Hi - We noticed some code was very slow in gfortran vs g77, almost a factor
of 7 times slower. This appears to be because gfortran uses the expf
liubrary function call for the fortran exp() function for single precision
numbers, but g77 uses the exp library function (i.e. the double precision
function).
You can see the very large speed differences by telling gfortran to always
use double precision numbers.
This can be replicated with this simple program:
test.f:
implicit none
integer i,j
real a, u
do j=1,10
do i=1,10000000
a=exp(1.0+1.0/i)
u = u+a
end do
end do
print *,u
end
Compiled with gfortran -O2:
real 0m29.921s
user 0m29.912s
sys 0m0.000s
Compiled with gfortran -O2 -fdefault-real-8:
real 0m4.306s
user 0m4.304s
sys 0m0.000s
This is with a newly built gcc 4.4.1 on Fedora 10 (glibc 2.9), x86-64.
As the difference comes down to the speed difference between exp and expf,
is it a gcc issue or a glibc issue? Should gcc be using this slow function?
It seems amazing that the same function call is 7 times slower for lower
precision numbers!
Thanks
Jeremy
--
Jeremy Sanders <j...@ast.cam.ac.uk> http://www-xray.ast.cam.ac.uk/~jss/
X-Ray Group, Institute of Astronomy, University of Cambridge, UK.
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