rtcd: serialize function pointer initialization

Ensure that RTCD function pointers are set at most once, to silence
some data race warnings. Implementation provided for POSIX threads and
Win32, with the prior unsynchronized behavior left in place for other
platforms.

Change-Id: I65c5856df43ef67043b3d5f26ddafddd8fcb2f7e

diff --git a/vp8/common/generic/systemdependent.c b/vp8/common/generic/systemdependent.c
index 39660ab..2a30166 100644 (file)
--- a/vp8/common/generic/systemdependent.c
+++ b/vp8/common/generic/systemdependent.c
@@ -82,6 +82,58 @@ static int get_cpu_count()
 }
 #endif
 
+
+#if HAVE_PTHREAD_H
+#include <pthread.h>
+static void once(void (*func)(void))
+{
+    static pthread_once_t lock = PTHREAD_ONCE_INIT;
+    pthread_once(&lock, func);
+}
+
+
+#elif defined(_WIN32)
+static void once(void (*func)(void))
+{
+    /* Using a static initializer here rather than InitializeCriticalSection()
+     * since there's no race-free context in which to execute it. Protecting
+     * it with an atomic op like InterlockedCompareExchangePointer introduces
+     * an x86 dependency, and InitOnceExecuteOnce requires Vista.
+     */
+    static CRITICAL_SECTION lock = {(void *)-1, -1, 0, 0, 0, 0};
+    static int done;
+
+    EnterCriticalSection(&lock);
+
+    if (!done)
+    {
+        func();
+        done = 1;
+    }
+
+    LeaveCriticalSection(&lock);
+}
+
+
+#else
+/* No-op version that performs no synchronization. vpx_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
+
+
 void vp8_machine_specific_config(VP8_COMMON *ctx)
 {
 #if CONFIG_MULTITHREAD
@@ -94,5 +146,5 @@ void vp8_machine_specific_config(VP8_COMMON *ctx)
     ctx->cpu_caps = x86_simd_caps();
 #endif
 
-    vpx_rtcd();
+    once(vpx_rtcd);
 }