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From: "linux at carewolf dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/65492] New: Bad optimization in -O3 on SSE intrinsics
Date: Fri, 20 Mar 2015 12:32:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65492

            Bug ID: 65492
           Summary: Bad optimization in -O3 on SSE intrinsics
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: linux at carewolf dot com

After investigating a loop using SSE intrinsics that was significantly faster
in clang than in gcc, I discovered gcc had the same performance as clang in
-O2, and only performed signficantly worse in -O3.

Disabling all the switches mentioned in the documentation as activates by -O3
(or enabling them for -O2), doesn't fully account for the difference, but the
switch -f(no-)tree-loop-vectorize accounts for roughly half of it.

I have attached the files I used to test it. Using gcc -O2 or clang -O2 or -O3,
it times in at 1.8s on my machine. Using g++ (4.9 or 5.0) -O3 it times in at
2.5s. Using -O3 -fno-tree-loop-vectorize it runs in 2.3s, and -O2
-ftree-vectorize at 2.25s.

Using callgrind, it seems the performance difference is mainly spend on the
accessing integers in the vector union.