Kaleidoscope: Conclusion and other useful LLVM tidbits

Tutorial Conclusion

Welcome to the final chapter of the "Implementing a language with LLVM" 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. :)

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, an interactive run-loop (with a JIT!), and emitted debug information in standalone executables - all in under 1000 lines of (non-comment/non-blank) code.

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.

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:

• global variables - 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 GlobalVariable class.
• typed variables - 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*.
• arrays, structs, vectors, etc - 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 getelementptr instruction works: it is so nifty/unconventional, it has its own FAQ!
• standard runtime - 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.
• memory management - 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 Accurate Garbage Collection including algorithms that move objects and need to scan/update the stack.
• exception handling support - LLVM supports generation of zero cost exceptions 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.
• object orientation, generics, database access, complex numbers, geometric programming, ... - Really, there is no end of crazy features that you can add to the language.
• unusual domains - 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?

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 llvm-dev mailing list: it has lots of people who are interested in languages and are often willing to help out.

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.

Properties of the LLVM IR

We have a couple of common questions about code in the LLVM IR form - let's just get these out of the way right now, shall we?

Target Independence

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).

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.

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?).

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:

#ifdef __i386__
  int X = 1;
#else
  int X = 42;
#endif

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.

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.

Safety Guarantees

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.

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 llvm-dev mailing list if you are interested in more details.

Language-Specific Optimizations

One thing about LLVM that turns off many people is that it does not solve all the world's problems in one system. One specific complaint is that people perceive LLVM as being incapable of performing high-level language-specific optimization: LLVM "loses too much information". Here are a few observations about this:

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.

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.

Third, it is possible and easy 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.

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 llvm-dev 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.

Tips and Tricks

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.

Implementing portable offsetof/sizeof

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.

Unfortunately, this can vary widely across targets: for example the width of a pointer is trivially target-specific. However, there is a clever way to use the getelementptr instruction that allows you to compute this in a portable way.

Garbage Collected Stack Frames

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 LLVM does support them, if you want. It requires your front-end to convert the code into Continuation Passing Style and the use of tail calls (which LLVM also supports).