diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index dd21fc9f55b..b5adb7df155 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2005-07-13  Paolo Bonzini  <bonzini@gnu.org>
+
+	PR tree-optimization/21921
+	* tree-iterator.c (tsi_link_before): Support the case when
+	tsi_end_p (tsi) == true.
+
 2005-07-12  Zdenek Dvorak  <dvorakz@suse.cz>
 
 	PR tree-optimization/22442
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index a2c26cee14e..ebc793e978d 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2005-07-13  Paolo Bonzini  <bonzini@gnu.org>
+
+	PR tree-optimization/21921
+	* gcc.dg/tree-ssa/pr21921.c: New.
+
 2005-07-12  Zdenek Dvorak  <dvorakz@suse.cz>
 
 	PR tree-optimizatio/22442
diff --git a/gcc/testsuite/gcc.dg/pr21921.c b/gcc/testsuite/gcc.dg/pr21921.c
new file mode 100644
index 00000000000..d6a8a2a94bf
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr21921.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -funsafe-math-optimizations" } */
+
+void
+Q (double *s, double h)
+{
+  int i;
+  if (h > 1)
+    h = h - 1;
+
+  for (i = 1; i < 3; i++)
+    if (s[i] / h > 0)
+      s[0] = h, s[i] = s[i] / h;
+}
diff --git a/gcc/tree-iterator.c b/gcc/tree-iterator.c
index 7913bdad7c8..3c2acd38ea7 100644
--- a/gcc/tree-iterator.c
+++ b/gcc/tree-iterator.c
@@ -110,8 +110,11 @@ tsi_link_before (tree_stmt_iterator *i, tree t, enum tsi_iterator_update mode)
     }
   else
     {
-      gcc_assert (!STATEMENT_LIST_TAIL (i->container));
-      STATEMENT_LIST_HEAD (i->container) = head;
+      head->prev = STATEMENT_LIST_TAIL (i->container);
+      if (head->prev)
+       head->prev->next = head;
+      else
+       STATEMENT_LIST_HEAD (i->container) = head;
       STATEMENT_LIST_TAIL (i->container) = tail;
     }
 
@@ -127,7 +130,6 @@ tsi_link_before (tree_stmt_iterator *i, tree t, enum tsi_iterator_update mode)
       i->ptr = tail;
       break;
     case TSI_SAME_STMT:
-      gcc_assert (cur);
       break;
     }
 }