Fortran: fix treatment of character, value, optional dummy arguments [PR107444]

Fix handling of character dummy arguments that have the optional+value
attribute.  Change name of internal symbols that carry the hidden presence
status of optional arguments to distinguish them from the internal hidden
character length.  Update documentation to clarify the gfortran ABI.

gcc/fortran/ChangeLog:

	PR fortran/107444
	* trans-decl.cc (create_function_arglist): Extend presence status
	to all intrinsic types, and change prefix of internal symbol to '.'.
	* trans-expr.cc (gfc_conv_expr_present): Align to changes in
	create_function_arglist.
	(gfc_conv_procedure_call): Fix generation of procedure arguments for
	the case of character dummy arguments with optional+value attribute.
	* trans-types.cc (gfc_get_function_type): Synchronize with changes
	to create_function_arglist.
	* doc/gfortran/naming-and-argument-passing-conventions.rst: Clarify
	the gfortran argument passing conventions with regard to OPTIONAL
	dummy arguments of intrinsic type.

gcc/testsuite/ChangeLog:

	PR fortran/107444
	* gfortran.dg/optional_absent_7.f90: Adjust regex.
	* gfortran.dg/optional_absent_8.f90: New test.

4 files changed, 30 insertions, 22 deletions

diff --git a/gcc/fortran/doc/gfortran/naming-and-argument-passing-conventions.rst b/gcc/fortran/doc/gfortran/naming-and-argument-passing-conventions.rst
index 4baaee9..fa999fa 100644
--- a/gcc/fortran/doc/gfortran/naming-and-argument-passing-conventions.rst
+++ b/gcc/fortran/doc/gfortran/naming-and-argument-passing-conventions.rst
@@ -142,8 +142,7 @@ is used for dummy arguments; with ``VALUE``, those variables are
 passed by value.
 
 For ``OPTIONAL`` dummy arguments, an absent argument is denoted
-by a NULL pointer, except for scalar dummy arguments of type
-``INTEGER``, ``LOGICAL``, ``REAL`` and ``COMPLEX``
+by a NULL pointer, except for scalar dummy arguments of intrinsic type
 which have the ``VALUE`` attribute.  For those, a hidden Boolean
 argument (``logical(kind=C_bool),value``) is used to indicate
 whether the argument is present.
diff --git a/gcc/fortran/trans-decl.cc b/gcc/fortran/trans-decl.cc
index 94988b8..217de6b 100644
--- a/gcc/fortran/trans-decl.cc
+++ b/gcc/fortran/trans-decl.cc
@@ -2708,16 +2708,16 @@ create_function_arglist (gfc_symbol * sym)
 		type = gfc_sym_type (f->sym);
 	    }
 	}
-      /* For noncharacter scalar intrinsic types, VALUE passes the value,
+      /* For scalar intrinsic types, VALUE passes the value,
 	 hence, the optional status cannot be transferred via a NULL pointer.
 	 Thus, we will use a hidden argument in that case.  */
-      else if (f->sym->attr.optional && f->sym->attr.value
-	       && !f->sym->attr.dimension && f->sym->ts.type != BT_CLASS
-	       && !gfc_bt_struct (f->sym->ts.type))
+      if (f->sym->attr.optional && f->sym->attr.value
+	  && !f->sym->attr.dimension && f->sym->ts.type != BT_CLASS
+	  && !gfc_bt_struct (f->sym->ts.type))
 	{
           tree tmp;
           strcpy (&name[1], f->sym->name);
-          name[0] = '_';
+	  name[0] = '.';
           tmp = build_decl (input_location,
 			    PARM_DECL, get_identifier (name),
 			    boolean_type_node);
diff --git a/gcc/fortran/trans-expr.cc b/gcc/fortran/trans-expr.cc
index f3fbb52..b95c5cf 100644
--- a/gcc/fortran/trans-expr.cc
+++ b/gcc/fortran/trans-expr.cc
@@ -1985,15 +1985,14 @@ gfc_conv_expr_present (gfc_symbol * sym, bool use_saved_desc)
 
   /* Intrinsic scalars with VALUE attribute which are passed by value
      use a hidden argument to denote the present status.  */
-  if (sym->attr.value && sym->ts.type != BT_CHARACTER
-      && sym->ts.type != BT_CLASS && sym->ts.type != BT_DERIVED
-      && !sym->attr.dimension)
+  if (sym->attr.value && !sym->attr.dimension
+      && sym->ts.type != BT_CLASS && !gfc_bt_struct (sym->ts.type))
     {
       char name[GFC_MAX_SYMBOL_LEN + 2];
       tree tree_name;
 
       gcc_assert (TREE_CODE (decl) == PARM_DECL);
-      name[0] = '_';
+      name[0] = '.';
       strcpy (&name[1], sym->name);
       tree_name = get_identifier (name);
 
@@ -6162,11 +6161,21 @@ gfc_conv_procedure_call (gfc_se * se, gfc_symbol * sym,
 		 value, pass "0" and a hidden argument gives the optional
 		 status.  */
 	      if (fsym && fsym->attr.optional && fsym->attr.value
-		  && !fsym->attr.dimension && fsym->ts.type != BT_CHARACTER
-		  && fsym->ts.type != BT_CLASS && fsym->ts.type != BT_DERIVED)
+		  && !fsym->attr.dimension && fsym->ts.type != BT_CLASS
+		  && !gfc_bt_struct (sym->ts.type))
 		{
-		  parmse.expr = fold_convert (gfc_sym_type (fsym),
-					      integer_zero_node);
+		  if (fsym->ts.type == BT_CHARACTER)
+		    {
+		      /* Pass a NULL pointer for an absent CHARACTER arg
+			 and a length of zero.  */
+		      parmse.expr = null_pointer_node;
+		      parmse.string_length
+			= build_int_cst (gfc_charlen_type_node,
+					 0);
+		    }
+		  else
+		    parmse.expr = fold_convert (gfc_sym_type (fsym),
+						integer_zero_node);
 		  vec_safe_push (optionalargs, boolean_false_node);
 		}
 	      else
diff --git a/gcc/fortran/trans-types.cc b/gcc/fortran/trans-types.cc
index 42907be..196f2ce 100644
--- a/gcc/fortran/trans-types.cc
+++ b/gcc/fortran/trans-types.cc
@@ -3225,15 +3225,15 @@ gfc_get_function_type (gfc_symbol * sym, gfc_actual_arglist *actual_args,
 
 	  vec_safe_push (hidden_typelist, type);
 	}
-      /* For noncharacter scalar intrinsic types, VALUE passes the value,
+      /* For scalar intrinsic types, VALUE passes the value,
 	 hence, the optional status cannot be transferred via a NULL pointer.
 	 Thus, we will use a hidden argument in that case.  */
-      else if (arg
-	       && arg->attr.optional
-	       && arg->attr.value
-	       && !arg->attr.dimension
-	       && arg->ts.type != BT_CLASS
-	       && !gfc_bt_struct (arg->ts.type))
+      if (arg
+	  && arg->attr.optional
+	  && arg->attr.value
+	  && !arg->attr.dimension
+	  && arg->ts.type != BT_CLASS
+	  && !gfc_bt_struct (arg->ts.type))
 	vec_safe_push (typelist, boolean_type_node);
       /* Coarrays which are descriptorless or assumed-shape pass with
 	 -fcoarray=lib the token and the offset as hidden arguments.  */