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

[Stéphane Letz]

Let's say we have the following scheme using C++ and virtual functions:


class DSP {

  public:

	DSP() {}
	virtual ~DSP() {}

	virtual int Compute(int count, float** in, float** out) = 0;
};

class CONCRETE_DSP : public DSP {

  public:

	CONCRETE_DSP():fValue() {}
	virtual ~CONCRETE_DSP() {}

	virtual int Compute(int count, float** in, float** out)
	{
		DoSomeProcess();
	}
};
			
		
class SEQ_DDSP : public DSP {

   private:
	
	DSP* 	fArg1;
	DSP* 	fArg2;
	
   public:

	SEQ_DDSP(DSP* a1, DSP* a2):fArg1(a1), fArg2(a2) {}
	virtual~SEQ_DDSP() {delete fArg1; delete fArg2;}
	
	virtual int Compute(int count, float** in, float** out)
	{
		// Some code that uses:
		fArg1->Compute(count, in, out);
		fArg2->Compute(count, in, out);
	}
};

class PAR_DSP : public DSP {

   private:
	
	DSP* 	fArg1;
	DSP* 	fArg2;
	
   public:

	PAR_DSP(DSP* a1, DSP* a2):fArg1(a1), fArg2(a2) {}
	virtual~PAR_DSP() {delete fArg1; delete fArg2;}
	
	virtual int Compute(int count, float** in, float** out)
	{
		// Some code that uses:
		fArg1->Compute(count, in, out);
		fArg2->Compute(count, in, out);
	}

};


void ProcessGraph (float** in, float** out)
{
	DSP* graph = new PAR_DSP(new SEQ_DDSP(new CONCRETE_DSP(), new  
CONCRETE_DSP()), new CONCRETE_DSP());
	graph->Compute(512, in, out);
	delete graph;
}

At runtime after a graph is created, one could imagine optimizing by   
resolving call to "virtual  Compute" and possibly get a more  
efficient Compute method for the entire graph, so that we could write:

DSP* graph = new PAR_DSP(new SEQ_DDSP(new CONCRETE_DSP(), new  
CONCRETE_DSP()), new CONCRETE_DSP());

graph->Optimize();

graph->Compute(512, in, out); possibly a lot of time.

Is there any possible method using LLVM that would help in this case?

Thanks

Stephane Letz