[llvm] [AMDGPU] Detect kills in register sets when trying to form V_CMPX instructions. (PR #68293)

[Thomas Symalla via llvm-commits]

tsymalla wrote:

> > > Can you just drop kill flags without prejudice? We shouldn't really need them anymore at this point
> > 
> > 
> > I am not sure about that.
> 
> In the general case we still use kill flags after this point in SIShrinkInstructions and SIPreEmitPeephole. More specifically can we drop kill flags from anything used by V_CMPX? I would say "yes", but I think that is basically what this patch is trying to do?

Yes, this is what this patch does (erasing the kill flags on everything that could block the V_CMPX operand from being live).
> 
> Perhaps Matt is suggesting that we permute all the sub and super registers of the registers which are passed to clearKillFlags, and call clearKillFlags on those as well.

Makes sense, but it seems more destructive than just dropping the flag from the relevant instructions (speaking of passes like SIShrinkInstructions and SIPreEmitPeephole).

https://github.com/llvm/llvm-project/pull/68293