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

[Ahmed Bougacha]

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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