diff --git a/llvm/unittests/Support/SwapByteOrderTest.cpp b/llvm/unittests/Support/SwapByteOrderTest.cpp
new file mode 100644
index 0000000..ac7974e
--- /dev/null
+++ b/llvm/unittests/Support/SwapByteOrderTest.cpp
@@ -0,0 +1,128 @@
+//===- unittests/Support/SwapByteOrderTest.cpp - swap byte order test -----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "gtest/gtest.h"
+#include "llvm/System/SwapByteOrder.h"
+#include <cstdlib>
+#include <ctime>
+using namespace llvm;
+
+#undef max
+
+namespace {
+
+// In these first two tests all of the origional_uintx values are truncated
+// except for 64. We could avoid this, but there's really no point.
+
+TEST(SwapByteOrder, UnsignedRoundTrip) {
+  // The point of the bit twiddling of magic is to test with and without bits
+  // in every byte.
+  uint64_t value = 1;
+  for (std::size_t i = 0; i <= sizeof(value); ++i) {
+    uint8_t origional_uint8 = static_cast<uint8_t>(value);
+    EXPECT_EQ(origional_uint8,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_uint8)));
+
+    uint16_t origional_uint16 = static_cast<uint16_t>(value);
+    EXPECT_EQ(origional_uint16,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_uint16)));
+
+    uint32_t origional_uint32 = static_cast<uint32_t>(value);
+    EXPECT_EQ(origional_uint32,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_uint32)));
+
+    uint64_t origional_uint64 = static_cast<uint64_t>(value);
+    EXPECT_EQ(origional_uint64,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_uint64)));
+
+    value = (value << 8) | 0x55; // binary 0101 0101.
+  }
+}
+
+TEST(SwapByteOrder, SignedRoundTrip) {
+  // The point of the bit twiddling of magic is to test with and without bits
+  // in every byte.
+  uint64_t value = 1;
+  for (std::size_t i = 0; i <= sizeof(value); ++i) {
+    int8_t origional_int8 = static_cast<int8_t>(value);
+    EXPECT_EQ(origional_int8,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_int8)));
+
+    int16_t origional_int16 = static_cast<int16_t>(value);
+    EXPECT_EQ(origional_int16,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_int16)));
+
+    int32_t origional_int32 = static_cast<int32_t>(value);
+    EXPECT_EQ(origional_int32,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_int32)));
+
+    int64_t origional_int64 = static_cast<int64_t>(value);
+    EXPECT_EQ(origional_int64,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_int64)));
+
+    // Test other sign.
+    value *= -1;
+
+    origional_int8 = static_cast<int8_t>(value);
+    EXPECT_EQ(origional_int8,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_int8)));
+
+    origional_int16 = static_cast<int16_t>(value);
+    EXPECT_EQ(origional_int16,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_int16)));
+
+    origional_int32 = static_cast<int32_t>(value);
+    EXPECT_EQ(origional_int32,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_int32)));
+
+    origional_int64 = static_cast<int64_t>(value);
+    EXPECT_EQ(origional_int64,
+      sys::SwapByteOrder(sys::SwapByteOrder(origional_int64)));
+
+    // Return to normal sign and twiddle.
+    value *= -1;
+    value = (value << 8) | 0x55; // binary 0101 0101.
+  }
+}
+
+TEST(SwapByteOrder, uint8_t) {
+  EXPECT_EQ(uint8_t(0x11), sys::SwapByteOrder<uint8_t>(0x11));
+}
+
+TEST(SwapByteOrder, uint16_t) {
+  EXPECT_EQ(uint16_t(0x1122), sys::SwapByteOrder<uint16_t>(0x2211));
+}
+
+TEST(SwapByteOrder, uint32_t) {
+  EXPECT_EQ(uint32_t(0x11223344), sys::SwapByteOrder<uint32_t>(0x44332211));
+}
+
+TEST(SwapByteOrder, uint64_t) {
+  EXPECT_EQ(uint64_t(0x1122334455667788),
+    sys::SwapByteOrder<uint64_t>(0x8877665544332211));
+}
+
+TEST(SwapByteOrder, int8_t) {
+  EXPECT_EQ(int8_t(0x11), sys::SwapByteOrder<int8_t>(0x11));
+}
+
+TEST(SwapByteOrder, int16_t) {
+  EXPECT_EQ(int16_t(0x1122), sys::SwapByteOrder<int16_t>(0x2211));
+}
+
+TEST(SwapByteOrder, int32_t) {
+  EXPECT_EQ(int32_t(0x11223344), sys::SwapByteOrder<int32_t>(0x44332211));
+}
+
+TEST(SwapByteOrder, int64_t) {
+  EXPECT_EQ(int64_t(0x1122334455667788),
+    sys::SwapByteOrder<int64_t>(0x8877665544332211));
+}
+
+}