Large slowdown with gfortran vs f77 (x7)

[Jeremy Sanders]

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 <jss@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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