Question

Is it easy to achieve high level of optimization with LLVM?

To give a concrete example let’s assume that I have a simple lanuage that I want to write a compiler for.

• simple functions
• simple structs
• tables
• pointers (with arithmetic)
• control structures
• etc.

I can quite easily create compilation-to-C backend and rely on clang -O3.
Is it as easy to use LLVM API for that purpose?

Answer 1

Except perhaps for a few high-level (as in, aware of high-level language features or details that aren’t encoded in LLVM IR) optimizations, Clang’s backend does little more than generate straightforward IR and run some set of LLVM optimization passes on it. All of these (or at least most) should be available trough the opt command and also as API calls when using the C++ libraries that all LLVM tools are built on. See the tutorial for a simple example. I see several advantages:

1. LLVM IR is far simpler than C and there’s already a convenient API for generating it programatically. To generate C, you either have lots of ugly and unreliable string fiddling or have to build an AST for the C language yourself. Or both.
2. You get to choose the set of optimizations yourself (it’s quite possible that Clang’s set of passes isn’t ideal for the code the language supports and the IR representation your compiler generates). This also means you can, during development, just run the passes checking for IR wellformedness (uncovering compiler bugs faster). You can just copy Clang’s pass order, but if you feel like it, you can also experiment.
3. It will allow better compile times. Clang is fast for a C compiler, but you’d be adding unnecessary overhead: You generate C code, then Clang parses it, converts it to IR, and goes on to do pretty much what you could do right away.
4. You may have access to a broader range of features, or at least you’d get them easier (i.e. without having to incorporate #defines, obscure pragmas, instrincts or command line options) to provide them. I’m talking about like vectors, guaranteed (well, more than in C anyway – AFAIK, some code generators ignore them) tail calls, pure/readonly functions, more control over memory layout and type conversions (for instance zero extending vs. sign extending). Granted, you may not need most of them.