Re: [PATCH, AArch64] Disable reg offset in quad-word store for Falkor.

[Jim Wilson <jim.wilson@linaro.org>]

On Fri, Sep 22, 2017 at 10:58 AM, Andrew Pinski <pinskia@gmail.com> wrote:
> Two overall comments:
> * What about splitting register_offset into two different elements,
> one for non 128bit modes and one for 128bit (and more; OI, etc.) modes
> so you get better address generation right away for the simd load
> cases rather than having LRA/reload having to reload the address into
> a register.

I'm not sure if changing register_offset cost would make a difference,
since costs are usually used during optimization, not during address
generation.  This is something that I didn't think to try though.  I
can try taking a look at this.

I did try writing a patch to modify predicates to disallow reg offset
for 128bit modes, and that got complicated, as I had to split apart a
number of patterns in the aarch64-simd.md file that accept both VD and
VQ modes.  I ended up with a patch 3-4 times as big as the one I
submitted, without any additional performance improvement, so it
wasn't worth the trouble.

> * Maybe adding a testcase to the testsuite to show this change.

Yes, I can add a testcase.

> One extra comment:
> * should we change the generic tuning to avoid reg+reg for 128bit modes?

Are there other targets with a similar problem?  I only know that it
is a problem for Falkor.  It might be a loss for some targets as it is
replacing one instruction with two.

Jim