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<title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="author" content="Chris Lattner">
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<link rel="stylesheet" href="../_static/llvm.css" type="text/css">
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</head>
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<body>
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<h1>Kaleidoscope: Conclusion and other useful LLVM tidbits</h1>
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<ul>
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<li><a href="index.html">Up to Tutorial Index</a></li>
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<li>Chapter 8
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<ol>
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<li><a href="#conclusion">Tutorial Conclusion</a></li>
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<li><a href="#llvmirproperties">Properties of LLVM IR</a>
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<ul>
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<li><a href="#targetindep">Target Independence</a></li>
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<li><a href="#safety">Safety Guarantees</a></li>
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<li><a href="#langspecific">Language-Specific Optimizations</a></li>
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</ul>
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</li>
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<li><a href="#tipsandtricks">Tips and Tricks</a>
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<ul>
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<li><a href="#offsetofsizeof">Implementing portable
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offsetof/sizeof</a></li>
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<li><a href="#gcstack">Garbage Collected Stack Frames</a></li>
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</ul>
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</li>
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</ol>
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</li>
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</ul>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="conclusion">Tutorial Conclusion</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>Welcome to the final chapter of the "<a href="index.html">Implementing a
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language with LLVM</a>" tutorial. In the course of this tutorial, we have grown
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our little Kaleidoscope language from being a useless toy, to being a
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semi-interesting (but probably still useless) toy. :)</p>
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<p>It is interesting to see how far we've come, and how little code it has
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taken. We built the entire lexer, parser, AST, code generator, and an
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interactive run-loop (with a JIT!) by-hand in under 700 lines of
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(non-comment/non-blank) code.</p>
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<p>Our little language supports a couple of interesting features: it supports
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user defined binary and unary operators, it uses JIT compilation for immediate
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evaluation, and it supports a few control flow constructs with SSA construction.
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</p>
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<p>Part of the idea of this tutorial was to show you how easy and fun it can be
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to define, build, and play with languages. Building a compiler need not be a
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scary or mystical process! Now that you've seen some of the basics, I strongly
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encourage you to take the code and hack on it. For example, try adding:</p>
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<ul>
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<li><b>global variables</b> - While global variables have questional value in
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modern software engineering, they are often useful when putting together quick
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little hacks like the Kaleidoscope compiler itself. Fortunately, our current
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setup makes it very easy to add global variables: just have value lookup check
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to see if an unresolved variable is in the global variable symbol table before
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rejecting it. To create a new global variable, make an instance of the LLVM
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<tt>GlobalVariable</tt> class.</li>
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<li><b>typed variables</b> - Kaleidoscope currently only supports variables of
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type double. This gives the language a very nice elegance, because only
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supporting one type means that you never have to specify types. Different
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languages have different ways of handling this. The easiest way is to require
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the user to specify types for every variable definition, and record the type
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of the variable in the symbol table along with its Value*.</li>
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<li><b>arrays, structs, vectors, etc</b> - Once you add types, you can start
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extending the type system in all sorts of interesting ways. Simple arrays are
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very easy and are quite useful for many different applications. Adding them is
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mostly an exercise in learning how the LLVM <a
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href="../LangRef.html#i_getelementptr">getelementptr</a> instruction works: it
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is so nifty/unconventional, it <a
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href="../GetElementPtr.html">has its own FAQ</a>! If you add support
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for recursive types (e.g. linked lists), make sure to read the <a
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href="../ProgrammersManual.html#TypeResolve">section in the LLVM
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Programmer's Manual</a> that describes how to construct them.</li>
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<li><b>standard runtime</b> - Our current language allows the user to access
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arbitrary external functions, and we use it for things like "printd" and
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"putchard". As you extend the language to add higher-level constructs, often
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these constructs make the most sense if they are lowered to calls into a
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language-supplied runtime. For example, if you add hash tables to the language,
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it would probably make sense to add the routines to a runtime, instead of
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inlining them all the way.</li>
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<li><b>memory management</b> - Currently we can only access the stack in
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Kaleidoscope. It would also be useful to be able to allocate heap memory,
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either with calls to the standard libc malloc/free interface or with a garbage
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collector. If you would like to use garbage collection, note that LLVM fully
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supports <a href="../GarbageCollection.html">Accurate Garbage Collection</a>
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including algorithms that move objects and need to scan/update the stack.</li>
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<li><b>debugger support</b> - LLVM supports generation of <a
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href="../SourceLevelDebugging.html">DWARF Debug info</a> which is understood by
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common debuggers like GDB. Adding support for debug info is fairly
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straightforward. The best way to understand it is to compile some C/C++ code
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with "<tt>llvm-gcc -g -O0</tt>" and taking a look at what it produces.</li>
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<li><b>exception handling support</b> - LLVM supports generation of <a
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href="../ExceptionHandling.html">zero cost exceptions</a> which interoperate
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with code compiled in other languages. You could also generate code by
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implicitly making every function return an error value and checking it. You
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could also make explicit use of setjmp/longjmp. There are many different ways
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to go here.</li>
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<li><b>object orientation, generics, database access, complex numbers,
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geometric programming, ...</b> - Really, there is
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no end of crazy features that you can add to the language.</li>
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<li><b>unusual domains</b> - We've been talking about applying LLVM to a domain
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that many people are interested in: building a compiler for a specific language.
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However, there are many other domains that can use compiler technology that are
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not typically considered. For example, LLVM has been used to implement OpenGL
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graphics acceleration, translate C++ code to ActionScript, and many other
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cute and clever things. Maybe you will be the first to JIT compile a regular
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expression interpreter into native code with LLVM?</li>
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</ul>
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<p>
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Have fun - try doing something crazy and unusual. Building a language like
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everyone else always has, is much less fun than trying something a little crazy
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or off the wall and seeing how it turns out. If you get stuck or want to talk
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about it, feel free to email the <a
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href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
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list</a>: it has lots of people who are interested in languages and are often
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willing to help out.
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</p>
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<p>Before we end this tutorial, I want to talk about some "tips and tricks" for generating
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LLVM IR. These are some of the more subtle things that may not be obvious, but
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are very useful if you want to take advantage of LLVM's capabilities.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="llvmirproperties">Properties of the LLVM IR</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>We have a couple common questions about code in the LLVM IR form - lets just
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get these out of the way right now, shall we?</p>
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<!-- ======================================================================= -->
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<h4><a name="targetindep">Target Independence</a></h4>
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<!-- ======================================================================= -->
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<div>
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<p>Kaleidoscope is an example of a "portable language": any program written in
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Kaleidoscope will work the same way on any target that it runs on. Many other
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languages have this property, e.g. lisp, java, haskell, javascript, python, etc
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(note that while these languages are portable, not all their libraries are).</p>
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<p>One nice aspect of LLVM is that it is often capable of preserving target
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independence in the IR: you can take the LLVM IR for a Kaleidoscope-compiled
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program and run it on any target that LLVM supports, even emitting C code and
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compiling that on targets that LLVM doesn't support natively. You can trivially
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tell that the Kaleidoscope compiler generates target-independent code because it
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never queries for any target-specific information when generating code.</p>
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<p>The fact that LLVM provides a compact, target-independent, representation for
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code gets a lot of people excited. Unfortunately, these people are usually
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thinking about C or a language from the C family when they are asking questions
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about language portability. I say "unfortunately", because there is really no
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way to make (fully general) C code portable, other than shipping the source code
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around (and of course, C source code is not actually portable in general
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either - ever port a really old application from 32- to 64-bits?).</p>
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<p>The problem with C (again, in its full generality) is that it is heavily
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laden with target specific assumptions. As one simple example, the preprocessor
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often destructively removes target-independence from the code when it processes
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the input text:</p>
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<div class="doc_code">
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<pre>
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#ifdef __i386__
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int X = 1;
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#else
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int X = 42;
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#endif
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</pre>
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</div>
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<p>While it is possible to engineer more and more complex solutions to problems
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like this, it cannot be solved in full generality in a way that is better than shipping
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the actual source code.</p>
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<p>That said, there are interesting subsets of C that can be made portable. If
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you are willing to fix primitive types to a fixed size (say int = 32-bits,
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and long = 64-bits), don't care about ABI compatibility with existing binaries,
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and are willing to give up some other minor features, you can have portable
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code. This can make sense for specialized domains such as an
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in-kernel language.</p>
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</div>
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<!-- ======================================================================= -->
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<h4><a name="safety">Safety Guarantees</a></h4>
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<!-- ======================================================================= -->
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<div>
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<p>Many of the languages above are also "safe" languages: it is impossible for
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a program written in Java to corrupt its address space and crash the process
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(assuming the JVM has no bugs).
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Safety is an interesting property that requires a combination of language
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design, runtime support, and often operating system support.</p>
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<p>It is certainly possible to implement a safe language in LLVM, but LLVM IR
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does not itself guarantee safety. The LLVM IR allows unsafe pointer casts,
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use after free bugs, buffer over-runs, and a variety of other problems. Safety
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needs to be implemented as a layer on top of LLVM and, conveniently, several
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groups have investigated this. Ask on the <a
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href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
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list</a> if you are interested in more details.</p>
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</div>
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<!-- ======================================================================= -->
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<h4><a name="langspecific">Language-Specific Optimizations</a></h4>
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<!-- ======================================================================= -->
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<div>
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<p>One thing about LLVM that turns off many people is that it does not solve all
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the world's problems in one system (sorry 'world hunger', someone else will have
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to solve you some other day). One specific complaint is that people perceive
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LLVM as being incapable of performing high-level language-specific optimization:
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LLVM "loses too much information".</p>
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<p>Unfortunately, this is really not the place to give you a full and unified
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version of "Chris Lattner's theory of compiler design". Instead, I'll make a
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few observations:</p>
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<p>First, you're right that LLVM does lose information. For example, as of this
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writing, there is no way to distinguish in the LLVM IR whether an SSA-value came
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from a C "int" or a C "long" on an ILP32 machine (other than debug info). Both
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get compiled down to an 'i32' value and the information about what it came from
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is lost. The more general issue here, is that the LLVM type system uses
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"structural equivalence" instead of "name equivalence". Another place this
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surprises people is if you have two types in a high-level language that have the
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same structure (e.g. two different structs that have a single int field): these
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types will compile down into a single LLVM type and it will be impossible to
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tell what it came from.</p>
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<p>Second, while LLVM does lose information, LLVM is not a fixed target: we
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continue to enhance and improve it in many different ways. In addition to
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adding new features (LLVM did not always support exceptions or debug info), we
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also extend the IR to capture important information for optimization (e.g.
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whether an argument is sign or zero extended, information about pointers
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aliasing, etc). Many of the enhancements are user-driven: people want LLVM to
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include some specific feature, so they go ahead and extend it.</p>
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<p>Third, it is <em>possible and easy</em> to add language-specific
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optimizations, and you have a number of choices in how to do it. As one trivial
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example, it is easy to add language-specific optimization passes that
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"know" things about code compiled for a language. In the case of the C family,
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there is an optimization pass that "knows" about the standard C library
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functions. If you call "exit(0)" in main(), it knows that it is safe to
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optimize that into "return 0;" because C specifies what the 'exit'
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function does.</p>
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<p>In addition to simple library knowledge, it is possible to embed a variety of
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other language-specific information into the LLVM IR. If you have a specific
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need and run into a wall, please bring the topic up on the llvmdev list. At the
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very worst, you can always treat LLVM as if it were a "dumb code generator" and
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implement the high-level optimizations you desire in your front-end, on the
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language-specific AST.
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</p>
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</div>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="tipsandtricks">Tips and Tricks</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>There is a variety of useful tips and tricks that you come to know after
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working on/with LLVM that aren't obvious at first glance. Instead of letting
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everyone rediscover them, this section talks about some of these issues.</p>
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<!-- ======================================================================= -->
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<h4><a name="offsetofsizeof">Implementing portable offsetof/sizeof</a></h4>
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<!-- ======================================================================= -->
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<div>
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<p>One interesting thing that comes up, if you are trying to keep the code
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generated by your compiler "target independent", is that you often need to know
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the size of some LLVM type or the offset of some field in an llvm structure.
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For example, you might need to pass the size of a type into a function that
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allocates memory.</p>
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<p>Unfortunately, this can vary widely across targets: for example the width of
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a pointer is trivially target-specific. However, there is a <a
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href="http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt">clever
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way to use the getelementptr instruction</a> that allows you to compute this
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in a portable way.</p>
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</div>
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<!-- ======================================================================= -->
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<h4><a name="gcstack">Garbage Collected Stack Frames</a></h4>
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<!-- ======================================================================= -->
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<div>
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<p>Some languages want to explicitly manage their stack frames, often so that
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they are garbage collected or to allow easy implementation of closures. There
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are often better ways to implement these features than explicit stack frames,
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but <a
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href="http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt">LLVM
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does support them,</a> if you want. It requires your front-end to convert the
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code into <a
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href="http://en.wikipedia.org/wiki/Continuation-passing_style">Continuation
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Passing Style</a> and the use of tail calls (which LLVM also supports).</p>
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</div>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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