it supports a few control flow constructs with SSA construction. 63 </p> 64 65 <p>Part of the idea of this tutorial was to show you how easy and fun it can be 66 to define, build, and play with languages. Building a compiler need not be a 67 scary or mystical process! Now that you've seen some of the basics, I strongly 68 encourage you to take the code and hack on it. For example, try adding:</p> 69 70 <ul> 71 <li><b>global variables</b> - While global variables have questional value in 72 modern software engineering, they are often useful when putting together quick 73 little hacks like the Kaleidoscope compiler itself. Fortunately, our current 74 setup makes it very easy to add global variables: just have value lookup check 75 to see if an unresolved variable is in the global variable symbol table before 76 rejecting it. To create a new global variable, make an instance of the LLVM 77 <tt>GlobalVariable</tt> class.</li> 78 79 <li><b>typed variables</b> - Kaleidoscope currently only supports variables of 80 type double. This gives the language a very nice elegance, because only 81 supporting one type means that you never have to specify types. Different 82 languages have different ways of handling this. The easiest way is to require 83 the user to specify types for every variable definition, and record the type 84 of the variable in the symbol table along with its Value*.</li> 85 86 <li><b>arrays, structs, vectors, etc</b> - Once you add types, you can start 87 extending the type system in all sorts of interesting ways. Simple arrays are 88 very easy and are quite useful for many different applications. Adding them is 89 mostly an exercise in learning how the LLVM <a 90 href="../LangRef.html#i_getelementptr">getelementptr</a> instruction works: it 91 is so nifty/unconventional, it <a 92 href="../GetElementPtr.html">has its own FAQ</a>! If you add support 93 for recursive types (e.g. linked lists), make sure to read the <a 94 href="../ProgrammersManual.html#TypeResolve">section in the LLVM 95 Programmer's Manual</a> that describes how to construct them.</li> 96 97 <li><b>standard runtime</b> - Our current language allows the user to access 98 arbitrary external functions, and we use it for things like "printd" and 99 "putchard". As you extend the language to add higher-level constructs, often 100 these constructs make the most sense if they are lowered to calls into a 101 language-supplied runtime. For example, if you add hash tables to the language, 102 it would probably make sense to add the routines to a runtime, instead of 103 inlining them all the way.</li> 104 105 <li><b>memory management</b> - Currently we can only access the stack in 106 Kaleidoscope. It would also be useful to be able to allocate heap memory, 107 either with calls to the standard libc malloc/free interface or with a garbage 108 collector. If you would like to use garbage collection, note that LLVM fully 109 supports <a href="../GarbageCollection.html">Accurate Garbage Collection</a> 110 including algorithms that move objects and need to scan/update the stack.</li> 111 112 <li><b>debugger support</b> - LLVM supports generation of <a 113 href="../SourceLevelDebugging.html">DWARF Debug info</a> which is understood by 114 common debuggers like GDB. Adding support for debug info is fairly 115 straightforward. The best way to understand it is to compile some C/C++ code 116 with "<tt>llvm-gcc -g -O0</tt>" and taking a look at what it produces.</li> 117 118 <li><b>exception handling support</b> - LLVM supports generation of <a 119 href="../ExceptionHandling.html">zero cost exceptions</a> which interoperate 120 with code compiled in other languages. You could also generate code by 121 implicitly making every function return an error value and checking it. You 122 could also make explicit use of setjmp/longjmp. There are many different ways 123 to go here.</li> 124 125 <li><b>object orientation, generics, database access, complex numbers, 126 geometric programming, ...</b> - Really, there is 127 no end of crazy features that you can add to the language.</li> 128 129 <li><b>unusual domains</b> - We've been talking about applying LLVM to a domain 130 that many people are interested in: building a compiler for a specific language. 131 However, there are many other domains that can use compiler technology that are 132 not typically considered. For example, LLVM has been used to implement OpenGL 133 graphics acceleration, translate C++ code to ActionScript, and many other 134 cute and clever things. Maybe you will be the first to JIT compile a regular 135 expression interpreter into native code with LLVM?</li> 136 137 </ul> 138 139 <p> 140 Have fun - try doing something crazy and unusual. Building a language like 141 everyone else always has, is much less fun than trying something a little crazy 142 or off the wall and seeing how it turns out. If you get stuck or want to talk 143 about it, feel free to email the <a 144 href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing 145 list</a>: it has lots of people who are interested in languages and are often 146 willing to help out. 147 </p> 148 149 <p>Before we end this tutorial, I want to talk about some "tips and tricks" for generating 150 LLVM IR. These are some of the more subtle things that may not be obvious, but 151 are very useful if you want to take advantage of LLVM's capabilities.</p> 152 153 </div> 154 155 <!-- *********************************************************************** --> 156 <h2><a name="llvmirproperties">Properties of the LLVM IR</a></h2> 157 <!-- *********************************************************************** --> 158 159 <div> 160 161 <p>We have a couple common questions about code in the LLVM IR form - lets just 162 get these out of the way right now, shall we?</p> 163 164 <!-- ======================================================================= --> 165 <h4><a name="targetindep">Target Independence</a></h4> 166 <!-- ======================================================================= --> 167 168 <div> 169 170 <p>Kaleidoscope is an example of a "portable language": any program written in 171 Kaleidoscope will work the same way on any target that it runs on. Many other 172 languages have this property, e.g. lisp, java, haskell, javascript, python, etc 173 (note that while these languages are portable, not all their libraries are).</p> 174 175 <p>One nice aspect of LLVM is that it is often capable of preserving target 176 independence in the IR: you can take the LLVM IR for a Kaleidoscope-compiled 177 program and run it on any target that LLVM supports, even emitting C code and 178 compiling that on targets that LLVM doesn't support natively. You can trivially 179 tell that the Kaleidoscope compiler generates target-independent code because it 180 never queries for any target-specific information when generating code.</p> 181 182 <p>The fact that LLVM provides a compact, target-independent, representation for 183 code gets a lot of people excited. Unfortunately, these people are usually 184 thinking about C or a language from the C family when they are asking questions 185 about language portability. I say "unfortunately", because there is really no 186 way to make (fully general) C code portable, other than shipping the source code 187 around (and of course, C source code is not actually portable in general 188 either - ever port a really old application from 32- to 64-bits?).</p> 189 190 <p>The problem with C (again, in its full generality) is that it is heavily 191 laden with target specific assumptions. As one simple example, the preprocessor 192 often destructively removes target-independence from the code when it processes 193 the input text:</p> 194 195 <div class="doc_code"> 196 <pre> 197 #ifdef __i386__ 198 int X = 1; 199 #else 200 int X = 42; 201 #endif 202 </pre> 203 </div> 204 205 <p>While it is possible to engineer more and more complex solutions to problems 206 like this, it cannot be solved in full generality in a way that is better than shipping 207 the actual source code.</p> 208 209 <p>That said, there are interesting subsets of C that can be made portable. If 210 you are willing to fix primitive types to a fixed size (say int = 32-bits, 211 and long = 64-bits), don't care about ABI compatibility with existing binaries, 212 and are willing to give up some other minor features, you can have portable 213 code. This can make sense for specialized domains such as an 214 in-kernel language.</p> 215 216 </div> 217 218 <!-- ======================================================================= --> 219 <h4><a name="safety">Safety Guarantees</a></h4> 220 <!-- ======================================================================= --> 221 222 <div> 223 224 <p>Many of the languages above are also "safe" languages: it is impossible for 225 a program written in Java to corrupt its address space and crash the process 226 (assuming the JVM has no bugs). 227 Safety is an interesting property that requires a combination of language 228 design, runtime support, and often operating system support.</p> 229 230 <p>It is certainly possible to implement a safe language in LLVM, but LLVM IR 231 does not itself guarantee safety. The LLVM IR allows unsafe pointer casts, 232 use after free bugs, buffer over-runs, and a variety of other problems. Safety 233 needs to be implemented as a layer on top of LLVM and, conveniently, several 234 groups have investigated this. Ask on the <a 235 href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing 236 list</a> if you are interested in more details.</p> 237 238 </div> 239 240 <!-- ======================================================================= --> 241 <h4><a name="langspecific">Language-Specific Optimizations</a></h4> 242 <!-- ======================================================================= --> 243 244 <div> 245 246 <p>One thing about LLVM that turns off many people is that it does not solve all 247 the world's problems in one system (sorry 'world hunger', someone else will have 248 to solve you some other day). One specific complaint is that people perceive 249 LLVM as being incapable of performing high-level language-specific optimization: 250 LLVM "loses too much information".</p> 251 252 <p>Unfortunately, this is really not the place to give you a full and unified 253 version of "Chris Lattner's theory of compiler design". Instead, I'll make a 254 few observations:</p> 255 256 <p>First, you're right that LLVM does lose information. For example, as of this 257 writing, there is no way to distinguish in the LLVM IR whether an SSA-value came 258 from a C "int" or a C "long" on an ILP32 machine (other than debug info). Both 259 get compiled down to an 'i32' value and the information about what it came from 260 is lost. The more general issue here, is that the LLVM type system uses 261 "structural equivalence" instead of "name equivalence". Another place this 262 surprises people is if you have two types in a high-level language that have the 263 same structure (e.g. two different structs that have a single int field): these 264 types will compile down into a single LLVM type and it will be impossible to 265 tell what it came from.</p> 266 267 <p>Second, while LLVM does lose information, LLVM is not a fixed target: we 268 continue to enhance and improve it in many different ways. In addition to 269 adding new features (LLVM did not always support exceptions or debug info), we 270 also extend the IR to capture important information for optimization (e.g. 271 whether an argument is sign or zero extended, information about pointers 272 aliasing, etc). Many of the enhancements are user-driven: people want LLVM to 273 include some specific feature, so they go ahead and extend it.</p> 274 275 <p>Third, it is <em>possible and easy</em> to add language-specific 276 optimizations, and you have a number of choices in how to do it. As one trivial 277 example, it is easy to add language-specific optimization passes that 278 "know" things about code compiled for a language. In the case of the C family, 279 there is an optimization pass that "knows" about the standard C library 280 functions. If you call "exit(0)" in main(), it knows that it is safe to 281 optimize that into "return 0;" because C specifies what the 'exit' 282 function does.</p> 283 284 <p>In addition to simple library knowledge, it is possible to embed a variety of 285 other language-specific information into the LLVM IR. If you have a specific 286 need and run into a wall, please bring the topic up on the llvmdev list. At the 287 very worst, you can always treat LLVM as if it were a "dumb code generator" and 288 implement the high-level optimizations you desire in your front-end, on the 289 language-specific AST. 290 </p> 291 292 </div> 293 294 </div> 295 296 <!-- *********************************************************************** --> 297 <h2><a name="tipsandtricks">Tips and Tricks</a></h2> 298 <!-- *********************************************************************** --> 299 300 <div> 301 302 <p>There is a variety of useful tips and tricks that you come to know after 303 working on/with LLVM that aren't obvious at first glance. Instead of letting 304 everyone rediscover them, this section talks about some of these issues.</p> 305 306 <!-- ======================================================================= --> 307 <h4><a name="offsetofsizeof">Implementing portable offsetof/sizeof</a></h4> 308 <!-- ======================================================================= --> 309 310 <div> 311 312 <p>One interesting thing that comes up, if you are trying to keep the code 313 generated by your compiler "target independent", is that you often need to know 314 the size of some LLVM type or the offset of some field in an llvm structure. 315 For example, you might need to pass the size of a type into a function that 316 allocates memory.</p> 317 318 <p>Unfortunately, this can vary widely across targets: for example the width of 319 a pointer is trivially target-specific. However, there is a <a 320 href="http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt">clever 321 way to use the getelementptr instruction</a> that allows you to compute this 322 in a portable way.</p> 323 324 </div> 325 326 <!-- ======================================================================= --> 327 <h4><a name="gcstack">Garbage Collected Stack Frames</a></h4> 328 <!-- ======================================================================= --> 329 330 <div> 331 332 <p>Some languages want to explicitly manage their stack frames, often so that 333 they are garbage collected or to allow easy implementation of closures. There 334 are often better ways to implement these features than explicit stack frames, 335 but <a 336 href="http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt">LLVM 337 does support them,</a> if you want. It requires your front-end to convert the 338 code into <a 339 href="http://en.wikipedia.org/wiki/Continuation-passing_style">Continuation 340 Passing Style</a> and the use of tail calls (which LLVM also supports).</p> 341 342 </div> 343 344 </div> 345 346 <!-- *********************************************************************** --> 347 <hr> 348 <address> 349 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 350 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a> 351 <a href="http://validator.w3.org/check/referer"><img 352 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a> 353 354 <a href="mailto:sabre (a] nondot.org">Chris Lattner</a><br> 355 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> 356 Last modified: $Date$ 357 </address> 358 </body> 359 </html> 360
