[Bug fortran/104696] [OpenMP] Implicit mapping breaks struct mapping

["burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104696

Tobias Burnus <burnus at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[12 Regression][OpenMP]     |[OpenMP] Implicit mapping
                   |Implicit mapping breaks     |breaks struct mapping
                   |struct mapping              |
           Keywords|                            |missed-optimization

--- Comment #1 from Tobias Burnus <burnus at gcc dot gnu.org> ---
Looking closer at it, I no longer think it is a regression (to be checked if
deemed important).

But it looks as if there are two problems - one wrong-code one and one
missed-optimization one.

Namely, I think reason for both issues is that 
     map(to:var3.r[1].d [len: 88]) ...
is not turned into
     map(struct:var3 [len: 1]) map(to:var3.r[1].d [len: 88])
but into 'to' + pointer assign. This does not even work when using the 'always'
modifier.

('struct:' appears when using 'Q' instead of 'R(2)'.)

That that the struct not detected seems to be because array and component refs
are mixed – hiding that the var is memory wise in the same struct.

I believe with that fixed, it would work correctly.

  * * *

For C/C++, it "works".

But: it still does not detect that the member is part of the whole struct - and
allocates pointlessly too much memory. To illustrate this, I added a large
'arr' element.

Otherwise, that the reason that it works in C/C++ is that ATTACH and not
pointer assign is used.

Namely:
------------ test.c--------------
struct s { int *d; };
struct s2 { struct s r[5], q, arr[1024][1024]; };

int main () {
  struct s2 x;
  x.q.d = __builtin_malloc(sizeof(int));
  x.r[1].d = __builtin_malloc(sizeof(int));

  #pragma omp target map(tofrom: x.q.d[:1])
   *x.q.d = 2;
  #pragma omp target map(tofrom: x.r[1].d[:1])
   *x.r[1].d = 3;

  __builtin_printf("%d, %d\n", *x.q.d, *x.r[1].d);
  return 0;
}
------------ test.c--------------

gives:

  #pragma omp target num_teams(1) thread_limit(0)
     map(tofrom:x [len: 8388656][implicit]) map(tofrom:*_3 [len: 4])
     map(attach:x.q.d [bias: 0])

  #pragma omp target num_teams(1) thread_limit(0)
     map(tofrom:x [len: 8388656][implicit])
     map(tofrom:*_4 [len: 4]) map(attach:x.r[1].d [bias: 0])