I'm having an issue with the debugger output in Visual Studio, which is showing uninitialized or in some cases undefined variables upon entering an OpenMP parallel section. This occurs despite declaring the variables as firstprivate. I've tested a couple scenarios and the issue occurs for subroutine arguments. I've put together a MWE to demonstrate that this problem does not appear for local variables or common variables, but it does exist for non-common and common variables alike that are passed in as arguments to a subroutine. This can be seen in Fig. 1 where execution can be seen paused at L43 by a breakpoint in the OMP parallel do section. The tested variables are as follows:
- arg1: test_arg passed in from main to feeder and subsequently into test_subroutine. (Problem)
- test_loc: local variable created in test_subroutine. (Good)
- test_glob1: common variable created in main and accessed in test_subroutine. (Good)
- arg2: test_glob2 defined in MWEh.for, initialized in init, and passed into test_subroutine from feeder. (Problem)
Fig. 1:
This MWE is as close of a program to the project I’m working on where I get similar behaviour, but variables in this other project actually become undefined in the debugger as opposed to garbage data. When I reduce the size of an array that appears as undefined from ~3k elements to 3 in this other project for example, the variable is shown as garbage data instead of undefined. This somewhat leads me to believe that the VS debugger is limited in memory, but this is only speculation. I could not "break" the MWE in such a way to show undefined variables however. The only apparent difference between the other project and the MWE shown here is the number of input arguments to any subroutine. The other project easily has 50+ input arguments to any given subroutine, which is part of the reason I thought a MWE was necessary to post here. Could this be a contributing factor for why variables are shown as undefined? I'll re-iterate that in this other project, I also use a parallel do loop and label the required variables as firstprivate as I did in the MWE.
Despite Visual Studio showing garbage data in the debugger, the values are printed correctly as shown in Fig. 2. I ran the program with 1 thread, but the problem occurs for any number of threads Is this something I should be concerned about? Even if it's not a concern, I'd appreciate any insight into why the debugger is behaving in this manner.
Fig. 2:
I'm using Visual Studio 2022 with Intel Fortran Compiler Classic 2021.5.0, although I first encountered this problem in Visual Studio 2012 with Intel Visual Fortran Compiler XE 14.0.2.176. I have /debug-parameters:used and /Qopenmp compiler flags activated.
Thank you in advance for any and all help provided.
MWE.for:
subroutine test_subroutine(arg1, arg2)
common /test_block/ test_glob1
character(20) :: fmt1, fmt2
real(4),intent(in)::arg1(3), arg2(100000)
real(4)::test_glob1(3)
real(4)::test_loc(3)
test_loc = (/4, 5, 6/)
fmt1 = "(A31,I8)"
print fmt1, 'Address of arg1 = ', loc(arg1)
print fmt1, 'Address of test_loc = ', loc(test_loc)
print fmt1, 'Address of test_glob1 = ', loc(test_glob1)
print fmt1, 'Address of test_glob2 (arg2) = ', loc(arg2)
print *, 'Entering do loop...'
!$OMP PARALLEL DO FIRSTPRIVATE(arg1, test_loc, test_glob1, arg2)
do i = 1, 3
print '( A5,I1,A17,F3.1,A12,I8)', 'arg1(', i, ') = ', arg1(i), '. Address = ', loc(arg1(i))
print '( A9,I1,A13,F3.1,A12,I8)', 'test_loc(', i, ') = ', test_loc(i), '. Address = ', loc(test_loc(i))
print '(A11,I1,A11,F3.1,A12,I8)', 'test_glob1(', i, ') = ', test_glob1(i), '. Address = ', loc(test_glob1(i))
print '(A11,I1,A11,F3.1,A12,I8)', 'test_glob2(', i, ') (arg2) = ', arg2(i), '. Address = ', loc(arg2(i))
end do
!$OMP END PARALLEL DO
return
end
subroutine feeder(test_arg)
include 'MWEh.for'
real(4),intent(in)::test_arg(3)
call test_subroutine(test_arg, test_glob2)
end
subroutine init
include 'MWEh.for'
do i = 1, n
test_glob2(i) = i
end do
end
program MWE
use omp_lib
! Variables
common /test_block/ test_glob1
real(4) :: test_glob1(3)
real(4) :: test_arg(3)
character(1) :: nthreads_string
integer :: rank, nthreads
call init
test_arg = (/1, 2, 3/)
test_glob1 = (/7, 8, 9/)
! Body of MWE
!DEC$ IF DEFINED (_OPENMP)
call get_command_argument(1,nthreads_string)
read (nthreads_string,*) nthreads
call OMP_SET_NUM_THREADS(nthreads)
print *, 'Running MWE in Parallel on', nthreads, 'threads'
!$OMP PARALLEL PRIVATE(rank)
rank = OMP_GET_THREAD_NUM()
!$OMP END PARALLEL
!DEC$ ELSE
pause 'This MWE must be run in parallel. Enable OpenMP'
call exit(0)
!DEC$ ENDIF
!$OMP BARRIER
call feeder(test_arg)
end program MWE
MWEh.for:
! Header file
parameter(n=100000)
common /extern_block/ test_glob2(n)
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