[LLVMdev] Some backend questions

[Vladimir Prus]

Ok, I'm now trying to write instruction selector and have some questions

1. The MachineInstrBuilder has methods to add register operand and immediate 
operand. However, what would be really nice is a method to add Value*.  So, I 
would write:

    BuildMI(*BB, NM::add, 1).add(I.getOperand(0), I.getOperand(1));

and depending on whether the passed Value* is contant or instruction, the add 
method would either add immediate constant or allocate/get new virtual 
register. 

2. Why SSARegMap is called this way. As far as I can see, it's does not 
implement any mapping, it simply allocates registers given a register class.

3. Maybe, the allocation of virtual registers for Value* should be made more 
reusable. The X86 backend has the code for that in getReg method in 
InstSelectSimple.cpp which:

   - uses SSARegMap instance
   - keeps internal Value* -> register mapping
   - copies constants into register when needed

At least first two things will be necessary for my backend too. I start to 
wonder if it makes sense to make a "BasicInstructionSelector" which will have 
such reusable logic?

Sorry if the above is not very clear -- I'm only starting to getting underting 
of LLVM codebase.

- Volodya