[LLVMdev] Questions on LLVM and binary translation

[Chris Lattner]

On Fri, 24 Feb 2006, Bao Yuncheng wrote:
> I'm currently investigating LLVM to see whether it can be used in dynamic 
> binary translation. My goal is to translate different "source" machine code 
> into the "target" machine code during runtime, e.g., MIPS -> x86. LLVM has a 
> well-defined intermediate representation to separate source and target 
> machine code (source -> LLVM Bytecode -> target), and is a quit extensible 
> and adaptable framework. So I consider LLVM as a good choice for building 
> advanced binary manipulation tools. But I have several questions in regard to 
> fit LLVM into dynamic binary translation use cases:

Ok.

> 1. The current JIT implementation assumes the bytecode file is fully 
> generated and should be read and parsed by [BytecodeFileReader] before JIT 
> (right? ). Can current LLVM be extended to support to parse bytecodes 
> just-in-time, that is, parse block of bytecode whenever available? I think it 
> may be a useful and interesting feature for LLVM.

Actually it already does this.  You can add functions to the Module 
whenever you'd like.  When you call the function added, they will be 
JIT'd.  The one feature that is missing that would be really nice is to be 
able to get a callback from the JIT when an external function is called, 
so that the .bc file could be generated in a 'pull' style instead of a 
push style.

> 2. Why are the current codegen passes per-function-a-time? I'd rather do it 
> per-BB-a-time because some BBs in the function may not be executed at all. Is 
> there any difficulty to do codegen per-BB-a-time?

The algorithms used are function-at-a-time, e.g. register allocation.

Note that an LLVM "Function" is just a unit of code with well defined 
inputs and outputs.  They wouldn't need to correspond to the functions in 
your input program.  If you turn each input basic block into a separate 
LLVM function, it should do what you want.

> Thank you for the attention, and any suggestions and comments on applying 
> LLVM on dynamic binary translation are mostly welcomed.

I don't have much experience with the field, maybe some other readers of 
this list do :)

-Chris

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