[PATCH] [CodeGen] Add hooks/combine to form vector extloads, and enable it on X86.

[Chandler Carruth]

Could you post the patches with Phabricator? That makes it so much easier
to review.

On Fri, Jan 9, 2015 at 10:27 AM, Ahmed Bougacha <ahmed.bougacha at gmail.com>
wrote:

> Hi all,
>
> As the last change in my extload series, here are 3 (WIP) patches to
> actually form extloads on vector types.
> They used to be disabled, because "None of the supported targets knows
> how to perform load and  sign extend on vectors in one instruction."
>
> The first patch enables the combine on legal vectors, but hides it
> behind a profitability callback.
> For instance, on ARM, several instructions have folded extload forms,
> so it's not always beneficial to create an extload node (and trying to
> match extloads is a whole 'nother can of worms).
>
> The second patch adds a combine to fold extloads of illegal
> (splittable) vector types, to replace it directly by multiple smaller
> extloads.  I'm not a big fan of this kind of pseudo-legalization in
> combines, but I tried the alternative: form illegal extloads, and
> later try to split them up, but then, you sometimes generate extloads
> that can't be split up, but have a valid ext+load expansion.  At
> vector-op legalization time, it's too late to generate this kind of
> thing, so it's better to just avoid creating egregiously illegal
> nodes.
>
>
> Finally, the last patches enables this all, unconditionally, on X86.
>
> Note that the splitting combine is happy with "custom" extloads.  As
> is, this bypasses the actual custom lowering, and just unrolls the
> extload.  But from what I've seen, this is still much better than the
> current custom lowering, which does some kind of unrolling at the end
> anyway (see for instance load_sext_4i8_to_4i64 on SSE2, and the added
> FIXME).
>
> Also note that there's a regression in the widen_load-2.ll test, where
> we can no longer fold the load. I'll look into that later.
>
>
> Anyway: as can be seen from the nice testcase cleanups, there's
> something to be done here.  The combines feel a bit dirty, but I don't
> see a better alternative.  Finally, I didn't see changes on the
> testsuite (SSE2 X86-64, I'll try SSE4.1 and AVX2 as well.)
>
> Feedback heartily welcome!
>
> Thanks,
>
> - Ahmed
>
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