diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index cc91a4c9413..487ce17aeab 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,11 @@
+2012-01-05  Richard Guenther  <rguenther@suse.de>
+
+	PR tree-optimization/51760
+	* tree-ssa-ccp.c (likely_value): Drop UNDEFINED to CONSTANT,
+	not VARYING.
+	(bit_value_unop): Handle UNDEFINED operands.
+	(bit_value_binop): Likewise.
+
 2012-01-05  Georg-Johann Lay  <avr@gjlay.de>
 
 	* config/avr/avr.c (avr_replace_prefix): Remove.
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 7b85906faf6..1064393e138 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2012-01-05  Richard Guenther  <rguenther@suse.de>
+
+	PR tree-optimization/51760
+	* gcc.dg/torture/pr51760.c: New testcase.
+
 2012-01-05  Aldy Hernandez  <aldyh@redhat.com>
 
 	PR middle-end/51472
diff --git a/gcc/testsuite/gcc.dg/torture/pr51760.c b/gcc/testsuite/gcc.dg/torture/pr51760.c
new file mode 100644
index 00000000000..ebff2c93043
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr51760.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+
+extern inline __attribute__ ((always_inline)) void *
+memmove (void *dest, const void *src, __SIZE_TYPE__ len)
+{
+  return __builtin___memmove_chk (dest, src, len,
+				  __builtin_object_size (dest, 0));
+}
+
+void
+foo (void)
+{
+  char a[64], *b;
+  for (;;)
+    {
+      memmove (a, b, 0);
+      b = a;
+    }
+}
diff --git a/gcc/tree-ssa-ccp.c b/gcc/tree-ssa-ccp.c
index 738606fad02..2080c06cce6 100644
--- a/gcc/tree-ssa-ccp.c
+++ b/gcc/tree-ssa-ccp.c
@@ -657,9 +657,10 @@ likely_value (gimple stmt)
 	}
     }
   /* If there was an UNDEFINED operand but the result may be not UNDEFINED
-     fall back to VARYING even if there were CONSTANT operands.  */
+     fall back to CONSTANT.  During iteration UNDEFINED may still drop
+     to CONSTANT.  */
   if (has_undefined_operand)
-    return VARYING;
+    return CONSTANT;
 
   /* We do not consider virtual operands here -- load from read-only
      memory may have only VARYING virtual operands, but still be
@@ -1368,6 +1369,10 @@ bit_value_unop (enum tree_code code, tree type, tree rhs)
   prop_value_t rval = get_value_for_expr (rhs, true);
   double_int value, mask;
   prop_value_t val;
+
+  if (rval.lattice_val == UNDEFINED)
+    return rval;
+
   gcc_assert ((rval.lattice_val == CONSTANT
 	       && TREE_CODE (rval.value) == INTEGER_CST)
 	      || double_int_minus_one_p (rval.mask));
@@ -1399,6 +1404,16 @@ bit_value_binop (enum tree_code code, tree type, tree rhs1, tree rhs2)
   prop_value_t r2val = get_value_for_expr (rhs2, true);
   double_int value, mask;
   prop_value_t val;
+
+  if (r1val.lattice_val == UNDEFINED
+      || r2val.lattice_val == UNDEFINED)
+    {
+      val.lattice_val = VARYING;
+      val.value = NULL_TREE;
+      val.mask = double_int_minus_one;
+      return val;
+    }
+
   gcc_assert ((r1val.lattice_val == CONSTANT
 	       && TREE_CODE (r1val.value) == INTEGER_CST)
 	      || double_int_minus_one_p (r1val.mask));