[Bug fortran/85364] New: -fopenmp should not put array in program on the stack

["tkoenig at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85364

            Bug ID: 85364
           Summary: -fopenmp should not put array in program on the stack
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkoenig at gcc dot gnu.org
  Target Milestone: ---

Currently, gfortran sometimes puts array in the main program on the stack
with -fopenmp.

This is not needed and makes no sense, because the main program in Fortran can
never be called by anything else.

It can also cause considerable confusion, as in PR85346.

Test case:

$ cat a.f90
! { dg-do  run }
! PR 83064 - this used to give wrong results.
! { dg-additional-options "-O1 -ftree-parallelize-loops=2" }
! Original test case by Christian Felter

program main
    use, intrinsic :: iso_fortran_env
    implicit none

    integer, parameter :: nsplit = 4
    integer(int64), parameter :: ne = 2**20
    integer(int64) :: stride, low(nsplit), high(nsplit), i
    integer(int64), dimension(ne) :: edof
    real(real64), dimension(nsplit) :: pi

    edof(1::4) = 1
    edof(2::4) = 2
    edof(3::4) = 3
    edof(4::4) = 4

    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
    if (abs (sum(pi) - atan(1.0d0)) > 1e-5) STOP 1

contains

    pure function compute( low, high ) result( ttt )        
        integer(int64), intent(in) :: low, high
        real(real64), dimension(nsplit) :: ttt
        integer(int64) :: j, k

        ttt = 0

        ! Unrolled loop
!         do j = low, high, 4
!             k = 1
!             ttt(k) = ttt(k) + (-1)**(j+1) / real( 2*j-1 )                     
!             k = 2
!             ttt(k) = ttt(k) + (-1)**(j+2) / real( 2*j+1 )                     
!             k = 3
!             ttt(k) = ttt(k) + (-1)**(j+3) / real( 2*j+3 )                     
!             k = 4
!             ttt(k) = ttt(k) + (-1)**(j+4) / real( 2*j+5 )                     
!         end do

        ! Loop with modulo operation
!         do j = low, high
!             k = mod( j, nsplit ) + 1
!             ttt(k) = ttt(k) + (-1)**(j+1) / real( 2*j-1 )                     
!         end do

        ! Loop with subscripting via host association
        do j = low, high
            k = edof(j)
            ttt(k) = ttt(k) + (-1.0_real64)**(j+1) / real( 2*j-1 )              
        end do
    end function

end program main
$ gfortran -fopenmp a.f90
$ ./a.out
Speicherzugriffsfehler (Speicherabzug geschrieben)