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Re: food for optimizer developers
Ralf W. Grosse-Kunstleve wrote:
Without knowing the compiler options, the results of any benchmark
gfortran -o dsyev_test_gfortran -O3 -ffast-math dsyev_test.f
If you work on a 32bit x86 system, you really should use -march=native
and possibly -mfpmath=sse - otherwise you might generate code which also
works with very old versions of x86 processors. (On x86-64 -march=native
is also useful albeit the effect is not as large; ifort's equivalent
option is -xHost. By default, ifort uses SSE and more modern processors
A possibly negligible effect is that ifort ignores parentheses by
default - to be standard conform, use ifort's -assume protect_parens.
(Or gfortran's -fno-protect-parens, though your gfortran might be too old.)
An interesting test could be to use Intel's libimf with
gfortran-compiled binaries by setting the
environmental variable (complete the path!) before running the binary of
your program. Another check could be to compile with the
-mveclibabi=svml and link then Intel's short vactor math library. That
way you make sure you compare the compiler itself and not the compiler +
In terms of options, I think -funroll-loops should also be used as it
usually improves performance (it is not enabled by any -O... option).
GCC 4.5/4.6: You could also try "-flto -fwhole-file". (Best to use 4.6