Hi,

I’m running into a problem with some code that makes use of the “sum” intrinsic in F90. The original code is quite long, but I’ve cut and pasted a shorter version which reproduces the error. I’ve compiled this code using pgf90 with no optimization on opterons (64 bit) running both SUSE 9.1 and Gentoo, & get the same behaviour. I’ve also compiled with ifort on a pentium and the code works fine.

The problem is one of the “mysterious” kind, where an unrelated command (in this case, a write statement to the screen) changes the final result of sum command.

The test code, shown below, consists of a subroutine which builds an array (it works fine) and the main body where the defective sum occurs. The goal of the code is to calculate and print the array m_inv. For the test case m_inv(:,1,1) should equal m_inv(:,2,2).

```
module type_defs
integer, parameter :: float=kind(1.d0)
complex (float), parameter :: i=(0d0,1d0)
contains
!***********************************************************************
subroutine make_h_index(ns,nbrs,n,h_index)
!***********************************************************************
implicit none
integer, intent (in) :: ns,nbrs,n(2)
integer, intent (out) :: h_index(ns,nbrs)
integer :: i0,i1,i2
! Make the index function which contains
! the nearest neighbours of the site i2.
! This matrix describes the ns x ns
! upper-left block of the total superconducting
! matrix.
do i0 = 1,n(1)
do i1 = 1,n(2)
i2 = i0 + (i1-1)*n(1)
h_index(i2,1) = i2 ! on-site
if (i0 /= n(1)) then ! neighbour in +x direction
h_index(i2,2) = i2 + 1
else
h_index(i2,2) = 1 + (i1-1)*n(1)
endif
if (i0 /= 1) then ! neighbour in -x direction
h_index(i2,3) = i2 - 1
else
h_index(i2,3) = n(1) + (i1-1)*n(1)
endif
if (i1 /= n(2)) then ! neighbour in +y direction
h_index(i2,4) = i2 + n(1)
else
h_index(i2,4) = i0
endif
if (i1 /= 1) then ! neighbour in -x direction
h_index(i2,5) = i2 - n(1)
else
h_index(i2,5) = i0 + (n(2)-1)*n(1)
endif
enddo
enddo
! Add next nearest and next-next nearest neighbours
do i0 = 1,ns
h_index(i0,6) = h_index(h_index(i0,2),4)
h_index(i0,7) = h_index(h_index(i0,3),4)
h_index(i0,8) = h_index(h_index(i0,3),5)
h_index(i0,9) = h_index(h_index(i0,2),5)
h_index(i0,10) = h_index(h_index(i0,2),2)
h_index(i0,11) = h_index(h_index(i0,3),3)
h_index(i0,12) = h_index(h_index(i0,4),4)
h_index(i0,13) = h_index(h_index(i0,5),5)
enddo
end subroutine make_h_index
end module type_defs
! MAIN PROGRAM
!********************************************************
program find_M_inv
use type_defs
implicit none
integer, parameter :: n=10,ns=n*n,ns2=2*ns,nbrs=13
integer :: h_index(ns,nbrs),nn(2)
real (float) :: H(ns,nbrs),pi
complex (float) :: Z(ns2,ns2)
complex (float) :: m_inv(ns2,2,2)
integer :: i0,i1,i2,i3,isn
complex (float) :: htk(ns,nbrs)
pi = 4d0*atan(1d0)
nn = (/ n,n /)
! Make the arrays needed to perform the inverse-mass calculation.
call make_h_index(ns,nbrs,nn,h_index)
htk = 0d0
htk(:,6:9) = 0.5d0
Z = 0d0
do i1 = 1,ns
do i2 = 1,ns
Z(i1,i2) = exp(i*2*pi*real(i2*i1)/real(ns))/sqrt(real(ns))
Z(i1+ns,i2+ns) = Z(i1,i2)
end do
end do
! Calculate m_inv
!
m_inv = 0d0
do i1 = 1,ns2
do i0 = 1,ns2
i2 = mod(i0-1,ns)+1
i3 = ns*((i0-1)/ns)
isn = -(-1)**((i0-1)/ns)
! **************
! When I uncomment this write statment, I get the correct values for
! M_inv(:,2,2). Obviously this makes no sense.
!write(6,*) abs(sum(Z(h_index(i2,4:5)+i3,i1)*htk(i2,4:5)))
! xx component:
m_inv(i1,1,1) = m_inv(i1,1,1) + isn*conjg(Z(i0,i1))*( &
sum(Z(h_index(i2,2:3)+i3,i1)*htk(i2,2:3))&
+sum(Z(h_index(i2,6:9)+i3,i1)*htk(i2,6:9))&
)
! yy component:
m_inv(i1,2,2) = m_inv(i1,2,2) + isn*conjg(Z(i0,i1))*( &
!
! When I comment out the next line, I get the correct answer for m_inv.
! It shouldn't make any difference since htk(i2,4:5) = 0
sum(Z(h_index(i2,4:5)+i3,i1)*htk(i2,4:5))&
!
! Alternatively, If I write the above line explicitly as
! Z(h_index(i2,4)+i3,i1)*htk(i2,4) &
! + Z(h_index(i2,5)+i3,i1)*htk(i2,5) &
! I get the correct answer for m_inv
!
+sum(Z(h_index(i2,6:9)+i3,i1)*htk(i2,6:9))&
)
end do
! These two number should always be equal:
write(6,*) real(m_inv(i1,1,1)),real(m_inv(i1,2,2))
! This code seems to return m_inv(:,1,1) correctly, but not m_inv(:,2,2).
! For reference, the last elements of the array should be
! m_inv(ns2,1,1)=m_inv(ns2,2,2)=2
end do
end program find_M_inv
```