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diff --git a/docs/tutorial/OCamlLangImpl8.html b/docs/tutorial/OCamlLangImpl8.html new file mode 100644 index 0000000..64a6200 --- /dev/null +++ b/docs/tutorial/OCamlLangImpl8.html @@ -0,0 +1,365 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" + "http://www.w3.org/TR/html4/strict.dtd"> + +<html> +<head> + <title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title> + <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> + <meta name="author" content="Chris Lattner"> + <link rel="stylesheet" href="../llvm.css" type="text/css"> +</head> + +<body> + +<div class="doc_title">Kaleidoscope: Conclusion and other useful LLVM + tidbits</div> + +<ul> +<li><a href="index.html">Up to Tutorial Index</a></li> +<li>Chapter 8 + <ol> + <li><a href="#conclusion">Tutorial Conclusion</a></li> + <li><a href="#llvmirproperties">Properties of LLVM IR</a> + <ul> + <li><a href="#targetindep">Target Independence</a></li> + <li><a href="#safety">Safety Guarantees</a></li> + <li><a href="#langspecific">Language-Specific Optimizations</a></li> + </ul> + </li> + <li><a href="#tipsandtricks">Tips and Tricks</a> + <ul> + <li><a href="#offsetofsizeof">Implementing portable + offsetof/sizeof</a></li> + <li><a href="#gcstack">Garbage Collected Stack Frames</a></li> + </ul> + </li> + </ol> +</li> +</ul> + + +<div class="doc_author"> + <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p> +</div> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="conclusion">Tutorial Conclusion</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<p>Welcome to the the final chapter of the "<a href="index.html">Implementing a +language with LLVM</a>" tutorial. In the course of this tutorial, we have grown +our little Kaleidoscope language from being a useless toy, to being a +semi-interesting (but probably still useless) toy. :)</p> + +<p>It is interesting to see how far we've come, and how little code it has +taken. We built the entire lexer, parser, AST, code generator, and an +interactive run-loop (with a JIT!) by-hand in under 700 lines of +(non-comment/non-blank) code.</p> + +<p>Our little language supports a couple of interesting features: it supports +user defined binary and unary operators, it uses JIT compilation for immediate +evaluation, and it supports a few control flow constructs with SSA construction. +</p> + +<p>Part of the idea of this tutorial was to show you how easy and fun it can be +to define, build, and play with languages. Building a compiler need not be a +scary or mystical process! Now that you've seen some of the basics, I strongly +encourage you to take the code and hack on it. For example, try adding:</p> + +<ul> +<li><b>global variables</b> - While global variables have questional value in +modern software engineering, they are often useful when putting together quick +little hacks like the Kaleidoscope compiler itself. Fortunately, our current +setup makes it very easy to add global variables: just have value lookup check +to see if an unresolved variable is in the global variable symbol table before +rejecting it. To create a new global variable, make an instance of the LLVM +<tt>GlobalVariable</tt> class.</li> + +<li><b>typed variables</b> - Kaleidoscope currently only supports variables of +type double. This gives the language a very nice elegance, because only +supporting one type means that you never have to specify types. Different +languages have different ways of handling this. The easiest way is to require +the user to specify types for every variable definition, and record the type +of the variable in the symbol table along with its Value*.</li> + +<li><b>arrays, structs, vectors, etc</b> - Once you add types, you can start +extending the type system in all sorts of interesting ways. Simple arrays are +very easy and are quite useful for many different applications. Adding them is +mostly an exercise in learning how the LLVM <a +href="../LangRef.html#i_getelementptr">getelementptr</a> instruction works: it +is so nifty/unconventional, it <a +href="../GetElementPtr.html">has its own FAQ</a>! If you add support +for recursive types (e.g. linked lists), make sure to read the <a +href="../ProgrammersManual.html#TypeResolve">section in the LLVM +Programmer's Manual</a> that describes how to construct them.</li> + +<li><b>standard runtime</b> - Our current language allows the user to access +arbitrary external functions, and we use it for things like "printd" and +"putchard". As you extend the language to add higher-level constructs, often +these constructs make the most sense if they are lowered to calls into a +language-supplied runtime. For example, if you add hash tables to the language, +it would probably make sense to add the routines to a runtime, instead of +inlining them all the way.</li> + +<li><b>memory management</b> - Currently we can only access the stack in +Kaleidoscope. It would also be useful to be able to allocate heap memory, +either with calls to the standard libc malloc/free interface or with a garbage +collector. If you would like to use garbage collection, note that LLVM fully +supports <a href="../GarbageCollection.html">Accurate Garbage Collection</a> +including algorithms that move objects and need to scan/update the stack.</li> + +<li><b>debugger support</b> - LLVM supports generation of <a +href="../SourceLevelDebugging.html">DWARF Debug info</a> which is understood by +common debuggers like GDB. Adding support for debug info is fairly +straightforward. The best way to understand it is to compile some C/C++ code +with "<tt>llvm-gcc -g -O0</tt>" and taking a look at what it produces.</li> + +<li><b>exception handling support</b> - LLVM supports generation of <a +href="../ExceptionHandling.html">zero cost exceptions</a> which interoperate +with code compiled in other languages. You could also generate code by +implicitly making every function return an error value and checking it. You +could also make explicit use of setjmp/longjmp. There are many different ways +to go here.</li> + +<li><b>object orientation, generics, database access, complex numbers, +geometric programming, ...</b> - Really, there is +no end of crazy features that you can add to the language.</li> + +<li><b>unusual domains</b> - We've been talking about applying LLVM to a domain +that many people are interested in: building a compiler for a specific language. +However, there are many other domains that can use compiler technology that are +not typically considered. For example, LLVM has been used to implement OpenGL +graphics acceleration, translate C++ code to ActionScript, and many other +cute and clever things. Maybe you will be the first to JIT compile a regular +expression interpreter into native code with LLVM?</li> + +</ul> + +<p> +Have fun - try doing something crazy and unusual. Building a language like +everyone else always has, is much less fun than trying something a little crazy +or off the wall and seeing how it turns out. If you get stuck or want to talk +about it, feel free to email the <a +href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing +list</a>: it has lots of people who are interested in languages and are often +willing to help out. +</p> + +<p>Before we end this tutorial, I want to talk about some "tips and tricks" for generating +LLVM IR. These are some of the more subtle things that may not be obvious, but +are very useful if you want to take advantage of LLVM's capabilities.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="llvmirproperties">Properties of the LLVM +IR</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<p>We have a couple common questions about code in the LLVM IR form - lets just +get these out of the way right now, shall we?</p> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsubsection"><a name="targetindep">Target +Independence</a></div> +<!-- ======================================================================= --> + +<div class="doc_text"> + +<p>Kaleidoscope is an example of a "portable language": any program written in +Kaleidoscope will work the same way on any target that it runs on. Many other +languages have this property, e.g. lisp, java, haskell, javascript, python, etc +(note that while these languages are portable, not all their libraries are).</p> + +<p>One nice aspect of LLVM is that it is often capable of preserving target +independence in the IR: you can take the LLVM IR for a Kaleidoscope-compiled +program and run it on any target that LLVM supports, even emitting C code and +compiling that on targets that LLVM doesn't support natively. You can trivially +tell that the Kaleidoscope compiler generates target-independent code because it +never queries for any target-specific information when generating code.</p> + +<p>The fact that LLVM provides a compact, target-independent, representation for +code gets a lot of people excited. Unfortunately, these people are usually +thinking about C or a language from the C family when they are asking questions +about language portability. I say "unfortunately", because there is really no +way to make (fully general) C code portable, other than shipping the source code +around (and of course, C source code is not actually portable in general +either - ever port a really old application from 32- to 64-bits?).</p> + +<p>The problem with C (again, in its full generality) is that it is heavily +laden with target specific assumptions. As one simple example, the preprocessor +often destructively removes target-independence from the code when it processes +the input text:</p> + +<div class="doc_code"> +<pre> +#ifdef __i386__ + int X = 1; +#else + int X = 42; +#endif +</pre> +</div> + +<p>While it is possible to engineer more and more complex solutions to problems +like this, it cannot be solved in full generality in a way that is better than shipping +the actual source code.</p> + +<p>That said, there are interesting subsets of C that can be made portable. If +you are willing to fix primitive types to a fixed size (say int = 32-bits, +and long = 64-bits), don't care about ABI compatibility with existing binaries, +and are willing to give up some other minor features, you can have portable +code. This can make sense for specialized domains such as an +in-kernel language.</p> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsubsection"><a name="safety">Safety Guarantees</a></div> +<!-- ======================================================================= --> + +<div class="doc_text"> + +<p>Many of the languages above are also "safe" languages: it is impossible for +a program written in Java to corrupt its address space and crash the process +(assuming the JVM has no bugs). +Safety is an interesting property that requires a combination of language +design, runtime support, and often operating system support.</p> + +<p>It is certainly possible to implement a safe language in LLVM, but LLVM IR +does not itself guarantee safety. The LLVM IR allows unsafe pointer casts, +use after free bugs, buffer over-runs, and a variety of other problems. Safety +needs to be implemented as a layer on top of LLVM and, conveniently, several +groups have investigated this. Ask on the <a +href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing +list</a> if you are interested in more details.</p> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsubsection"><a name="langspecific">Language-Specific +Optimizations</a></div> +<!-- ======================================================================= --> + +<div class="doc_text"> + +<p>One thing about LLVM that turns off many people is that it does not solve all +the world's problems in one system (sorry 'world hunger', someone else will have +to solve you some other day). One specific complaint is that people perceive +LLVM as being incapable of performing high-level language-specific optimization: +LLVM "loses too much information".</p> + +<p>Unfortunately, this is really not the place to give you a full and unified +version of "Chris Lattner's theory of compiler design". Instead, I'll make a +few observations:</p> + +<p>First, you're right that LLVM does lose information. For example, as of this +writing, there is no way to distinguish in the LLVM IR whether an SSA-value came +from a C "int" or a C "long" on an ILP32 machine (other than debug info). Both +get compiled down to an 'i32' value and the information about what it came from +is lost. The more general issue here, is that the LLVM type system uses +"structural equivalence" instead of "name equivalence". Another place this +surprises people is if you have two types in a high-level language that have the +same structure (e.g. two different structs that have a single int field): these +types will compile down into a single LLVM type and it will be impossible to +tell what it came from.</p> + +<p>Second, while LLVM does lose information, LLVM is not a fixed target: we +continue to enhance and improve it in many different ways. In addition to +adding new features (LLVM did not always support exceptions or debug info), we +also extend the IR to capture important information for optimization (e.g. +whether an argument is sign or zero extended, information about pointers +aliasing, etc). Many of the enhancements are user-driven: people want LLVM to +include some specific feature, so they go ahead and extend it.</p> + +<p>Third, it is <em>possible and easy</em> to add language-specific +optimizations, and you have a number of choices in how to do it. As one trivial +example, it is easy to add language-specific optimization passes that +"know" things about code compiled for a language. In the case of the C family, +there is an optimization pass that "knows" about the standard C library +functions. If you call "exit(0)" in main(), it knows that it is safe to +optimize that into "return 0;" because C specifies what the 'exit' +function does.</p> + +<p>In addition to simple library knowledge, it is possible to embed a variety of +other language-specific information into the LLVM IR. If you have a specific +need and run into a wall, please bring the topic up on the llvmdev list. At the +very worst, you can always treat LLVM as if it were a "dumb code generator" and +implement the high-level optimizations you desire in your front-end, on the +language-specific AST. +</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="tipsandtricks">Tips and Tricks</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<p>There is a variety of useful tips and tricks that you come to know after +working on/with LLVM that aren't obvious at first glance. Instead of letting +everyone rediscover them, this section talks about some of these issues.</p> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsubsection"><a name="offsetofsizeof">Implementing portable +offsetof/sizeof</a></div> +<!-- ======================================================================= --> + +<div class="doc_text"> + +<p>One interesting thing that comes up, if you are trying to keep the code +generated by your compiler "target independent", is that you often need to know +the size of some LLVM type or the offset of some field in an llvm structure. +For example, you might need to pass the size of a type into a function that +allocates memory.</p> + +<p>Unfortunately, this can vary widely across targets: for example the width of +a pointer is trivially target-specific. However, there is a <a +href="http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt">clever +way to use the getelementptr instruction</a> that allows you to compute this +in a portable way.</p> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsubsection"><a name="gcstack">Garbage Collected +Stack Frames</a></div> +<!-- ======================================================================= --> + +<div class="doc_text"> + +<p>Some languages want to explicitly manage their stack frames, often so that +they are garbage collected or to allow easy implementation of closures. There +are often better ways to implement these features than explicit stack frames, +but <a +href="http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt">LLVM +does support them,</a> if you want. It requires your front-end to convert the +code into <a +href="http://en.wikipedia.org/wiki/Continuation-passing_style">Continuation +Passing Style</a> and the use of tail calls (which LLVM also supports).</p> + +</div> + +<!-- *********************************************************************** --> +<hr> +<address> + <a href="http://jigsaw.w3.org/css-validator/check/referer"><img + src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a> + <a href="http://validator.w3.org/check/referer"><img + src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a> + + <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> + <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br> + Last modified: $Date$ +</address> +</body> +</html> |