[FFmpeg-devel] [PATCH] Stop using _explicit atomic operations where not necessary.

[Anton Khirnov]

Memory ordering constraints other than the default (sequentially
consistent) can behave in very unintuitive and unexpected ways, and so
should be avoided unless there is a strong (typically performance)
reason for using them. This holds especially for memory_order_relaxed,
which imposes no ordering constraints.
---
 libavformat/fifo.c |  8 ++++----
 libavutil/cpu.c    |  8 ++++----
 libavutil/mem.c    | 10 +++++-----
 3 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/libavformat/fifo.c b/libavformat/fifo.c
index 50656f78b7..2ac14207b8 100644
--- a/libavformat/fifo.c
+++ b/libavformat/fifo.c
@@ -185,7 +185,7 @@ static int fifo_thread_write_packet(FifoThreadContext *ctx, AVPacket *pkt)
     int ret, s_idx;
 
     if (fifo->timeshift && pkt->dts != AV_NOPTS_VALUE)
-        atomic_fetch_sub_explicit(&fifo->queue_duration, next_duration(avf, pkt, &ctx->last_received_dts), memory_order_relaxed);
+        atomic_fetch_sub(&fifo->queue_duration, next_duration(avf, pkt, &ctx->last_received_dts));
 
     if (ctx->drop_until_keyframe) {
         if (pkt->flags & AV_PKT_FLAG_KEY) {
@@ -454,7 +454,7 @@ static void *fifo_consumer_thread(void *data)
             av_log(avf, AV_LOG_INFO, "FIFO queue flushed\n");
 
         if (fifo->timeshift)
-            while (atomic_load_explicit(&fifo->queue_duration, memory_order_relaxed) < fifo->timeshift)
+            while (atomic_load(&fifo->queue_duration) < fifo->timeshift)
                 av_usleep(10000);
 
         ret = av_thread_message_queue_recv(queue, &msg, 0);
@@ -594,7 +594,7 @@ static int fifo_write_packet(AVFormatContext *avf, AVPacket *pkt)
     }
 
     if (fifo->timeshift && pkt && pkt->dts != AV_NOPTS_VALUE)
-        atomic_fetch_add_explicit(&fifo->queue_duration, next_duration(avf, pkt, &fifo->last_sent_dts), memory_order_relaxed);
+        atomic_fetch_add(&fifo->queue_duration, next_duration(avf, pkt, &fifo->last_sent_dts));
 
     return ret;
 fail:
@@ -621,7 +621,7 @@ static int fifo_write_trailer(AVFormatContext *avf)
             } else {
                 now += delay;
             }
-            atomic_fetch_add_explicit(&fifo->queue_duration, delay, memory_order_relaxed);
+            atomic_fetch_add(&fifo->queue_duration, delay);
             elapsed += delay;
             if (elapsed > fifo->timeshift)
                 break;
diff --git a/libavutil/cpu.c b/libavutil/cpu.c
index 8960415d00..8a6af81ae4 100644
--- a/libavutil/cpu.c
+++ b/libavutil/cpu.c
@@ -89,15 +89,15 @@ void av_force_cpu_flags(int arg){
         arg |= AV_CPU_FLAG_MMX;
     }
 
-    atomic_store_explicit(&cpu_flags, arg, memory_order_relaxed);
+    atomic_store(&cpu_flags, arg);
 }
 
 int av_get_cpu_flags(void)
 {
-    int flags = atomic_load_explicit(&cpu_flags, memory_order_relaxed);
+    int flags = atomic_load(&cpu_flags);
     if (flags == -1) {
         flags = get_cpu_flags();
-        atomic_store_explicit(&cpu_flags, flags, memory_order_relaxed);
+        atomic_store(&cpu_flags, flags);
     }
     return flags;
 }
@@ -221,7 +221,7 @@ int av_cpu_count(void)
     nb_cpus = sysinfo.dwNumberOfProcessors;
 #endif
 
-    if (!atomic_exchange_explicit(&printed, 1, memory_order_relaxed))
+    if (!atomic_exchange(&printed, 1))
         av_log(NULL, AV_LOG_DEBUG, "detected %d logical cores\n", nb_cpus);
 
     return nb_cpus;
diff --git a/libavutil/mem.c b/libavutil/mem.c
index a52d33d4a6..c216c0314f 100644
--- a/libavutil/mem.c
+++ b/libavutil/mem.c
@@ -72,14 +72,14 @@ void  free(void *ptr);
 static atomic_size_t max_alloc_size = ATOMIC_VAR_INIT(INT_MAX);
 
 void av_max_alloc(size_t max){
-    atomic_store_explicit(&max_alloc_size, max, memory_order_relaxed);
+    atomic_store(&max_alloc_size, max);
 }
 
 void *av_malloc(size_t size)
 {
     void *ptr = NULL;
 
-    if (size > atomic_load_explicit(&max_alloc_size, memory_order_relaxed))
+    if (size > atomic_load(&max_alloc_size))
         return NULL;
 
 #if HAVE_POSIX_MEMALIGN
@@ -135,7 +135,7 @@ void *av_malloc(size_t size)
 void *av_realloc(void *ptr, size_t size)
 {
     void *ret;
-    if (size > atomic_load_explicit(&max_alloc_size, memory_order_relaxed))
+    if (size > atomic_load(&max_alloc_size))
         return NULL;
 
 #if HAVE_ALIGNED_MALLOC
@@ -489,7 +489,7 @@ void *av_fast_realloc(void *ptr, unsigned int *size, size_t min_size)
     if (min_size <= *size)
         return ptr;
 
-    max_size = atomic_load_explicit(&max_alloc_size, memory_order_relaxed);
+    max_size = atomic_load(&max_alloc_size);
 
     if (min_size > max_size) {
         *size = 0;
@@ -521,7 +521,7 @@ static inline void fast_malloc(void *ptr, unsigned int *size, size_t min_size, i
         return;
     }
 
-    max_size = atomic_load_explicit(&max_alloc_size, memory_order_relaxed);
+    max_size = atomic_load(&max_alloc_size);
 
     if (min_size > max_size) {
         av_freep(ptr);
-- 
2.30.2