[Bug libgomp/94648] New: [OpenMP] DEVICE_HOST_FALLBACK when no actual device + omp_target_alloc + omp_get_default_device

[burnus at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94648

            Bug ID: 94648
           Summary: [OpenMP] DEVICE_HOST_FALLBACK when no actual device +
                    omp_target_alloc + omp_get_default_device
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: openmp, wrong-code
          Severity: normal
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

That's with sollve_vv, tests/4.5/target_data/test_target_data_map_alloc.c
https://github.com/SOLLVE/sollve_vv.git

If one runs that test case on a system without offloading support compiled it,
it fails:

  int *d_sum = (int *)omp_target_alloc(sizeof(int), omp_get_default_device());
  #pragma omp target is_device_ptr(d_sum)

* The omp_get_default_device () function returns '0'
* omp_target_alloc contains:

  if (device_num == GOMP_DEVICE_HOST_FALLBACK)
    return malloc (size);
  struct gomp_device_descr *devicep = resolve_device (device_num);
  if (devicep == NULL)
    return NULL;

As devicep == NULL, it fails.

I wonder whether
  omp_get_default_device ()
should return -2 == GOMP_DEVICE_HOST_FALLBACK in this case – namely, if no
devices are available and, possibly, if OMP_DEFAULT_DEVICE is not explicitly
set.

At least that seems to make more sense than "0" and then failing to
return omp_target_alloc.

"TABLE2.1:ICV Initial Values" lists
  default-device-var  OMP_DEFAULT_DEVICE  Implementation defined
Thus, I think it would be permitted. (But I might have missed some fine print.)

 * * *

#include <assert.h>
#include <omp.h>

int main()
{
  int sum = 0;
  int *d_sum = (int *) omp_target_alloc (sizeof (int),
                                         omp_get_default_device ());

  assert (d_sum);

#pragma omp target is_device_ptr(d_sum) map(from:sum)
  {
    *d_sum = 42;
    sum = *d_sum;
  }
  assert (sum == 42);
  return 0;
}