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.
