[Bug target/38306] [4.4/4.5 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[jv244 at cam dot ac dot uk]

------- Comment #16 from jv244 at cam dot ac dot uk  2009-09-01 09:13 -------
(In reply to comment #15)
> Please try -O2 and -O2 -funroll-loops too, since -O3 is not always good for
> speed.  (It would be even better if -O2 is not slower and you can find out what
> the culprit is at -O3; this is not necessarily possible though).

you're right that, without -fschedule-insns -O2 is faster than -O3 on this
case, but nothing comes close to 4.3 performance. adding '-fschedule-insns' to
the fastest -O2 choice makes it 20% slower.

All numbers with trunk:

 -O2 -march=native -funroll-loops  -ffast-math: 4.032
 -O2 -march=native -funroll-loops  -ffast-math -fschedule-insns: 4.712
 -O3 -march=native -funroll-loops  -ffast-math: 4.408
 -O2 -march=native -ffast-math: 11.373
 -O2 -march=native -ffast-math -fschedule-insns: 11.409
 -O3 -march=native -ffast-math: 4.296
 -O3 -march=native -ffast-math -fschedule-insns: 4.656

I can test other flags if you've a hint


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306