* gcc 4.6.1 fpmath options produce unexpected results
@ 2012-02-01 21:12 tomsen_san
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From: tomsen_san @ 2012-02-01 21:12 UTC (permalink / raw)
To: gcc-help
Hi there,
I am running some numerical benchmarking on Ubuntu 11.10 with gcc 4.6.1 on
an Athlon XP (Barton).
I noticed that -O3 -march=athlon-xp -mfpmath=sse|both produces results
twice as fast for double-precision FP-ops as -O3 -march=athlon-xp
-mfpmath=387.
I looked at the assembly code and found the reason to be inefficient code in
the latter case.
For a CPU that does not support SSE2 and above this result is slightly
disconcerting as this makes selecting the correct options for producing fast
code hard. I documented this here:
http://mandelperformance.blogspot.com/2012/01/magic-gcc-options.html
http://mandelperformance.blogspot.com/2012/01/magic-gcc-options.html
It would be nice if someone commented the fact that the SSE option produces
perfect 387 code while the 387 option does not :-)
best regards
-tomsen
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