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[Ada] Add warning for explicit by-reference mechanism

This instructs gigi to issue a warning when an explicit by-reference mechanism 
specified by means of pragma Export_Function cannot be honored.

Tested on x86_64-suse-linux, applied on the mainline.

2019-08-30  Eric Botcazou  <>

	* gcc-interface/ada-tree.h (DECL_FORCED_BY_REF_P): New macro.
	* gcc-interface/decl.c (gnat_to_gnu_param): Set it on parameters
	whose mechanism was forced to by-reference.
	* gcc-interface/trans.c (Call_to_gnu): Do not issue a warning about a
	misaligned actual parameter if it is based on a CONSTRUCTOR.  Remove
 	obsolete warning for users of Starlet.  Issue a warning if a temporary
	is make around the call for a parameter with DECL_FORCED_BY_REF_P set.
	(addressable_p): Return true for REAL_CST and ADDR_EXPR.

Eric Botcazou
Index: gcc-interface/ada-tree.h
--- gcc-interface/ada-tree.h	(revision 275062)
+++ gcc-interface/ada-tree.h	(working copy)
@@ -482,6 +482,9 @@ do {						   \
    value of a function call or 'reference to a function call.  */
+/* Nonzero in a PARM_DECL if its mechanism was forced to by-reference.  */
 /* In a FIELD_DECL corresponding to a discriminant, contains the
    discriminant number.  */
Index: gcc-interface/decl.c
--- gcc-interface/decl.c	(revision 275196)
+++ gcc-interface/decl.c	(working copy)
@@ -5208,6 +5208,7 @@ gnat_to_gnu_param (Entity_Id gnat_param, tree gnu_
   bool ro_param = in_param && !Address_Taken (gnat_param);
   bool by_return = false, by_component_ptr = false;
   bool by_ref = false;
+  bool forced_by_ref = false;
   bool restricted_aliasing_p = false;
   location_t saved_location = input_location;
   tree gnu_param;
@@ -5235,7 +5236,11 @@ gnat_to_gnu_param (Entity_Id gnat_param, tree gnu_
   /* Or else, see if a Mechanism was supplied that forced this parameter
      to be passed one way or another.  */
   else if (mech == Default || mech == By_Copy || mech == By_Reference)
-    ;
+    forced_by_ref
+      = (mech == By_Reference
+	 && !foreign
+	 && !TYPE_IS_BY_REFERENCE_P (gnu_param_type)
+	 && !Is_Aliased (gnat_param));
   /* Positive mechanism means by copy for sufficiently small parameters.  */
   else if (mech > 0)
@@ -5368,6 +5373,7 @@ gnat_to_gnu_param (Entity_Id gnat_param, tree gnu_
   gnu_param = create_param_decl (gnu_param_name, gnu_param_type);
   TREE_READONLY (gnu_param) = ro_param || by_ref || by_component_ptr;
   DECL_BY_REF_P (gnu_param) = by_ref;
+  DECL_FORCED_BY_REF_P (gnu_param) = forced_by_ref;
   DECL_BY_COMPONENT_PTR_P (gnu_param) = by_component_ptr;
     = (ro_param && (by_ref || by_component_ptr));
Index: gcc-interface/trans.c
--- gcc-interface/trans.c	(revision 275197)
+++ gcc-interface/trans.c	(working copy)
@@ -5257,30 +5257,20 @@ Call_to_gnu (Node_Id gnat_node, tree *gnu_result_t
 	  /* Do not issue warnings for CONSTRUCTORs since this is not a copy
 	     but sort of an instantiation for them.  */
-	  if (TREE_CODE (gnu_name) == CONSTRUCTOR)
+	  if (TREE_CODE (remove_conversions (gnu_name, true)) == CONSTRUCTOR)
-	  /* If the type is passed by reference, a copy is not allowed.  */
-	  else if (TYPE_IS_BY_REFERENCE_P (gnu_formal_type))
+	  /* If the formal is passed by reference, a copy is not allowed.  */
+	  else if (TYPE_IS_BY_REFERENCE_P (gnu_formal_type)
+		   || Is_Aliased (gnat_formal))
 	    post_error ("misaligned actual cannot be passed by reference",
-	  /* For users of Starlet we issue a warning because the interface
-	     apparently assumes that by-ref parameters outlive the procedure
-	     invocation.  The code still will not work as intended, but we
-	     cannot do much better since low-level parts of the back-end
-	     would allocate temporaries at will because of the misalignment
-	     if we did not do so here.  */
-	  else if (Is_Valued_Procedure (Entity (Name (gnat_node))))
-	    {
-	      post_error
-		("?possible violation of implicit assumption", gnat_actual);
-	      post_error_ne
-		("?made by pragma Import_Valued_Procedure on &", gnat_actual,
-		 Entity (Name (gnat_node)));
-	      post_error_ne ("?because of misalignment of &", gnat_actual,
-			     gnat_formal);
-	    }
+	  /* If the mechanism was forced to by-ref, a copy is not allowed but
+	     we issue only a warning because this case is not strict Ada.  */
+	  else if (DECL_FORCED_BY_REF_P (gnu_formal))
+	    post_error ("misaligned actual cannot be passed by reference??",
+			gnat_actual);
 	  /* If the actual type of the object is already the nominal type,
 	     we have nothing to do, except if the size is self-referential
@@ -10394,6 +10384,7 @@ addressable_p (tree gnu_expr, tree gnu_type)
     case STRING_CST:
     case INTEGER_CST:
+    case REAL_CST:
       /* Taking the address yields a pointer to the constant pool.  */
       return true;
@@ -10403,6 +10394,7 @@ addressable_p (tree gnu_expr, tree gnu_type)
       return TREE_STATIC (gnu_expr) ? true : false;
     case NULL_EXPR:
+    case ADDR_EXPR:
     case SAVE_EXPR:
     case CALL_EXPR:
     case PLUS_EXPR:

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