[Bug fortran/83017] New: DO CONCURRENT not parallelizing

["cfztol at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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