[FFmpeg-devel] [PATCH] w32pthreads: add pthread_once emulation

[Hendrik Leppkes]

The emulation uses native InitOnce* APIs on Windows Vista+, and a
lock-free/allocation-free approach using atomics and spinning for Windows XP.
---

This is in preparation to use pthread_once for global static init functions,
and eventually removing the global lock in avcodec_open2

compat/w32pthreads.h | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/compat/w32pthreads.h b/compat/w32pthreads.h
index deb1c53..8523976 100644
--- a/compat/w32pthreads.h
+++ b/compat/w32pthreads.h
@@ -154,6 +154,19 @@ static inline int pthread_cond_signal(pthread_cond_t *cond)
     return 0;
 }
 
+typedef INIT_ONCE pthread_once_t;
+#define PTHREAD_ONCE_INIT INIT_ONCE_STATIC_INIT
+
+static av_unused int pthread_once(pthread_once_t *once_control, void (*init_routine)(void))
+{
+    BOOL pending = FALSE;
+    InitOnceBeginInitialize(once_control, 0, &pending, NULL);
+    if (pending)
+        init_routine();
+    InitOnceComplete(once_control, 0, NULL);
+    return 0;
+}
+
 #else // _WIN32_WINNT < 0x0600
 /* for pre-Windows 6.0 platforms we need to define and use our own condition
  * variable and api */
@@ -304,6 +317,57 @@ static av_unused int pthread_cond_signal(pthread_cond_t *cond)
     pthread_mutex_unlock(&win32_cond->mtx_broadcast);
     return 0;
 }
+
+/* for pre-Windows 6.0 platforms, define INIT_ONCE struct,
+ * compatible to the one used in the native API */
+
+typedef union pthread_once_t  {
+    void * Ptr;    ///< For the Windows 6.0+ native functions
+    LONG state;    ///< For the pre-Windows 6.0 compat code
+} pthread_once_t;
+
+#define PTHREAD_ONCE_INIT {0}
+
+/* function pointers to init once API on windows 6.0+ kernels */
+static BOOL (WINAPI *initonce_begin)(pthread_once_t *lpInitOnce, DWORD dwFlags, BOOL *fPending, void **lpContext);
+static BOOL (WINAPI *initonce_complete)(pthread_once_t *lpInitOnce, DWORD dwFlags, void *lpContext);
+
+static av_unused int pthread_once(pthread_once_t *once_control, void (*init_routine)(void))
+{
+    /* Use native functions on Windows 6.0+ */
+    if (initonce_begin && initonce_complete)
+    {
+        BOOL pending = FALSE;
+        initonce_begin(once_control, 0, &pending, NULL);
+        if (pending)
+            init_routine();
+        initonce_complete(once_control, 0, NULL);
+        return 0;
+    }
+
+    /* pre-Windows 6.0 compat using a spin-lock */
+    switch (InterlockedCompareExchange(&once_control->state, 1, 0))
+    {
+        /* Initial run */
+        case 0:
+            init_routine();
+            InterlockedExchange(&once_control->state, 2);
+            break;
+        /* Another thread is running init */
+        case 1:
+            while (1) {
+                MemoryBarrier();
+                if (once_control->state == 2)
+                    break;
+                Sleep(0);
+            }
+            break;
+        /* Initialization complete */
+        case 2:
+            break;
+    }
+    return 0;
+}
 #endif
 
 static av_unused void w32thread_init(void)
@@ -319,6 +383,10 @@ static av_unused void w32thread_init(void)
         (void*)GetProcAddress(kernel_dll, "WakeConditionVariable");
     cond_wait      =
         (void*)GetProcAddress(kernel_dll, "SleepConditionVariableCS");
+    initonce_begin =
+        (void*)GetProcAddress(kernel_dll, "InitOnceBeginInitialize");
+    initonce_complete =
+        (void*)GetProcAddress(kernel_dll, "InitOnceComplete");
 #endif
 
 }
-- 
2.5.3.windows.1