Fortran: Skip bound conv in gfc_conv_gfc_desc_to_cfi_desc with intent(out) ptr [PR108621]

When the dummy argument of the bind(C) proc is 'pointer, intent(out)', the conversion
of the GFC to the CFI bounds can be skipped: it is not needed and avoids issues with
noninit memory.

Note that the 'cfi->base_addr = gfc->addr' assignment is kept as the C code of a user
might assume that a nullified pointer arrives as NULL (or even a specific value).
For instance, gfortran.dg/c-interop/section-{1,2}.f90 assumes the value NULL.

Note 2: The PR is about a may-be-uninitialized warning with intent(out). In the PR's
testcase, the pointer was nullified and should not have produced that warning.
That is a diagnostic issue, now tracked as PR middle-end/108906 as the issue in principle
still exists (e.g. with 'intent(inout)'). [But no longer for intent(out).]

Note 3: With undefined pointers and no 'intent', accessing uninit memory is unavoidable
on the caller side as the compiler cannot know what the C function does (but this usage
determines whether the pointer is permitted be undefined or whether the bounds must be
gfc-to-cfi converted).

gcc/fortran/ChangeLog:

	PR fortran/108621
	* trans-expr.cc (gfc_conv_gfc_desc_to_cfi_desc): Skip setting of
	bounds of CFI desc for 'pointer,intent(out)'.

gcc/testsuite/ChangeLog:

	PR fortran/108621
	* gfortran.dg/c-interop/fc-descriptor-pr108621.f90: New test.

1 files changed, 6 insertions, 0 deletions

diff --git a/gcc/fortran/trans-expr.cc b/gcc/fortran/trans-expr.cc
index e85b53f..045c8b0 100644
--- a/gcc/fortran/trans-expr.cc
+++ b/gcc/fortran/trans-expr.cc
@@ -5673,6 +5673,9 @@ gfc_conv_gfc_desc_to_cfi_desc (gfc_se *parmse, gfc_expr *e, gfc_symbol *fsym)
   gfc_add_modify (&block, tmp,
 		  build_int_cst (TREE_TYPE (tmp), attr));
 
+  /* The cfi-base_addr assignment could be skipped for 'pointer, intent(out)'.
+     That is very sensible for undefined pointers, but the C code might assume
+     that the pointer retains the value, in particular, if it was NULL.  */
   if (e->rank == 0)
     {
       tmp = gfc_get_cfi_desc_base_addr (cfi);
@@ -5695,6 +5698,9 @@ gfc_conv_gfc_desc_to_cfi_desc (gfc_se *parmse, gfc_expr *e, gfc_symbol *fsym)
       gfc_add_modify (&block, tmp2, fold_convert (TREE_TYPE (tmp2), tmp));
     }
 
+  if (fsym->attr.pointer && fsym->attr.intent == INTENT_OUT)
+    goto done;
+
   /* When allocatable + intent out, free the cfi descriptor.  */
   if (fsym->attr.allocatable && fsym->attr.intent == INTENT_OUT)
     {