Large slowdown with gfortran vs f77 (x7)

[James Cloos]

>>>>> "Jeremy" == Jeremy Sanders <jss@ast.cam.ac.uk> writes:

Jeremy> You can see the very large speed differences by telling gfortran
Jeremy> to always use double precision numbers.

Jeremy> This can be replicated with this simple program:

Jeremy> Compiled with gfortran -O2:
Jeremy> real    0m29.921s
Jeremy> user    0m29.912s
Jeremy> sys     0m0.000s

Jeremy> Compiled with gfortran -O2 -fdefault-real-8:
Jeremy> real    0m4.306s
Jeremy> user    0m4.304s
Jeremy> sys     0m0.000s

Jeremy> This is with a newly built gcc 4.4.1 on Fedora 10 (glibc 2.9), x86-64.

I tried it on my 1Ghz PIII laptop running Gentoo with gcc 4.1.1 and
glibc 2.10.1.

I added -march=pentium3 to the gcc cmd line; that probably made little
difference.  (glibc was also compiled with -march=pentium3 -O2).

I got nearly identical user times for the two compiles; user time was
always within 0.03 of 23.40 over multiple runs.

Incidently, while the real-8 compile output 271828665.96115601, the
real4 compile output 67108976.

This does, then, seem to be an x86-64 issue.

-JimC
-- 
James Cloos <cloos@jhcloos.com>         OpenPGP: 1024D/ED7DAEA6