[llvm] [AMDGPU] Use correct VGPR threshold for flagging ExcessRP regions in unified register file case (PR #85860)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> > This was done conservatively on purpose. That is true we have total of 512 VGPRs, but only half of these a general purpose. Doubling it will result in a higher register pressure on some programs and spilling because most of the instructions may not use AGPRs.
> 
> I'm afraid I don't follow. This is not doubling, but rather halving from 512 to 256. The goal is to reduce RP and avoid spilling.
> 
> This will make the scheduler more conservative in terms of RP. AVReg are tracked as VGPR, so a limit of 256 will only be accurate if 100% of AVRegs are allocated as VGPR.
> 
> I don't understand how the current implementation is supposed to work. It seems to me, comparing pressure from RegKind::VGPR32 to 512 (assuming min-waves is 1) only makes sense if we add pressure from RegKind::AGPR32 , but even then, that would assume perfect allocation of AVRegs. We don't do that, though. We compare pressure from RegKind::VGPR32 to 512, then we compare pressure from RegKind::AGPR32 to 512

I see. Then I misread the description. Halving should be fine.

https://github.com/llvm/llvm-project/pull/85860