[LLVMdev] Can I use Clang to parse snippets of C++ code?

[Félix Cloutier]

I believe that what I'm trying to do with Clang is fairly simple; the final goal, however, might be a little harder.

Knowing myself, there are chances I'll never go through with this project (like Mikael who posted earlier, I'm nothing but an enthusiastic student with lots of time on my hands), but it feels cool enough to me to announce my idea. Besides, I'll probably need help from more knowledgeable people anyways.

The way LLVM works makes it pretty easy and straightforward to generate code from nested structures (like ASTs), which is totally commendable since LLVM is a compiler back-end. However, in the past months I've set myself to make an emulator back-end with LLVM that would translate machine code to LLBC then compile it to native code with the JIT, and my experience haven't been so great, especially because of the following:
it's stupid-hard to debug just-in-time generated code with the version of gdb that ships with Xcode (it repeatedly crashed on me);
the sheer number of cases to treat is, in itself, rebuking enough: a NES 6502 is 'fine' with just less than 60 distinct operations, but the full-fledged PowerPC you get with a GameCube has roughly 6 times more;
when faced with subtle bugs, it's much easier to deal with C++ code representing what you want to do (like interpreter code) than IRBuilder::Create* calls.

I figured that while I can't do much about the first, if I could get LLVM to generate code that would generate code, the two others would be much less cumbersome.

So my plan is to write a tool that accepts a specification of how instructions should be interpreted, with handlers written in C++, and turn that into an usable recompiler (that would also use LLVM libraries). The grammar would be a shell for C++ code, and I'd use Clang to turn the actual code into LLBC; then, I would pass through the code (à la llvm2cpp), and create calls to an IRBuilder to generate equivalent code. Once this generated class compiled (through regular means), clients would call the appropriate methods on the object to generate code, and will finally be able to get a Function to use with the JIT.

I've joined an example grammar and an example expected output (made on the train, it's not actually working, but it gives a good idea).



Branch management (except for the branching itself) would have to be completely managed by the client, but this is a minor hurt compared to what it is right now.

As a side-effect of using C++ to make instruction handlers, such a solution could almost be a drop-in replacement for existing interpreters.

So this is what I'd like to be able to compile with Clang: snippets of C++, with a few symbols declared beforehand.

Félix

Le 2011-02-21 à 23:35:55, Chris Lattner a écrit :

> 
> On Feb 21, 2011, at 6:31 PM, Félix Cloutier wrote:
> 
>> Hello guys,
>> 
>> I'd like to use Clang to parse snippets of (and emit bytecode for) C++ code that come from larger files that don't contain only C++, but looking at the clang interpreter example, either I didn't get it, or it looks like the driver expects only files, and not strings or char buffers.
>> 
>> Is there a simple way to achieve this? Do I have to split my input into small files and pass them to clang, or is there a better way?
> 
> It depends on exactly what you're trying to achieve.  LLDB uses clang to parse individual expressions, and does this by hooking into various name lookup routines to dynamically/lazily populate symbol tables from debug info.
> 
> This is all possible, but it's a nontrivial amount of work.
> 
> -Chris

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