This fixes PR21419 by checking that read-only expressions are not given 
as outputs for an asm statement.  The gimplifier was already performed 
consistency tests on asm statements, so this looked like a good place to 
make this check.

--- gcc/gcc/gimplify.c	14 Sep 2005 06:59:10 -0000
+++ gcc/gcc/gimplify.c	25 Sep 2005 12:32:41 -0000
@@ -3543,6 +3543,12 @@ gimplify_asm_expr (tree *expr_p, tree *p
       parse_output_constraint (&constraint, i, 0, 0,
 			       &allows_mem, &allows_reg, &is_inout);
 
+      if (TYPE_READONLY (TREE_TYPE (TREE_VALUE (link))))
+	{
+	  error ("invalid lvalue in asm output %d", i);
+	  ret = GS_ERROR;
+	}
+
       if (!allows_reg && allows_mem)
 	lang_hooks.mark_addressable (TREE_VALUE (link));