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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104408] SLP discovery fails due to -Ofast
 rewriting
Date: Mon, 07 Feb 2022 09:58:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104408

--- Comment #4 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> match.pd just does canonicalization here.  SLP discovery could handle this
> in the existing swap operands or reassoc support but I guess the desire h=
ere
> is to pull out a Complex SLP pattern.

Yes, though also to optimize the case where you don't have the optab, curre=
ntly
the generated code is much worse at -Ofast.

>=20
> So - no perfect idea yet how to reliably match a Complex pattern here but
> trying to attack this from the match.pd side sounds wrong.

Well the problem is that the scalar code is suboptimal too. even without
matching a complex pattern, so the epilogue here does an extra sub on each
unrolled step.

So I initially figured we'd want to not perform the canonization if it's co=
ming
at the expense of sharing. However that looks harder than I though at first=
 as
there are multiple points in const-fold.c that will try and force this form.

I can probably fix the epilogue post vectorization but that seemed like a w=
orse
solution.=