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[Bug middle-end/46837] induct compiled with -ffast-math -fschedule-insns -fsched-pressure -O3 ~30% slower
- From: "howarth at nitro dot med.uc.edu" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Tue, 7 Dec 2010 16:48:14 +0000
- Subject: [Bug middle-end/46837] induct compiled with -ffast-math -fschedule-insns -fsched-pressure -O3 ~30% slower
- Auto-submitted: auto-generated
- References: <bug-46837-4@http.gcc.gnu.org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46837
--- Comment #2 from Jack Howarth <howarth at nitro dot med.uc.edu> 2010-12-07 16:48:11 UTC ---
The runtime performance reduction from...
gfortran -mtune=core2 -fschedule-insns -ffast-math -O3 induct.f90 -o induct
compared to...
gfortran -mtune=core2 -ffast-math -O3 induct.f90 -o induct
...is very confusing. According to the man page...
-fschedule-insns
If supported for the target machine, attempt to reorder instructions
to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating
point or memory load instructions by allowing other
instructions to be issued until the result of the load or floating
point instruction is required.
Enabled at levels -O2, -O3.
So shouldn't this already be defaulted on by -O3 in the second case?