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From: "law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101895] [11/12 Regression] SLP Vectorizer
 change pushes VEC_PERM_EXPR into bad location spoiling further optimization
 opportunities
Date: Wed, 25 Aug 2021 04:40:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101895

--- Comment #3 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Understood WRT phase ordering.  That was fully expected.

What I still don't understand is why moving the permute down is profitable =
here
or generally why moving a permute into a dependency chain is profitable.

ISTM that hoisting a permute up a dependency chain so that it's fed by a lo=
ad
is profitable.  Similarly sinking a permute to the end of a dependency chai=
n so
that it feeds a store is profitable.  Moving a permute so that it feeds or =
is
fed by another permute is probably profitable too.  Otherwise moving a perm=
ute
into the middle of a dependency chain seems like a pessimization.=