We encountered a possible problem with OpenMP SIMD vectorization.
Here is the simplest piece of code which generates the error:
program test
implicit none
integer, parameter :: n=4
integer i,j,k,iv(2)
real(8) a(n*n)
a(:)=0.d0
do j=1,n
k=(j-1)*n
!$OMP SIMD PRIVATE(iv)
do i=1,n
k=k+1
a(k)=1
iv(:)=1
end do
end do
print *,a
end program
The program should fill the matrix a with ones. Instead the variable k is corrupted by the assignment iv(:)=1 even though iv is not used.
If iv has dimension 1 then the problem disappears. Removing the PRIVATE clause also fixes the problem.
(Do I understand correctly that any variable not declared as PRIVATE in a SIMD loop is automatically shared and updated atomically? In other words, does the OpenMP SIMD standard ensure that the variable k is incremented correctly?)
The problem occurs with Intel Fortran versions 17 and 18. GNU Fortran version 8.2.1 works fine.