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