[openacc] Fix acc declare for VLAs

[openacc] Fix acc declare for VLAs

Consider test-case test.c, with VLA A:
...
int main (void) {
  int N = 1000;
  int A[N];
  #pragma acc declare copy(A)
  return 0;
}
...
compiled using:
...
$ gcc test.c -fopenacc -S -fdump-tree-all
...

At original, we have:
...
  #pragma acc declare map(tofrom:A);
...
but at gimple, we have a map (to:A.1), but not a map (from:A.1):
...
  int[0:D.2074] * A.1;

  {
    int A[0:D.2074] [value-expr: *A.1];

    saved_stack.2 = __builtin_stack_save ();
    try
      {
        A.1 = __builtin_alloca_with_align (D.2078, 32);
        #pragma omp target oacc_declare map(to:(*A.1) [len: D.2076])
      }
    finally
      {
        __builtin_stack_restore (saved_stack.2);
      }
  }
...

This is caused by the following incompatibility.  When storing the desired
from clause in oacc_declare_returns, we use 'A.1' as the key:
...
10898                 oacc_declare_returns->put (decl, c);
(gdb) call debug_generic_expr (decl)
A.1
(gdb) call debug_generic_expr (c)
map(from:(*A.1))
...
but when looking it up, we use 'A' as the key:
...
(gdb)
1471                  tree *c = oacc_declare_returns->get (t);
(gdb) call debug_generic_expr (t)
A
...

Fix this by extracing the 'A.1' lookup key from 'A' using the decl-expr.

In addition, unshare the looked up value, to fix avoid running into
an "incorrect sharing of tree nodes" error.

Using these two fixes, we get our desired:
...
     finally
       {
+        #pragma omp target oacc_declare map(from:(*A.1))
         __builtin_stack_restore (saved_stack.2);
       }
...

Build on x86_64-linux with nvptx accelerator, tested libgomp.

gcc/ChangeLog:

2020-10-06  Tom de Vries  <tdevries@suse.de>

	PR middle-end/90861
	* gimplify.c (gimplify_bind_expr): Handle lookup in
	oacc_declare_returns using key with decl-expr.

libgomp/ChangeLog:

2020-10-06  Tom de Vries  <tdevries@suse.de>

	PR middle-end/90861
	* testsuite/libgomp.oacc-c-c++-common/declare-vla.c: Remove xfail.