atomics: switch to C11 stdatomic.h

In my opinion, we shouldn't use atomics at all, but ok.

This switches the mpv code to use C11 stdatomic.h, and for compilers
that don't support stdatomic.h yet, we emulate the subset used by mpv
using the builtins commonly provided by gcc and clang.

This supersedes an earlier similar attempt by Kovensky. That attempt
unfortunately relied on a big copypasted freebsd header (which also
depended on much more highly compiler-specific functionality, defined
reserved symbols, etc.), so it had to be NIH'ed.

Some issues:
- C11 says default initialization of atomics "produces a valid state",
  but it's not sure whether the stored value is really 0. But we rely on
  this.
- I'm pretty sure our use of the __atomic... builtins is/was incorrect.
  We don't use atomic load/store intrinsics, and access stuff directly.
- Our wrapper actually does stricter typechecking than the stdatomic.h
  implementation by gcc 4.9. We make the atomic types incompatible with
  normal types by wrapping them into structs. (The FreeBSD wrapper does
  the same.)
- I couldn't test on MinGW.

1 files changed, 3 insertions, 6 deletions

diff --git a/audio/out/ao_wasapi.c b/audio/out/ao_wasapi.c
index 3d59554dcd..431b24c3a1 100644
--- a/audio/out/ao_wasapi.c
+++ b/audio/out/ao_wasapi.c
@@ -33,7 +33,6 @@
 #include "audio/out/ao_wasapi_utils.h"
 
 #include "audio/format.h"
-#include "compat/atomics.h"
 #include "osdep/timer.h"
 #include "osdep/io.h"
 
@@ -43,7 +42,7 @@
               do { if ((unk) != NULL) { release; (unk) = NULL; } } while(0)
 
 static double get_device_delay(struct wasapi_state *state) {
-    UINT64 sample_count = state->sample_count;
+    UINT64 sample_count = atomic_load(&state->sample_count);
     UINT64 position, qpc_position;
     HRESULT hr;
 
@@ -100,8 +99,7 @@ static void thread_feed(struct ao *ao)
                                           frame_count, 0);
     EXIT_ON_ERROR(hr);
 
-    mp_atomic_add_and_fetch(&state->sample_count, frame_count);
-    mp_memory_barrier();
+    atomic_fetch_add(&state->sample_count, frame_count);
 
     return;
 exit_label:
@@ -318,8 +316,7 @@ static void audio_pause(struct ao *ao)
 
     IAudioClient_Stop(state->pAudioClientProxy);
     IAudioClient_Reset(state->pAudioClientProxy);
-    state->sample_count = 0;
-    mp_memory_barrier();
+    atomic_store(&state->sample_count, 0);
 }
 
 static void audio_resume(struct ao *ao)