Add missing pointer map in test_declare_mapper_present.c

[abhinavgaba]

The test was mapping a pointee array-section s.data[0:N], without mapping the base-pointer s.data itself, and then expecting s.data to be accessible on the device.

The fix is to map s.data as well.

[abhinavgaba]

We could add map(present, alloc: v.data) on the mapper as well, but it's not necessary for correctness.

[abhinavgaba]

@spophale, @tob2, please take a look.

[colleeneb]

Out of curiosity, is this change specific to s being a struct and so for structs we will need to allocate members like s.data on the device? Or is this general to any array section mapping, like for:

   int * s_array = (int *) malloc(N*sizeof(int)) 

  #pragma omp target enter data map(to: s_array[0:s.len]) # do we now also need map(alloc: s_array)?
   ….
  // code uses s_array[0] in a target region
  ….

Do we now also need map(alloc: s_array)?

And would map(to: s.data) have worked instead of map(alloc: s.data)? I’m asking in case this comes up in some other code in the future.

Thanks!

[abhinavgaba]

Or is this general to any array section mapping, like for

Yes, this is applicable to all array-sections with pointer bases. For any map(s_array[...]), the pointer s_array is not supposed to be mapped, only the pointee array-section. The user has control over whether s_array itself should be mapped.

  int * s_array = (int *) malloc(N*sizeof(int));
  #pragma omp target enter data map(to: s_array[0:s.len]) // needs map(alloc: s_array)

  printf("%d\n", omp_target_is_present(&s_array, omp_get_default_device()); // Should print 0 when s_array is not mapped.

And would map(to: s.data) have worked instead of map(alloc: s.data)?

Yes, map(to: s.data) would have worked, but the to is redundant, as it would have been overwritten by the pointer-attachment that happens between s.data and s.data[...] because they are being newly mapped on the directive.

[abhinavgaba]

Note that for scalar pointers, there is an implicit assumed-size-array-map on target constructs, so even if the pointer is not mapped, it may still be legal to access the pointee through it. e.g.

  int * s_array = (int *) malloc(N*sizeof(int));
  #pragma omp target enter data map(to: s_array[0:s.len])

  #pragma omp target // implies map(s_array[:]) (or s_array[0:0] as per OpenMP 5.2)
  {
    s_array[0] = 111; // This is OK
  }

  #pragma omp target map(present, alloc: s_array) // But this present-check should fail
  {}

[colleeneb]

Got it, thank you!