diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index fb6640308a1fdfb407240d02d4239348389970a8..2d793e03d12b3f299a50f750f6d366685275babf 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2005-09-29  Paolo Bonzini  <bonzini@gnu.org>
+
+	PR c/21419
+	* gimplify.c (gimplify_asm_expr): Raise an error if an output is
+	read-only.
+
 2005-09-29  Steven Bosscher  <stevenb@suse.de>
 
 	PR tree-optimization/23911
diff --git a/gcc/gimplify.c b/gcc/gimplify.c
index 42b8740d08330b910adfb545be5853152b82636d..e5d50e29ee77c04bfc7710b1d4c2bd6a86220396 100644
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -3544,6 +3544,12 @@ gimplify_asm_expr (tree *expr_p, tree *pre_p, tree *post_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));
 
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 6146c2387f668a682bf621c3516ff7d9fc9478b0..e279840cbb21cec1d0bedc2f7e535826525014c7 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2005-09-29  Paolo Bonzini  <bonzini@gnu.org>
+
+        PR c/21419
+        * gcc.dg/pr21419.c: New test.
+
 2005-09-29  Steven Bosscher  <stevenb@suse.de>
 
 	* gcc.dg/pr23911.c: New test.
diff --git a/gcc/testsuite/gcc.dg/pr21419.c b/gcc/testsuite/gcc.dg/pr21419.c
new file mode 100644
index 0000000000000000000000000000000000000000..f1e9cb924683ec9dd8fea96ef95a1418109c0451
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr21419.c
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+const int i = 0;
+
+void f(void)
+{
+  __asm__ __volatile__ ("" : "=m" (i)); /* { dg-error "invalid lvalue in asm output" } */
+
+}
+
+void g(const int set)
+{
+  __asm__ __volatile__ ("" : "=r" (set)); /* { dg-error "invalid lvalue in asm output" } */
+}
+
+