[LLVMdev] Register Coalescer does not preserve TargetFlag

[Jakob Stoklund Olesen]

On Aug 6, 2012, at 11:00 AM, Vincent Lejeune <vljn at ovi.com> wrote:

> Ok.
> 
> I tried to do it using a pass after register allocation, lowering NEG/ABS instructions.
> However I met a problem : apparently getNextOperandForReg() can returns a MachineOperand before the one I'm processing.
> 
> The following code snippet : 
> 
> 
> void R600ModifiersPropagation::substituteReg(MachineOperand &def_MO, unsigned new_reg, unsigned char flag) {
>   MachineOperand * MO = def_MO.getNextOperandForReg();
>   while (MO && MO->isUse()) {
>     MachineOperand *next_MO = MO->getNextOperandForReg();
>     MO->dump();
> 
>     MO = next_MO;
>   }
> }
> 
> displays instructions that are before the one I'm passing as def_MO. I'm not sure if I should call some Analysis pass to enforce MachineOperand order or not.

We make no guarantees about the order of operands in the use-def chains, and there are no analyses that can reorder them.

I would suggest that you read the code in other targets to see how things are usually done.

/jakob