https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83017
Bug ID: 83017
Summary: DO CONCURRENT not parallelizing
Product: gcc
Version: 6.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: cfztol at hotmail dot com
Target Milestone: ---
I found a strange behaviour, which I think is a bug. I'm trying to split a loop
with DO CONCURRENT and compile flag -ftree-parallelize-loops=2 . However,
whether parallelization actually happens depends on whether I use a computed
vector index in an assignment or a constant index. The indexing happens inside
a pure function, so I believe that should not affect the outer do concurrent
loop (?)
Here's my test code
! compile:
! gfortran -Ofast -march=native -ftree-parallelize-loops=2 -o main main.f90
! run;
! time ./main
program main
use, intrinsic :: iso_fortran_env
implicit none
integer, parameter :: nsplit = 4
integer(int64), parameter :: ne = 2000000000
integer(int64) :: stride, low(nsplit), high(nsplit), i
real, dimension(nsplit) :: pi
stride = ceiling(real(ne)/nsplit)
do i = 1, nsplit
high(i) = stride*i
end do
do i = 2, nsplit
low(i) = high(i-1) + 1
end do
low(1) = 1
high(nsplit) = ne
pi = 0
do concurrent (i = 1:nsplit)
pi(i) = sum(compute( low(i), high(i) ))
end do
print *, "PI", 4*sum(pi)
print *, "PI", 4*atan(1.0)
contains
pure function compute( low, high ) result( tmp )
integer(int64), intent(in) :: low, high
real, dimension(nsplit) :: tmp
integer(int64) :: j, k
tmp = 0
! With this loop no parallelization happens
do j = low, high
k = mod( j, nsplit ) + 1
tmp(k) = tmp(k) + (-1)**(j+1) / real( 2*j-1 )
end do
! With this loop the code is parallelized (apparent from time study)
! do j = low, high, 4 ! nsplit is equal to 4
! k = 1
! tmp(k) = tmp(k) + (-1)**(j+1) / real( 2*j-1 )
! k = 2
! tmp(k) = tmp(k) + (-1)**(j+2) / real( 2*j+1 )
! k = 3
! tmp(k) = tmp(k) + (-1)**(j+3) / real( 2*j+3 )
! k = 4
! tmp(k) = tmp(k) + (-1)**(j+4) / real( 2*j+5 )
! end do
end function
end program main
