Index: explow.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/explow.c,v
retrieving revision 1.118
diff -u -p -r1.118 explow.c
--- explow.c	13 Dec 2003 04:11:20 -0000	1.118
+++ explow.c	19 Mar 2004 19:18:45 -0000
@@ -597,18 +597,9 @@ maybe_set_unchanging (rtx ref, tree t)
   /* We can set RTX_UNCHANGING_P from TREE_READONLY for decls whose
      initialization is only executed once, or whose initializer always
      has the same value.  Currently we simplify this to PARM_DECLs in the
-     first case, and decls with TREE_CONSTANT initializers in the second.
-
-     We cannot do this for non-static aggregates, because of the double
-     writes that can be generated by store_constructor, depending on the
-     contents of the initializer.  Yes, this does eliminate a good fraction
-     of the number of uses of RTX_UNCHANGING_P for a language like Ada.
-     It also eliminates a good quantity of bugs.  Let this be incentive to
-     eliminate RTX_UNCHANGING_P entirely in favor of a more reliable
-     solution, perhaps based on alias sets.  */
+     first case, and decls with TREE_CONSTANT initializers in the second.  */
 
   if ((TREE_READONLY (t) && DECL_P (t)
-       && (TREE_STATIC (t) || ! AGGREGATE_TYPE_P (TREE_TYPE (t)))
        && (TREE_CODE (t) == PARM_DECL
 	   || (DECL_INITIAL (t) && TREE_CONSTANT (DECL_INITIAL (t)))))
       || TREE_CODE_CLASS (TREE_CODE (t)) == 'c')
Index: alias.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/alias.c,v
retrieving revision 1.209.2.4
diff -u -p -r1.209.2.4 alias.c
--- alias.c	12 Feb 2004 23:28:27 -0000	1.209.2.4
+++ alias.c	19 Mar 2004 19:18:53 -0000
@@ -312,6 +312,26 @@ readonly_fields_p (tree type)
 
   return 0;
 }
+
+/* Same as above but for read-write fields.  */
+
+int
+readwrite_fields_p (tree type)
+{
+  tree field;
+
+  if (TREE_CODE (type) != RECORD_TYPE && TREE_CODE (type) != UNION_TYPE
+      && TREE_CODE (type) != QUAL_UNION_TYPE)
+    return 0;
+
+  for (field = TYPE_FIELDS (type); field != 0; field = TREE_CHAIN (field))
+    if (TREE_CODE (field) == FIELD_DECL
+	&& (! TREE_READONLY (field)
+	    || readwrite_fields_p (TREE_TYPE (field))))
+      return 1;
+
+  return 0;
+}
 
 /* Return 1 if any MEM object of type T1 will always conflict (using the
    dependency routines in this file) with any MEM object of type T2.
Index: expr.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/expr.c,v
retrieving revision 1.615.4.9
diff -u -p -r1.615.4.9 expr.c
--- expr.c	13 Mar 2004 18:26:23 -0000	1.615.4.9
+++ expr.c	19 Mar 2004 19:19:19 -0000
@@ -4513,6 +4513,7 @@ store_constructor (tree exp, rtx target,
   if (TREE_CODE (type) == RECORD_TYPE || TREE_CODE (type) == UNION_TYPE
       || TREE_CODE (type) == QUAL_UNION_TYPE)
     {
+      bool readwrite_fields_marked_unchanging_p = false;
       tree elt;
 
       /* If size is zero or the target is already cleared, do nothing.  */
@@ -4552,15 +4553,28 @@ store_constructor (tree exp, rtx target,
 	{
 	  rtx xtarget = target;
 
-	  if (readonly_fields_p (type))
+	  if (readonly_fields_p (type) && ! RTX_UNCHANGING_P (xtarget))
 	    {
 	      xtarget = copy_rtx (xtarget);
 	      RTX_UNCHANGING_P (xtarget) = 1;
+	      if (readwrite_fields_p (type))
+		readwrite_fields_marked_unchanging_p = true;
 	    }
 
 	  clear_storage (xtarget, GEN_INT (size));
 	  cleared = 1;
 	}
+
+      /* ??? Emit a blockage to prevent the scheduler from swapping the
+	 memory write issued above with the /u flag and memory writes
+	 that may be issued later without it.  Note that the clearing
+	 above cannot be simply disabled in the unsafe cases because
+	 the C front-end relies on it to implement the semantics of
+	 constructors for automatic objects.  However, not all machine
+	 descriptions define a blockage insn, so emit an ASM_INPUT to
+	 act as one.  */
+      if (readwrite_fields_marked_unchanging_p)
+	emit_insn (gen_rtx_ASM_INPUT (VOIDmode, ""));
 
       if (! cleared)
 	emit_insn (gen_rtx_CLOBBER (VOIDmode, target));
Index: tree.h
===================================================================
RCS file: /cvs/gcc/gcc/gcc/tree.h,v
retrieving revision 1.458.2.4
diff -u -p -r1.458.2.4 tree.h
--- tree.h	8 Feb 2004 01:52:43 -0000	1.458.2.4
+++ tree.h	19 Mar 2004 19:19:27 -0000
@@ -2828,6 +2828,7 @@ extern void record_component_aliases (tr
 extern HOST_WIDE_INT get_alias_set (tree);
 extern int alias_sets_conflict_p (HOST_WIDE_INT, HOST_WIDE_INT);
 extern int readonly_fields_p (tree);
+extern int readwrite_fields_p (tree);
 extern int objects_must_conflict_p (tree, tree);
 
 /* In tree.c */