summaryrefslogtreecommitdiffstats
path: root/third_party/aom/aom_ports/aom_once.h
diff options
context:
space:
mode:
Diffstat (limited to 'third_party/aom/aom_ports/aom_once.h')
-rw-r--r--third_party/aom/aom_ports/aom_once.h141
1 files changed, 141 insertions, 0 deletions
diff --git a/third_party/aom/aom_ports/aom_once.h b/third_party/aom/aom_ports/aom_once.h
new file mode 100644
index 000000000..3cfd2fd95
--- /dev/null
+++ b/third_party/aom/aom_ports/aom_once.h
@@ -0,0 +1,141 @@
+/*
+ * 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_PORTS_AOM_ONCE_H_
+#define AOM_PORTS_AOM_ONCE_H_
+
+#include "aom_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:
+ * 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, 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 // AOM_PORTS_AOM_ONCE_H_