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gfortran and power calculation
- From: jbijak <jbijak at imc dot pcz dot czest dot pl>
- To: gcc at gcc dot gnu dot org
- Date: Wed, 04 Apr 2007 15:59:56 +0200
- Subject: gfortran and power calculation
Hi,
I tried to calculate the relations:
a=b**1.5
and
a=sqrt(b)**3
they are the same in the mathematical point of view but the second
version works 7 times faster. I tried it with various optimization
levels without big difference.
I think that it is easy to optimize no the code level.
I'm not a member of the group so if you need more details write to me
directly.
bellow is the piece of code I have used to check the problem
program aaaa
integer, parameter:: N=10000000
real*8 a(N)
integer i
real czas0,czas1 !time0, time1
! some initial values
do i=1,N
a(i)=real(i,8)*0.1
enddo
! the first version
call cpu_time(czas0)
do i=1,N
a(i)=a(i)**1.5
enddo
call cpu_time(czas1)
print*,"pow 1.5",czas1-czas0
! the second version
call cpu_time(czas0)
do i=1,N
a(i)=sqrt(a(i))**3
enddo
call cpu_time(czas1)
print*,"sqrt/pow",czas1-czas0
end
See you