[llvm-commits] [llvm] r98043 - in /llvm/trunk/lib/CodeGen: LLVMTargetMachine.cpp MachineCSE.cpp

[Jakob Stoklund Olesen]

On Mar 8, 2010, at 7:21 PM, Evan Cheng wrote:

> @@ -91,14 +92,17 @@
>     unsigned SrcReg, DstReg, SrcSubIdx, DstSubIdx;
>     if (TII->isMoveInstr(*DefMI, SrcReg, DstReg, SrcSubIdx, DstSubIdx) &&
>         TargetRegisterInfo::isVirtualRegister(SrcReg) &&
> -        MRI->getRegClass(SrcReg) == MRI->getRegClass(Reg) &&
>         !SrcSubIdx && !DstSubIdx) {
> -      DEBUG(dbgs() << "Coalescing: " << *DefMI);
> -      DEBUG(dbgs() << "*** to: " << *MI);
> -      MO.setReg(SrcReg);
> -      DefMI->eraseFromParent();
> -      ++NumCoalesces;
> -      Changed = true;
> +      const TargetRegisterClass *SRC = MRI->getRegClass(SrcReg);
> +      const TargetRegisterClass *RC  = MRI->getRegClass(Reg);
> +      if (SRC == RC || SRC->hasSubClass(RC) || RC->hasSubClass(SRC)) {
> +        DEBUG(dbgs() << "Coalescing: " << *DefMI);
> +        DEBUG(dbgs() << "*** to: " << *MI);
> +        MO.setReg(SrcReg);
> +        DefMI->eraseFromParent();
> +        ++NumCoalesces;
> +        Changed = true;
> +      }
>     }
>   }

The case "SRC->hasSubClass(RC)" is not valid. MO requires an RC operand, and SRC is a larger register class. It may contain registers that are illegal for MO.

If you don't want to change the register class of SrcReg, you have to make do with:

> +      if (SRC == RC || RC->hasSubClass(SRC)) {

If you are not afraid of switching register classes, you can go:

> +      const TargetRegisterClass *SRC = MRI->getRegClass(SrcReg);
> +      const TargetRegisterClass *RC  = MRI->getRegClass(Reg);
> +      const TargetRegisterClass *NewRC  = getCommonSubClass(SRC, RC);
> +      if (NewRC) {
> +        DEBUG(dbgs() << "Coalescing: " << *DefMI);
> +        DEBUG(dbgs() << "*** to: " << *MI);
> +        MO.setReg(SrcReg);
> +        if (SRC != NewRC)
> +          MRI->setRegClass(SrcReg, NewRC);

The problem with both approaches is that you are producing a longer live range of a smaller register class: Higher register pressure.

/jakob