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[Bug middle-end/46837] induct compiled with -ffast-math -fschedule-insns -fsched-pressure -O3 ~30% slower


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?


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