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diff --git a/third_party/aom/aom_ports/aom_once.h b/third_party/aom/aom_ports/aom_once.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_AOM_PORTS_AOM_ONCE_H_
+#define AOM_AOM_PORTS_AOM_ONCE_H_
+
+#include "config/aom_config.h"
+
+/* Implement a function wrapper to guarantee initialization
+ * thread-safety for library singletons.
+ *
+ * NOTE: This function uses static locks, and can only be
+ * used with one common argument per compilation unit. So
+ *
+ * file1.c:
+ *   aom_once(foo);
+ *   ...
+ *   aom_once(foo);
+ *
+ * file2.c:
+ *   aom_once(bar);
+ *
+ * will ensure foo() and bar() are each called only once, but in
+ *
+ * file1.c:
+ *   aom_once(foo);
+ *   aom_once(bar):
+ *
+ * bar() will never be called because the lock is used up
+ * by the call to foo().
+ */
+
+#if CONFIG_MULTITHREAD && defined(_WIN32)
+#include <windows.h>
+#include <stdlib.h>
+/* Declare a per-compilation-unit state variable to track the progress
+ * of calling func() only once. This must be at global scope because
+ * local initializers are not thread-safe in MSVC prior to Visual
+ * Studio 2015.
+ *
+ * As a static, aom_once_state will be zero-initialized as program start.
+ */
+static LONG aom_once_state;
+static void aom_once(void (*func)(void)) {
+  /* Try to advance aom_once_state from its initial value of 0 to 1.
+   * Only one thread can succeed in doing so.
+   */
+  if (InterlockedCompareExchange(&aom_once_state, 1, 0) == 0) {
+    /* We're the winning thread, having set aom_once_state to 1.
+     * Call our function. */
+    func();
+    /* Now advance aom_once_state to 2, unblocking any other threads. */
+    InterlockedIncrement(&aom_once_state);
+    return;
+  }
+
+  /* We weren't the winning thread, but we want to block on
+   * the state variable so we don't return before func()
+   * has finished executing elsewhere.
+   *
+   * Try to advance aom_once_state from 2 to 2, which is only possible
+   * after the winning thead advances it from 1 to 2.
+   */
+  while (InterlockedCompareExchange(&aom_once_state, 2, 2) != 2) {
+    /* State isn't yet 2. Try again.
+     *
+     * We are used for singleton initialization functions,
+     * which should complete quickly. Contention will likewise
+     * be rare, so it's worthwhile to use a simple but cpu-
+     * intensive busy-wait instead of successive backoff,
+     * waiting on a kernel object, or another heavier-weight scheme.
+     *
+     * We can at least yield our timeslice.
+     */
+    Sleep(0);
+  }
+
+  /* We've seen aom_once_state advance to 2, so we know func()
+   * has been called. And we've left aom_once_state as we found it,
+   * so other threads will have the same experience.
+   *
+   * It's safe to return now.
+   */
+  return;
+}
+
+#elif CONFIG_MULTITHREAD && defined(__OS2__)
+#define INCL_DOS
+#include <os2.h>
+static void aom_once(void (*func)(void)) {
+  static int done;
+
+  /* If the initialization is complete, return early. */
+  if (done) return;
+
+  /* Causes all other threads in the process to block themselves
+   * and give up their time slice.
+   */
+  DosEnterCritSec();
+
+  if (!done) {
+    func();
+    done = 1;
+  }
+
+  /* Restores normal thread dispatching for the current process. */
+  DosExitCritSec();
+}
+
+#elif CONFIG_MULTITHREAD && HAVE_PTHREAD_H
+#include <pthread.h>
+static void aom_once(void (*func)(void)) {
+  static pthread_once_t lock = PTHREAD_ONCE_INIT;
+  pthread_once(&lock, func);
+}
+
+#else
+/* Default version that performs no synchronization. */
+
+static void aom_once(void (*func)(void)) {
+  static int done;
+
+  if (!done) {
+    func();
+    done = 1;
+  }
+}
+#endif
+
+#endif  // AOM_AOM_PORTS_AOM_ONCE_H_