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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/81594] Optimize PowerPC vector set and store
Date: Wed, 25 Mar 2020 18:40:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D81594

--- Comment #7 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Peepholes catch fewer cases, and it is very hard to write correct peepholes.
The only reason to use peepholes is when the other passes leave some import=
ant
optimisation on the table, and you cannot feasibly fix that problem.

They are not a substitute for proper optimisation (for example, almost all
"interesting" optimisation happen before it, so you cannot rely on combine
or cse or *prop etc. to do anything -- so, if you want some simple
optimisations
with it you need to write that manually (an exponential amount of work).  T=
his
even applies to "trivial" things like constant arguments.

Peepholes are nice for mopping up those cases that for one reason or the ot=
her
the other compiler passes cannot / do not get right.  They can "tune" the
compiler output to be just a teeny bit better.  They cannot do anything more
than that.=