Re: [PATCH] expand: Convert cst - x into cst xor x.

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Sep 7, 2022 at 2:20 PM Robin Dapp <rdapp@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