On Wed, Sep 7, 2022 at 2:20 PM Robin Dapp <rd...@linux.ibm.com> wrote: > > > The question is really whether xor or sub is "better" statically. I can't > > think of any reasons. On s390, why does xor end up "better"? > > There is an xor with immediate (as opposed to no "subtract from > immediate") which saves an instruction, usually. On x86, I think the > usual argument for xor is that it's shorter (if flags etc. are not needed). > > It's not that I don't want to implement it in the backend, just that I > understood the original PR in a way that it would make sense to have > this conversion available for more targets. If there are too many > confounding factors that prevent this situation from being statically > costed properly, then sure, not much use in implementing it generally.
Do we have evidence that targets properly cost XOR vs SUB RTXen? It might actually be a reload optimization - when the constant is available in a register use 'sub', when it needs to be reloaded use 'xor'? That said, I wonder if the fallout of changing some SUB to XOR is bigger than the benefit when we do it early (missed combines, etc.)? > Regards > Robin
