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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)
+#undef WIN32_LEAN_AND_MEAN
+#define WIN32_LEAN_AND_MEAN
+#include <windows.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.
+ */
+static INIT_ONCE aom_init_once = INIT_ONCE_STATIC_INIT;
+
+static void aom_once(void (*func)(void)) {
+  BOOL pending;
+  InitOnceBeginInitialize(&aom_init_once, 0, &pending, NULL);
+  if (!pending) {
+    // Initialization has already completed.
+    return;
+  }
+  func();
+  InitOnceComplete(&aom_init_once, 0, NULL);
+}
+
+#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 volatile int done;
+
+  if (!done) {
+    func();
+    done = 1;
+  }
+}
+#endif
+
+#endif  // AOM_AOM_PORTS_AOM_ONCE_H_