http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48329
Summary: Program takes twice as long *without* -fopenmp than
with 1 OpenMP thread
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Keywords: missed-optimization, openmp
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: bur...@gcc.gnu.org
Program taken from http://openmp.org/forum/viewtopic.php?f=3&t=1123
System: Intel Xeon X5570 @ 2.93GHz, SUSE SLES 11 (x86_64) [glibc-2.11.1]
No OpenMP:
gfortran -O3 -ffast-math test2.f90
time ./a.out -> 14.44user 0.00system 0:14.46elapsed 99%CPU
With OpenMP and OMP_NUM_THREADS=1
gfortran -fopenmp -O3 -ffast-math test2.f90
time ./a.out -> 7.22user 0.00system 0:07.23elapsed 99%CPU
Using gfortran 4.3.4, I get the 7s result also without -fopenmp; ditto with
ifort 11.1. With OpenMP the run time of GCC 4.6 and ifort is exactly the same
[modulo noise] for 1 and 2 threads.
program calcpi
USE omp_lib
implicit none
double precision:: h,x,sum,pi
integer:: n,i
double precision:: f
f(x) = 4.0/(1.0+x**2)
n = 2100000000
h= 1.0 / dble(n)
sum = 0.0
!$OMP PARALLEL DO DEFAULT(NONE) &
!$OMP SHARED(n,h) PRIVATE(x) &
!$OMP REDUCTION(+:sum)
DO i=1, n
x = h * (dble(i)-0.5)
sum = sum + f(x)
END DO
!$OMP END PARALLEL DO
pi = h * sum
write(*,*) 'Pi=',pi
end program calcpi
