1 files changed, 150 insertions, 0 deletions
diff --git a/media/libvpx/vpx_ports/vpx_once.h b/media/libvpx/vpx_ports/vpx_once.h
new file mode 100644
index 000000000..da04db459
--- /dev/null
+++ b/media/libvpx/vpx_ports/vpx_once.h
@@ -0,0 +1,150 @@
+/*
+ *  Copyright (c) 2015 The WebM project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef VPX_PORTS_VPX_ONCE_H_
+#define VPX_PORTS_VPX_ONCE_H_
+
+#include "vpx_config.h"
+
+/* Implement a function wrapper to guarantee initialization
+ * thread-safety for library singletons.
+ *
+ * NOTE: These functions use static locks, and can only be
+ * used with one common argument per compilation unit. So
+ *
+ * file1.c:
+ *   vpx_once(foo);
+ *   ...
+ *   vpx_once(foo);
+ *
+ *   file2.c:
+ *     vpx_once(bar);
+ *
+ * will ensure foo() and bar() are each called only once, but in
+ *
+ * file1.c:
+ *   vpx_once(foo);
+ *   vpx_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, once_state will be zero-initialized as program start.
+ */
+static LONG once_state;
+static void once(void (*func)(void))
+{
+    /* Try to advance once_state from its initial value of 0 to 1.
+     * Only one thread can succeed in doing so.
+     */
+    if (InterlockedCompareExchange(&once_state, 1, 0) == 0) {
+        /* We're the winning thread, having set once_state to 1.
+         * Call our function. */
+        func();
+        /* Now advance once_state to 2, unblocking any other threads. */
+        InterlockedIncrement(&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 once_state from 2 to 2, which is only possible
+     * after the winning thead advances it from 1 to 2.
+     */
+    while (InterlockedCompareExchange(&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 once_state advance to 2, so we know func()
+     * has been called. And we've left 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 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 once(void (*func)(void))
+{
+    static pthread_once_t lock = PTHREAD_ONCE_INIT;
+    pthread_once(&lock, func);
+}
+
+
+#else
+/* No-op version that performs no synchronization. *_rtcd() is idempotent,
+ * so as long as your platform provides atomic loads/stores of pointers
+ * no synchronization is strictly necessary.
+ */
+
+static void once(void (*func)(void))
+{
+    static int done;
+
+    if(!done)
+    {
+        func();
+        done = 1;
+    }
+}
+#endif
+
+#endif  // VPX_PORTS_VPX_ONCE_H_