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Subject: [Bug target/38306] [4.4/4.5/4.6/4.7 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures
Date: Sat, 10 Sep 2011 09:52:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D38306

Manuel L=C3=B3pez-Ib=C3=A1=C3=B1ez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |manu at gcc dot gnu.org

--- Comment #25 from Manuel L=C3=B3pez-Ib=C3=A1=C3=B1ez <manu at gcc dot gn=
u.org> 2011-09-10 09:43:58 UTC ---
(In reply to comment #24)
>=20
> The issue is that at -O3 the subroutine PD2VAL is not vectorized, while i=
t is
> at -O2.

If you are interested in investigating why this is so by yourself, I would
suggest that you use the various -fdump- options to check what GCC is doing
differently between the two variants.=20

1) Dump everything you can dump.

2) Then find the earliest optimization pass where they differ (you may even=
 use
diff to make this faster).

3) Check subsequent dumps to see if that difference is actually what makes =
-O3
to not vectorize. (At this point you can play with -f* -fno-* to reduce the
differences further and isolate the trigger).