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, 14 insertions, 17 deletions

diff --git a/common/msg.c b/common/msg.c
index 11fee518d2..d2e728b585 100644
--- a/common/msg.c
+++ b/common/msg.c
@@ -66,7 +66,7 @@ struct mp_log_root {
     /* This is incremented every time the msglevels must be reloaded.
      * (This is perhaps better than maintaining a globally accessible and
      * synchronized mp_log tree.) */
-    int64_t reload_counter;
+    atomic_ulong reload_counter;
 };
 
 struct mp_log {
@@ -75,7 +75,7 @@ struct mp_log {
     const char *verbose_prefix;
     int level;                  // minimum log level for any outputs
     int terminal_level;         // minimum log level for terminal output
-    int64_t reload_counter;
+    atomic_ulong reload_counter;
 };
 
 struct mp_log_buffer {
@@ -119,7 +119,7 @@ static void update_loglevel(struct mp_log *log)
         log->level = MPMAX(log->level, log->root->buffers[n]->level);
     if (log->root->stats_file)
         log->level = MPMAX(log->level, MSGL_STATS);
-    log->reload_counter = log->root->reload_counter;
+    atomic_store(&log->reload_counter, atomic_load(&log->root->reload_counter));
     pthread_mutex_unlock(&mp_msg_lock);
 }
 
@@ -127,10 +127,10 @@ static void update_loglevel(struct mp_log *log)
 // Thread-safety: see mp_msg().
 bool mp_msg_test(struct mp_log *log, int lev)
 {
-    mp_memory_barrier();
-    if (!log->root || log->root->mute)
+    struct mp_log_root *root = log->root;
+    if (!root || root->mute)
         return false;
-    if (log->reload_counter != log->root->reload_counter)
+    if (atomic_load(&log->reload_counter) != atomic_load(&root->reload_counter))
         update_loglevel(log);
     return lev <= log->level;
 }
@@ -395,9 +395,11 @@ void mp_msg_init(struct mpv_global *global)
     assert(!global->log);
 
     struct mp_log_root *root = talloc_zero(NULL, struct mp_log_root);
-    root->global = global;
-    root->header = true;
-    root->reload_counter = 1;
+    *root = (struct mp_log_root){
+        .global = global,
+        .header = true,
+        .reload_counter = ATOMIC_VAR_INIT(1),
+    };
 
     struct mp_log dummy = { .root = root };
     struct mp_log *log = mp_log_new(root, &dummy, "");
@@ -429,8 +431,7 @@ void mp_msg_update_msglevels(struct mpv_global *global)
     talloc_free(root->msglevels);
     root->msglevels = talloc_strdup(root, global->opts->msglevels);
 
-    mp_atomic_add_and_fetch(&root->reload_counter, 1);
-    mp_memory_barrier();
+    atomic_fetch_add(&root->reload_counter, 1);
     pthread_mutex_unlock(&mp_msg_lock);
 }
 
@@ -475,9 +476,7 @@ struct mp_log_buffer *mp_msg_log_buffer_new(struct mpv_global *global,
 
     MP_TARRAY_APPEND(root, root->buffers, root->num_buffers, buffer);
 
-    mp_atomic_add_and_fetch(&root->reload_counter, 1);
-    mp_memory_barrier();
-
+    atomic_fetch_add(&root->reload_counter, 1);
     pthread_mutex_unlock(&mp_msg_lock);
 
     return buffer;
@@ -510,9 +509,7 @@ found:
     }
     talloc_free(buffer);
 
-    mp_atomic_add_and_fetch(&root->reload_counter, 1);
-    mp_memory_barrier();
-
+    atomic_fetch_add(&root->reload_counter, 1);
     pthread_mutex_unlock(&mp_msg_lock);
 }