alias.c (true_dependence): Allow non-unchanging read to conflict with unchanging write.

	* alias.c (true_dependence): Allow non-unchanging read to conflict
	with unchanging write.

From-SVN: r36476

gcc/ChangeLog

@@ -13,6 +13,9 @@ Sun Sep 17 18:48:32 2000  Denis Chertykov  <denisc@overta.ru>
 
 Sun Sep 17 10:46:17 2000  Richard Kenner  <kenner@vlsi1.ultra.nyu.edu>
 
+	* alias.c (true_dependence): Allow non-unchanging read to conflict
+	with unchanging write.
+
 	* expr.c (store_constructor_field): Accept ALIAS_SET arg.
 	(store_constructor): Pass alias_set to it.
 

gcc/alias.c

@@ -1600,10 +1600,13 @@ true_dependence (mem, mem_mode, x, varies)
      A non-unchanging read can conflict with a non-unchanging write.
      An unchanging read can conflict with an unchanging write since
      there may be a single store to this address to initialize it.
+     Note that an unchanging store can conflict with a non-unchanging read
+     since we have to make conservative assumptions when we have a
+     record with readonly fields and we are copying the whole thing.
      Just fall through to the code below to resolve potential conflicts.
      This won't handle all cases optimally, but the possible performance
      loss should be negligible.  */
-  if (RTX_UNCHANGING_P (x) != RTX_UNCHANGING_P (mem))
+  if (RTX_UNCHANGING_P (x) && ! RTX_UNCHANGING_P (mem))
     return 0;
 
   if (mem_mode == VOIDmode)