lavu/atomic: add support for the new memory model aware gcc built-ins

__sync built-ins are considered legacy and will be deprecated.
These new memory model aware built-ins have been available since GCC 4.7.0

Use them by default when available except for __atomic_compare_exchange_n(),
which is slower, and is instead implemented as a fallback for when and if gcc
removes the legacy __sync built-ins.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -1602,6 +1602,7 @@ ARCH_FEATURES="
 
 BUILTIN_LIST="
     atomic_cas_ptr
+    atomic_compare_exchange
     machine_rw_barrier
     MemoryBarrier
     mm_empty
@@ -2028,7 +2029,7 @@ simd_align_16_if_any="altivec neon sse"
 symver_if_any="symver_asm_label symver_gnu_asm"
 
 # threading support
-atomics_gcc_if="sync_val_compare_and_swap"
+atomics_gcc_if_any="sync_val_compare_and_swap atomic_compare_exchange"
 atomics_suncc_if="atomic_cas_ptr machine_rw_barrier"
 atomics_win32_if="MemoryBarrier"
 atomics_native_if_any="$ATOMICS_LIST"
@@ -4681,6 +4682,7 @@ if ! disabled network; then
 fi
 
 check_builtin atomic_cas_ptr atomic.h "void **ptr; void *oldval, *newval; atomic_cas_ptr(ptr, oldval, newval)"
+check_builtin atomic_compare_exchange "" "int *ptr, *oldval; int newval; __atomic_compare_exchange_n(ptr, oldval, newval, 0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)"
 check_builtin machine_rw_barrier mbarrier.h "__machine_rw_barrier()"
 check_builtin MemoryBarrier windows.h "MemoryBarrier()"
 check_builtin sarestart signal.h "SA_RESTART"

libavutil/atomic_gcc.h

@@ -28,27 +28,40 @@
 #define avpriv_atomic_int_get atomic_int_get_gcc
 static inline int atomic_int_get_gcc(volatile int *ptr)
 {
+#if HAVE_ATOMIC_COMPARE_EXCHANGE
+    return __atomic_load_n(ptr, __ATOMIC_SEQ_CST);
+#else
     __sync_synchronize();
     return *ptr;
+#endif
 }
 
 #define avpriv_atomic_int_set atomic_int_set_gcc
 static inline void atomic_int_set_gcc(volatile int *ptr, int val)
 {
+#if HAVE_ATOMIC_COMPARE_EXCHANGE
+    __atomic_store_n(ptr, val, __ATOMIC_SEQ_CST);
+#else
     *ptr = val;
     __sync_synchronize();
+#endif
 }
 
 #define avpriv_atomic_int_add_and_fetch atomic_int_add_and_fetch_gcc
 static inline int atomic_int_add_and_fetch_gcc(volatile int *ptr, int inc)
 {
+#if HAVE_ATOMIC_COMPARE_EXCHANGE
+    return __atomic_add_fetch(ptr, inc, __ATOMIC_SEQ_CST);
+#else
     return __sync_add_and_fetch(ptr, inc);
+#endif
 }
 
 #define avpriv_atomic_ptr_cas atomic_ptr_cas_gcc
 static inline void *atomic_ptr_cas_gcc(void * volatile *ptr,
                                        void *oldval, void *newval)
 {
+#if HAVE_SYNC_VAL_COMPARE_AND_SWAP
 #ifdef __ARMCC_VERSION
     // armcc will throw an error if ptr is not an integer type
     volatile uintptr_t *tmp = (volatile uintptr_t*)ptr;
@@ -56,6 +69,10 @@ static inline void *atomic_ptr_cas_gcc(void * volatile *ptr,
 #else
     return __sync_val_compare_and_swap(ptr, oldval, newval);
 #endif
+#else
+    __atomic_compare_exchange_n(ptr, &oldval, newval, 0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
+    return oldval;
+#endif
 }
 
 #endif /* AVUTIL_ATOMIC_GCC_H */