Re: [PATCH] combine: Try harder to form zero_extends [PR106594]

[Segher Boessenkool <segher@kernel.crashing.org>]

On Mon, Mar 06, 2023 at 04:34:59PM +0000, Richard Sandiford wrote:
> Jakub Jelinek <jakub@redhat.com> writes:
> > Could we have a target hook to canonicalize memory addresses for combiner,
> > like we have that targetm.canonicalize_comparison ?
> 
> I don't think a hook makes sense as a long-term design decision.
> The canonicalisation we're doing here isn't logically AArch64-specific,
> and in general, the less variation in RTL rules between targets, the better.

Some targets do not want all insasnity allowed for other targets.  We
have quite a few exampples of this already.  But of course a hook like
the proposed one can be abused a lot to do completely unrelated things.
We'll just have to trust target maintainers to have good taste and some
wisdom (because not everyine else looks at all target patches).  What
else is new :-)

> But if you mean adding target control as a GCC 13 hack, to avoid any
> effect on other targets, then TBH, I'd prefer just sticking it in an
> #ifdef GCC_AARCH64_H :-)

And I will NAK that for all the same reasons: it is unmaintainable, it
makes things harder instead of solving problems, it is a completely
ad-hoc code change.

> That makes it 100% clear that it's a
> temporary hack to restrict the target impact rather than something
> based on fundamentals.  We can then revisit for GCC 14.

And that will never happen, you know this as well as anyone else :-(

Most importantly, what makes you think this is a problem for aarch64
only?  If it actually is, you can fix it in the aarch64 config!  Either
with or without new hooks, whatever works best.


Segher