[LLVMdev] Runtime optimization of C++ code with virtual functions

[Chris Lattner]

On Wed, 20 Jun 2007, Maurizio Vitale wrote:
>>> Is there any possible method using LLVM that would help in this case?
>>
>> LLVM won't help in this case.
>
> Is that so or it means that LLVM wouldn't have a prebuilt solution?

It means that LLVM doesn't have any trivial builtin solution.

> I'm asking because (without having ever looked seriously into LLVM) I 
> was thinking to experiment along these lines:
>
> class Source {
> 	void send (T data) {
> 		invoke_jit_magic();
> 		transport (data);
>         }
> }
>
> transport() would be a virtual method like the original posting. In my 
> case send() would be part of the framework, so it is not a problem to 
> add the invoke_jit_magic. In other case it might be trickier.

Ok.

> On the first call, invoke_jit_magic gains control, traverse the binary 
> converting (a subset of) what it finds to LLVM IR, until it gets to the 
> concrete target. It may have to do a bit of work to understand how 
> parameters are passed to the transport code (it is a virtual function 
> call and might be messy in presence of multiple/virtual inheritance. 
> After that LLVM jit can be used to replace the original binary fragment 
> with something faster.

Ok.

> I agree with the suggestion of using templates when possible. In my case 
> it is not doable because transport would be propietary and the code 
> containing it distributed only as binary.

Ok.

> I understand that the disassemblying portion need to be rewritten. Is 
> there anything else that would prevent this approach from working? 
> Again, haven't looked into LLVM yet, so I can immagine there might be 
> problems in describing physical registers in the IR and at some point 
> stuff must be exactly where the pre-existing code expects it. I don;t 
> want to take your time, but if you could elaborate a bit it might 
> prevent me from going down the wrong path.

This should work, I don't expect you to run into any significant problems. 
When you're rewriting the LLVM IR for the indirect call, you can just 
replace it with a direct call to the native code.

-Chris

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