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diff --git a/third_party/aom/test/register_state_check.h b/third_party/aom/test/register_state_check.h
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+/*
+ * Copyright (c) 2016, Alliance for Open Media. All rights reserved
+ *
+ * This source code is subject to the terms of the BSD 2 Clause License and
+ * the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
+ * was not distributed with this source code in the LICENSE file, you can
+ * obtain it at www.aomedia.org/license/software. If the Alliance for Open
+ * Media Patent License 1.0 was not distributed with this source code in the
+ * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
+ */
+
+#ifndef AOM_TEST_REGISTER_STATE_CHECK_H_
+#define AOM_TEST_REGISTER_STATE_CHECK_H_
+
+#include "third_party/googletest/src/googletest/include/gtest/gtest.h"
+
+#include "config/aom_config.h"
+
+#include "aom/aom_integer.h"
+
+// API_REGISTER_STATE_CHECK(function)
+//   Validates the environment pre & post function execution to ensure the
+//   environment is in a consistent state. This should be used with API
+//   function sand assembly functions which are not expected to fully restore
+//   the system state.
+//   See platform implementations of RegisterStateCheck and
+//   RegisterStateCheckMMX for details.
+
+#if defined(_WIN64) && AOM_ARCH_X86_64
+
+#undef NOMINMAX
+#define NOMINMAX
+#undef WIN32_LEAN_AND_MEAN
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#include <winnt.h>
+
+inline bool operator==(const M128A &lhs, const M128A &rhs) {
+  return (lhs.Low == rhs.Low && lhs.High == rhs.High);
+}
+
+namespace libaom_test {
+
+// Compares the state of xmm[6-15] at construction with their state at
+// destruction. These registers should be preserved by the callee on
+// Windows x64.
+class RegisterStateCheck {
+ public:
+  RegisterStateCheck() { initialized_ = StoreRegisters(&pre_context_); }
+  ~RegisterStateCheck() { Check(); }
+
+ private:
+  static bool StoreRegisters(CONTEXT *const context) {
+    const HANDLE this_thread = GetCurrentThread();
+    EXPECT_NE(this_thread, nullptr);
+    context->ContextFlags = CONTEXT_FLOATING_POINT;
+    const bool context_saved = GetThreadContext(this_thread, context) == TRUE;
+    EXPECT_TRUE(context_saved) << "GetLastError: " << GetLastError();
+    return context_saved;
+  }
+
+  // Compares the register state. Returns true if the states match.
+  void Check() const {
+    ASSERT_TRUE(initialized_);
+    CONTEXT post_context;
+    ASSERT_TRUE(StoreRegisters(&post_context));
+
+    const M128A *xmm_pre = &pre_context_.Xmm6;
+    const M128A *xmm_post = &post_context.Xmm6;
+    for (int i = 6; i <= 15; ++i) {
+      EXPECT_EQ(*xmm_pre, *xmm_post) << "xmm" << i << " has been modified!";
+      ++xmm_pre;
+      ++xmm_post;
+    }
+  }
+
+  bool initialized_;
+  CONTEXT pre_context_;
+};
+}  // namespace libaom_test
+
+#else
+
+namespace libaom_test {
+
+class RegisterStateCheck {};
+}  // namespace libaom_test
+
+#endif  // _WIN64 && AOM_ARCH_X86_64
+
+#if (AOM_ARCH_X86 || AOM_ARCH_X86_64) && defined(__GNUC__)
+namespace libaom_test {
+
+// Checks the FPU tag word pre/post execution to ensure emms has been called.
+class RegisterStateCheckMMX {
+ public:
+  RegisterStateCheckMMX() {
+    __asm__ volatile("fstenv %0" : "=rm"(pre_fpu_env_));
+  }
+  ~RegisterStateCheckMMX() { Check(); }
+
+ private:
+  // Checks the FPU tag word pre/post execution, returning false if not cleared
+  // to 0xffff.
+  void Check() const {
+    EXPECT_EQ(0xffff, pre_fpu_env_[4])
+        << "FPU was in an inconsistent state prior to call";
+
+    uint16_t post_fpu_env[14];
+    __asm__ volatile("fstenv %0" : "=rm"(post_fpu_env));
+    EXPECT_EQ(0xffff, post_fpu_env[4])
+        << "FPU was left in an inconsistent state after call";
+  }
+
+  uint16_t pre_fpu_env_[14];
+};
+}  // namespace libaom_test
+
+#else
+namespace libaom_test {
+
+class RegisterStateCheckMMX {};
+}  // namespace libaom_test
+
+#endif  // (AOM_ARCH_X86 || AOM_ARCH_X86_64) && defined(__GNUC__)
+
+#define API_REGISTER_STATE_CHECK(statement)           \
+  do {                                                \
+    libaom_test::RegisterStateCheck reg_check;        \
+    libaom_test::RegisterStateCheckMMX reg_check_mmx; \
+    statement;                                        \
+    (void)reg_check_mmx;                              \
+    (void)reg_check;                                  \
+  } while (false)
+
+#endif  // AOM_TEST_REGISTER_STATE_CHECK_H_