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260 lines
13 KiB
260 lines
13 KiB
======================================================
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Kaleidoscope: Conclusion and other useful LLVM tidbits
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======================================================
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.. contents::
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:local:
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Tutorial Conclusion
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===================
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Welcome to the final chapter of the "`Implementing a language with
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LLVM <index.html>`_" tutorial. In the course of this tutorial, we have
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grown our little Kaleidoscope language from being a useless toy, to
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being a semi-interesting (but probably still useless) toy. :)
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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, an
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interactive run-loop (with a JIT!), and emitted debug information in
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standalone executables - all in under 1000 lines of (non-comment/non-blank)
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code.
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Our little language supports a couple of interesting features: it
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supports user defined binary and unary operators, it uses JIT
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compilation for immediate evaluation, and it supports a few control flow
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constructs with SSA construction.
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Part of the idea of this tutorial was to show you how easy and fun it
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can be to define, build, and play with languages. Building a compiler
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need not be a scary or mystical process! Now that you've seen some of
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the basics, I strongly encourage you to take the code and hack on it.
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For example, try adding:
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- **global variables** - While global variables have questional value
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in modern software engineering, they are often useful when putting
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together quick little hacks like the Kaleidoscope compiler itself.
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Fortunately, our current setup makes it very easy to add global
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variables: just have value lookup check to see if an unresolved
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variable is in the global variable symbol table before rejecting it.
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To create a new global variable, make an instance of the LLVM
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``GlobalVariable`` class.
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- **typed variables** - Kaleidoscope currently only supports variables
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of type double. This gives the language a very nice elegance, because
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only supporting one type means that you never have to specify types.
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Different languages have different ways of handling this. The easiest
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way is to require the user to specify types for every variable
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definition, and record the type of the variable in the symbol table
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along with its Value\*.
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- **arrays, structs, vectors, etc** - Once you add types, you can start
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extending the type system in all sorts of interesting ways. Simple
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arrays are very easy and are quite useful for many different
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applications. Adding them is mostly an exercise in learning how the
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LLVM `getelementptr <../LangRef.html#getelementptr-instruction>`_ instruction
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works: it is so nifty/unconventional, it `has its own
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FAQ <../GetElementPtr.html>`_!
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- **standard runtime** - Our current language allows the user to access
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arbitrary external functions, and we use it for things like "printd"
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and "putchard". As you extend the language to add higher-level
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constructs, often these constructs make the most sense if they are
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lowered to calls into a language-supplied runtime. For example, if
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you add hash tables to the language, it would probably make sense to
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add the routines to a runtime, instead of inlining them all the way.
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- **memory management** - Currently we can only access the stack in
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Kaleidoscope. It would also be useful to be able to allocate heap
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memory, either with calls to the standard libc malloc/free interface
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or with a garbage collector. If you would like to use garbage
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collection, note that LLVM fully supports `Accurate Garbage
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Collection <../GarbageCollection.html>`_ including algorithms that
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move objects and need to scan/update the stack.
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- **exception handling support** - LLVM supports generation of `zero
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cost exceptions <../ExceptionHandling.html>`_ which interoperate with
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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
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it. You could also make explicit use of setjmp/longjmp. There are
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many different ways to go here.
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- **object orientation, generics, database access, complex numbers,
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geometric programming, ...** - Really, there is no end of crazy
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features that you can add to the language.
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- **unusual domains** - We've been talking about applying LLVM to a
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domain that many people are interested in: building a compiler for a
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specific language. However, there are many other domains that can use
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compiler technology that are not typically considered. For example,
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LLVM has been used to implement OpenGL graphics acceleration,
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translate C++ code to ActionScript, and many other cute and clever
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things. Maybe you will be the first to JIT compile a regular
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expression interpreter into native code with LLVM?
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Have fun - try doing something crazy and unusual. Building a language
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like everyone else always has, is much less fun than trying something a
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little crazy or off the wall and seeing how it turns out. If you get
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stuck or want to talk about it, feel free to email the `llvm-dev mailing
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list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_: it has lots
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of people who are interested in languages and are often willing to help
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out.
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Before we end this tutorial, I want to talk about some "tips and tricks"
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for generating LLVM IR. These are some of the more subtle things that
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may not be obvious, but are very useful if you want to take advantage of
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LLVM's capabilities.
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Properties of the LLVM IR
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=========================
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We have a couple of common questions about code in the LLVM IR form -
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let's just get these out of the way right now, shall we?
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Target Independence
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-------------------
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Kaleidoscope is an example of a "portable language": any program written
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in Kaleidoscope will work the same way on any target that it runs on.
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Many other languages have this property, e.g. lisp, java, haskell,
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javascript, python, etc (note that while these languages are portable,
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not all their libraries are).
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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
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Kaleidoscope-compiled program and run it on any target that LLVM
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supports, even emitting C code and compiling that on targets that LLVM
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doesn't support natively. You can trivially tell that the Kaleidoscope
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compiler generates target-independent code because it never queries for
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any target-specific information when generating code.
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The fact that LLVM provides a compact, target-independent,
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representation for code gets a lot of people excited. Unfortunately,
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these people are usually thinking about C or a language from the C
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family when they are asking questions about language portability. I say
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"unfortunately", because there is really no way to make (fully general)
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C code portable, other than shipping the source code around (and of
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course, C source code is not actually portable in general either - ever
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port a really old application from 32- to 64-bits?).
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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
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preprocessor often destructively removes target-independence from the
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code when it processes the input text:
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.. code-block:: c
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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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While it is possible to engineer more and more complex solutions to
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problems like this, it cannot be solved in full generality in a way that
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is better than shipping the actual source code.
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That said, there are interesting subsets of C that can be made portable.
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If you are willing to fix primitive types to a fixed size (say int =
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32-bits, and long = 64-bits), don't care about ABI compatibility with
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existing binaries, and are willing to give up some other minor features,
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you can have portable code. This can make sense for specialized domains
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such as an in-kernel language.
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Safety Guarantees
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-----------------
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Many of the languages above are also "safe" languages: it is impossible
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for a program written in Java to corrupt its address space and crash the
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process (assuming the JVM has no bugs). Safety is an interesting
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property that requires a combination of language design, runtime
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support, and often operating system support.
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It is certainly possible to implement a safe language in LLVM, but LLVM
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IR does not itself guarantee safety. The LLVM IR allows unsafe pointer
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casts, use after free bugs, buffer over-runs, and a variety of other
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problems. Safety needs to be implemented as a layer on top of LLVM and,
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conveniently, several groups have investigated this. Ask on the `llvm-dev
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mailing list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_ if
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you are interested in more details.
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Language-Specific Optimizations
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-------------------------------
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One thing about LLVM that turns off many people is that it does not
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solve all the world's problems in one system (sorry 'world hunger',
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someone else will have to solve you some other day). One specific
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complaint is that people perceive LLVM as being incapable of performing
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high-level language-specific optimization: LLVM "loses too much
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information".
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Unfortunately, this is really not the place to give you a full and
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unified version of "Chris Lattner's theory of compiler design". Instead,
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I'll make a few observations:
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First, you're right that LLVM does lose information. For example, as of
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this writing, there is no way to distinguish in the LLVM IR whether an
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SSA-value came from a C "int" or a C "long" on an ILP32 machine (other
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than debug info). Both get compiled down to an 'i32' value and the
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information about what it came from is lost. The more general issue
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here, is that the LLVM type system uses "structural equivalence" instead
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of "name equivalence". Another place this surprises people is if you
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have two types in a high-level language that have the same structure
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(e.g. two different structs that have a single int field): these types
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will compile down into a single LLVM type and it will be impossible to
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tell what it came from.
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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
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to adding new features (LLVM did not always support exceptions or debug
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info), we also extend the IR to capture important information for
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optimization (e.g. whether an argument is sign or zero extended,
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information about pointers aliasing, etc). Many of the enhancements are
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user-driven: people want LLVM to include some specific feature, so they
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go ahead and extend it.
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Third, it is *possible and easy* to add language-specific optimizations,
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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
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family, there is an optimization pass that "knows" about the standard C
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library functions. If you call "exit(0)" in main(), it knows that it is
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safe to optimize that into "return 0;" because C specifies what the
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'exit' function does.
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In addition to simple library knowledge, it is possible to embed a
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variety of other language-specific information into the LLVM IR. If you
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have a specific need and run into a wall, please bring the topic up on
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the llvm-dev list. At the very worst, you can always treat LLVM as if it
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were a "dumb code generator" and implement the high-level optimizations
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you desire in your front-end, on the language-specific AST.
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Tips and Tricks
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===============
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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
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letting everyone rediscover them, this section talks about some of these
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issues.
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Implementing portable offsetof/sizeof
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-------------------------------------
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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
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to know the size of some LLVM type or the offset of some field in an
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llvm structure. For example, you might need to pass the size of a type
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into a function that allocates memory.
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Unfortunately, this can vary widely across targets: for example the
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width of a pointer is trivially target-specific. However, there is a
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`clever way to use the getelementptr
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instruction <http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt>`_
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that allows you to compute this in a portable way.
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Garbage Collected Stack Frames
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------------------------------
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Some languages want to explicitly manage their stack frames, often so
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that they are garbage collected or to allow easy implementation of
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closures. There are often better ways to implement these features than
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explicit stack frames, but `LLVM does support
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them, <http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt>`_
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if you want. It requires your front-end to convert the code into
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`Continuation Passing
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Style <http://en.wikipedia.org/wiki/Continuation-passing_style>`_ and
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the use of tail calls (which LLVM also supports).
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