1====================================================== 2Kaleidoscope: Conclusion and other useful LLVM tidbits 3====================================================== 4 5.. contents:: 6 :local: 7 8Tutorial Conclusion 9=================== 10 11Welcome to the final chapter of the "`Implementing a language with 12LLVM <index.html>`_" tutorial. In the course of this tutorial, we have 13grown our little Kaleidoscope language from being a useless toy, to 14being a semi-interesting (but probably still useless) toy. :) 15 16It is interesting to see how far we've come, and how little code it has 17taken. We built the entire lexer, parser, AST, code generator, an 18interactive run-loop (with a JIT!), and emitted debug information in 19standalone executables - all in under 1000 lines of (non-comment/non-blank) 20code. 21 22Our little language supports a couple of interesting features: it 23supports user defined binary and unary operators, it uses JIT 24compilation for immediate evaluation, and it supports a few control flow 25constructs with SSA construction. 26 27Part of the idea of this tutorial was to show you how easy and fun it 28can be to define, build, and play with languages. Building a compiler 29need not be a scary or mystical process! Now that you've seen some of 30the basics, I strongly encourage you to take the code and hack on it. 31For example, try adding: 32 33- **global variables** - While global variables have questional value 34 in modern software engineering, they are often useful when putting 35 together quick little hacks like the Kaleidoscope compiler itself. 36 Fortunately, our current setup makes it very easy to add global 37 variables: just have value lookup check to see if an unresolved 38 variable is in the global variable symbol table before rejecting it. 39 To create a new global variable, make an instance of the LLVM 40 ``GlobalVariable`` class. 41- **typed variables** - Kaleidoscope currently only supports variables 42 of type double. This gives the language a very nice elegance, because 43 only supporting one type means that you never have to specify types. 44 Different languages have different ways of handling this. The easiest 45 way is to require the user to specify types for every variable 46 definition, and record the type of the variable in the symbol table 47 along with its Value\*. 48- **arrays, structs, vectors, etc** - Once you add types, you can start 49 extending the type system in all sorts of interesting ways. Simple 50 arrays are very easy and are quite useful for many different 51 applications. Adding them is mostly an exercise in learning how the 52 LLVM `getelementptr <../LangRef.html#getelementptr-instruction>`_ instruction 53 works: it is so nifty/unconventional, it `has its own 54 FAQ <../GetElementPtr.html>`_! If you add support for recursive types 55 (e.g. linked lists), make sure to read the `section in the LLVM 56 Programmer's Manual <../ProgrammersManual.html#TypeResolve>`_ that 57 describes how to construct them. 58- **standard runtime** - Our current language allows the user to access 59 arbitrary external functions, and we use it for things like "printd" 60 and "putchard". As you extend the language to add higher-level 61 constructs, often these constructs make the most sense if they are 62 lowered to calls into a language-supplied runtime. For example, if 63 you add hash tables to the language, it would probably make sense to 64 add the routines to a runtime, instead of inlining them all the way. 65- **memory management** - Currently we can only access the stack in 66 Kaleidoscope. It would also be useful to be able to allocate heap 67 memory, either with calls to the standard libc malloc/free interface 68 or with a garbage collector. If you would like to use garbage 69 collection, note that LLVM fully supports `Accurate Garbage 70 Collection <../GarbageCollection.html>`_ including algorithms that 71 move objects and need to scan/update the stack. 72- **exception handling support** - LLVM supports generation of `zero 73 cost exceptions <../ExceptionHandling.html>`_ which interoperate with 74 code compiled in other languages. You could also generate code by 75 implicitly making every function return an error value and checking 76 it. You could also make explicit use of setjmp/longjmp. There are 77 many different ways to go here. 78- **object orientation, generics, database access, complex numbers, 79 geometric programming, ...** - Really, there is no end of crazy 80 features that you can add to the language. 81- **unusual domains** - We've been talking about applying LLVM to a 82 domain that many people are interested in: building a compiler for a 83 specific language. However, there are many other domains that can use 84 compiler technology that are not typically considered. For example, 85 LLVM has been used to implement OpenGL graphics acceleration, 86 translate C++ code to ActionScript, and many other cute and clever 87 things. Maybe you will be the first to JIT compile a regular 88 expression interpreter into native code with LLVM? 89 90Have fun - try doing something crazy and unusual. Building a language 91like everyone else always has, is much less fun than trying something a 92little crazy or off the wall and seeing how it turns out. If you get 93stuck or want to talk about it, feel free to email the `llvm-dev mailing 94list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_: it has lots 95of people who are interested in languages and are often willing to help 96out. 97 98Before we end this tutorial, I want to talk about some "tips and tricks" 99for generating LLVM IR. These are some of the more subtle things that 100may not be obvious, but are very useful if you want to take advantage of 101LLVM's capabilities. 102 103Properties of the LLVM IR 104========================= 105 106We have a couple common questions about code in the LLVM IR form - lets 107just get these out of the way right now, shall we? 108 109Target Independence 110------------------- 111 112Kaleidoscope is an example of a "portable language": any program written 113in Kaleidoscope will work the same way on any target that it runs on. 114Many other languages have this property, e.g. lisp, java, haskell, 115javascript, python, etc (note that while these languages are portable, 116not all their libraries are). 117 118One nice aspect of LLVM is that it is often capable of preserving target 119independence in the IR: you can take the LLVM IR for a 120Kaleidoscope-compiled program and run it on any target that LLVM 121supports, even emitting C code and compiling that on targets that LLVM 122doesn't support natively. You can trivially tell that the Kaleidoscope 123compiler generates target-independent code because it never queries for 124any target-specific information when generating code. 125 126The fact that LLVM provides a compact, target-independent, 127representation for code gets a lot of people excited. Unfortunately, 128these people are usually thinking about C or a language from the C 129family when they are asking questions about language portability. I say 130"unfortunately", because there is really no way to make (fully general) 131C code portable, other than shipping the source code around (and of 132course, C source code is not actually portable in general either - ever 133port a really old application from 32- to 64-bits?). 134 135The problem with C (again, in its full generality) is that it is heavily 136laden with target specific assumptions. As one simple example, the 137preprocessor often destructively removes target-independence from the 138code when it processes the input text: 139 140.. code-block:: c 141 142 #ifdef __i386__ 143 int X = 1; 144 #else 145 int X = 42; 146 #endif 147 148While it is possible to engineer more and more complex solutions to 149problems like this, it cannot be solved in full generality in a way that 150is better than shipping the actual source code. 151 152That said, there are interesting subsets of C that can be made portable. 153If you are willing to fix primitive types to a fixed size (say int = 15432-bits, and long = 64-bits), don't care about ABI compatibility with 155existing binaries, and are willing to give up some other minor features, 156you can have portable code. This can make sense for specialized domains 157such as an in-kernel language. 158 159Safety Guarantees 160----------------- 161 162Many of the languages above are also "safe" languages: it is impossible 163for a program written in Java to corrupt its address space and crash the 164process (assuming the JVM has no bugs). Safety is an interesting 165property that requires a combination of language design, runtime 166support, and often operating system support. 167 168It is certainly possible to implement a safe language in LLVM, but LLVM 169IR does not itself guarantee safety. The LLVM IR allows unsafe pointer 170casts, use after free bugs, buffer over-runs, and a variety of other 171problems. Safety needs to be implemented as a layer on top of LLVM and, 172conveniently, several groups have investigated this. Ask on the `llvm-dev 173mailing list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_ if 174you are interested in more details. 175 176Language-Specific Optimizations 177------------------------------- 178 179One thing about LLVM that turns off many people is that it does not 180solve all the world's problems in one system (sorry 'world hunger', 181someone else will have to solve you some other day). One specific 182complaint is that people perceive LLVM as being incapable of performing 183high-level language-specific optimization: LLVM "loses too much 184information". 185 186Unfortunately, this is really not the place to give you a full and 187unified version of "Chris Lattner's theory of compiler design". Instead, 188I'll make a few observations: 189 190First, you're right that LLVM does lose information. For example, as of 191this writing, there is no way to distinguish in the LLVM IR whether an 192SSA-value came from a C "int" or a C "long" on an ILP32 machine (other 193than debug info). Both get compiled down to an 'i32' value and the 194information about what it came from is lost. The more general issue 195here, is that the LLVM type system uses "structural equivalence" instead 196of "name equivalence". Another place this surprises people is if you 197have two types in a high-level language that have the same structure 198(e.g. two different structs that have a single int field): these types 199will compile down into a single LLVM type and it will be impossible to 200tell what it came from. 201 202Second, while LLVM does lose information, LLVM is not a fixed target: we 203continue to enhance and improve it in many different ways. In addition 204to adding new features (LLVM did not always support exceptions or debug 205info), we also extend the IR to capture important information for 206optimization (e.g. whether an argument is sign or zero extended, 207information about pointers aliasing, etc). Many of the enhancements are 208user-driven: people want LLVM to include some specific feature, so they 209go ahead and extend it. 210 211Third, it is *possible and easy* to add language-specific optimizations, 212and you have a number of choices in how to do it. As one trivial 213example, it is easy to add language-specific optimization passes that 214"know" things about code compiled for a language. In the case of the C 215family, there is an optimization pass that "knows" about the standard C 216library functions. If you call "exit(0)" in main(), it knows that it is 217safe to optimize that into "return 0;" because C specifies what the 218'exit' function does. 219 220In addition to simple library knowledge, it is possible to embed a 221variety of other language-specific information into the LLVM IR. If you 222have a specific need and run into a wall, please bring the topic up on 223the llvm-dev list. At the very worst, you can always treat LLVM as if it 224were a "dumb code generator" and implement the high-level optimizations 225you desire in your front-end, on the language-specific AST. 226 227Tips and Tricks 228=============== 229 230There is a variety of useful tips and tricks that you come to know after 231working on/with LLVM that aren't obvious at first glance. Instead of 232letting everyone rediscover them, this section talks about some of these 233issues. 234 235Implementing portable offsetof/sizeof 236------------------------------------- 237 238One interesting thing that comes up, if you are trying to keep the code 239generated by your compiler "target independent", is that you often need 240to know the size of some LLVM type or the offset of some field in an 241llvm structure. For example, you might need to pass the size of a type 242into a function that allocates memory. 243 244Unfortunately, this can vary widely across targets: for example the 245width of a pointer is trivially target-specific. However, there is a 246`clever way to use the getelementptr 247instruction <http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt>`_ 248that allows you to compute this in a portable way. 249 250Garbage Collected Stack Frames 251------------------------------ 252 253Some languages want to explicitly manage their stack frames, often so 254that they are garbage collected or to allow easy implementation of 255closures. There are often better ways to implement these features than 256explicit stack frames, but `LLVM does support 257them, <http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt>`_ 258if you want. It requires your front-end to convert the code into 259`Continuation Passing 260Style <http://en.wikipedia.org/wiki/Continuation-passing_style>`_ and 261the use of tail calls (which LLVM also supports). 262 263