diff -r 000000000000 -r 6474c204b198 js/src/jsapi-tests/testToIntWidth.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jsapi-tests/testToIntWidth.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,68 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <math.h>
+
+#include "jsapi-tests/tests.h"
+#include "vm/NumericConversions.h"
+
+using js::detail::ToIntWidth;
+using js::detail::ToUintWidth;
+
+BEGIN_TEST(testToUint8TwiceUint8Range)
+{
+    double d = -256;
+    uint8_t expected = 0;
+    do {
+        CHECK(ToUintWidth<uint8_t>(d) == expected);
+
+        d++;
+        expected++;
+    } while (d <= 256);
+    return true;
+}
+END_TEST(testToUint8TwiceUint8Range)
+
+BEGIN_TEST(testToInt8)
+{
+    double d = -128;
+    int8_t expected = -128;
+    do {
+        CHECK(ToIntWidth<int8_t>(d) == expected);
+
+        d++;
+        expected++;
+    } while (expected < 127);
+    return true;
+}
+END_TEST(testToInt8)
+
+BEGIN_TEST(testToUint32Large)
+{
+    CHECK(ToUintWidth<uint32_t>(pow(2.0, 83)) == 0);
+    CHECK(ToUintWidth<uint32_t>(pow(2.0, 83) + pow(2.0, 31)) == (1U << 31));
+    CHECK(ToUintWidth<uint32_t>(pow(2.0, 83) + 2 * pow(2.0, 31)) == 0);
+    CHECK(ToUintWidth<uint32_t>(pow(2.0, 83) + 3 * pow(2.0, 31)) == (1U << 31));
+    CHECK(ToUintWidth<uint32_t>(pow(2.0, 84)) == 0);
+    CHECK(ToUintWidth<uint32_t>(pow(2.0, 84) + pow(2.0, 31)) == 0);
+    CHECK(ToUintWidth<uint32_t>(pow(2.0, 84) + pow(2.0, 32)) == 0);
+    return true;
+}
+END_TEST(testToUint32Large)
+
+BEGIN_TEST(testToUint64Large)
+{
+    CHECK(ToUintWidth<uint64_t>(pow(2.0, 115)) == 0);
+    CHECK(ToUintWidth<uint64_t>(pow(2.0, 115) + pow(2.0, 63)) == (1ULL << 63));
+    CHECK(ToUintWidth<uint64_t>(pow(2.0, 115) + 2 * pow(2.0, 63)) == 0);
+    CHECK(ToUintWidth<uint64_t>(pow(2.0, 115) + 3 * pow(2.0, 63)) == (1ULL << 63));
+    CHECK(ToUintWidth<uint64_t>(pow(2.0, 116)) == 0);
+    CHECK(ToUintWidth<uint64_t>(pow(2.0, 116) + pow(2.0, 63)) == 0);
+    CHECK(ToUintWidth<uint64_t>(pow(2.0, 116) + pow(2.0, 64)) == 0);
+    return true;
+}
+END_TEST(testToUint64Large)