Add a compat stdatomic.h implementation based on windows atomics

Adapted from the code by Rémi Denis-Courmont from VLC

This merges libav commit c2755864.
Signed-off-by: Wan-Teh Chang <wtc@google.com>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

compat/atomics/win32/stdatomic.h

+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef FFMPEG_COMPAT_ATOMICS_WIN32_STDATOMIC_H
+#define FFMPEG_COMPAT_ATOMICS_WIN32_STDATOMIC_H
+
+#include <stddef.h>
+#include <stdint.h>
+#include <windows.h>
+
+#define ATOMIC_FLAG_INIT 0
+
+#define ATOMIC_VAR_INIT(value) (value)
+
+#define atomic_init(obj, value) \
+do {                            \
+    *(obj) = (value);           \
+} while(0)
+
+#define kill_dependency(y) ((void)0)
+
+#define atomic_thread_fence(order) \
+    MemoryBarrier();
+
+#define atomic_signal_fence(order) \
+    ((void)0)
+
+#define atomic_is_lock_free(obj) 0
+
+typedef intptr_t atomic_flag;
+typedef intptr_t atomic_bool;
+typedef intptr_t atomic_char;
+typedef intptr_t atomic_schar;
+typedef intptr_t atomic_uchar;
+typedef intptr_t atomic_short;
+typedef intptr_t atomic_ushort;
+typedef intptr_t atomic_int;
+typedef intptr_t atomic_uint;
+typedef intptr_t atomic_long;
+typedef intptr_t atomic_ulong;
+typedef intptr_t atomic_llong;
+typedef intptr_t atomic_ullong;
+typedef intptr_t atomic_wchar_t;
+typedef intptr_t atomic_int_least8_t;
+typedef intptr_t atomic_uint_least8_t;
+typedef intptr_t atomic_int_least16_t;
+typedef intptr_t atomic_uint_least16_t;
+typedef intptr_t atomic_int_least32_t;
+typedef intptr_t atomic_uint_least32_t;
+typedef intptr_t atomic_int_least64_t;
+typedef intptr_t atomic_uint_least64_t;
+typedef intptr_t atomic_int_fast8_t;
+typedef intptr_t atomic_uint_fast8_t;
+typedef intptr_t atomic_int_fast16_t;
+typedef intptr_t atomic_uint_fast16_t;
+typedef intptr_t atomic_int_fast32_t;
+typedef intptr_t atomic_uint_fast32_t;
+typedef intptr_t atomic_int_fast64_t;
+typedef intptr_t atomic_uint_fast64_t;
+typedef intptr_t atomic_intptr_t;
+typedef intptr_t atomic_uintptr_t;
+typedef intptr_t atomic_size_t;
+typedef intptr_t atomic_ptrdiff_t;
+typedef intptr_t atomic_intmax_t;
+typedef intptr_t atomic_uintmax_t;
+
+#define atomic_store(object, desired)   \
+do {                                    \
+    *(object) = (desired);              \
+    MemoryBarrier();                    \
+} while (0)
+
+#define atomic_store_explicit(object, desired, order) \
+    atomic_store(object, desired)
+
+#define atomic_load(object) \
+    (MemoryBarrier(), *(object))
+
+#define atomic_load_explicit(object, order) \
+    atomic_load(object)
+
+#define atomic_exchange(object, desired) \
+    InterlockedExchangePointer(object, desired);
+
+#define atomic_exchange_explicit(object, desired, order) \
+    atomic_exchange(object, desired)
+
+static inline int atomic_compare_exchange_strong(intptr_t *object, intptr_t *expected,
+                                                 intptr_t desired)
+{
+    intptr_t old = *expected;
+    *expected = InterlockedCompareExchangePointer(object, desired, old);
+    return *expected == old;
+}
+
+#define atomic_compare_exchange_strong_explicit(object, expected, desired, success, failure) \
+    atomic_compare_exchange_strong(object, expected, desired)
+
+#define atomic_compare_exchange_weak(object, expected, desired) \
+    atomic_compare_exchange_strong(object, expected, desired)
+
+#define atomic_compare_exchange_weak_explicit(object, expected, desired, success, failure) \
+    atomic_compare_exchange_weak(object, expected, desired)
+
+#ifdef _WIN64
+#define atomic_fetch_add(object, operand) \
+    InterlockedExchangeAdd64(object, operand)
+
+#define atomic_fetch_sub(object, operand) \
+    InterlockedExchangeAdd64(object, -(operand))
+
+#define atomic_fetch_or(object, operand) \
+    InterlockedOr64(object, operand)
+
+#define atomic_fetch_xor(object, operand) \
+    InterlockedXor64(object, operand)
+
+#define atomic_fetch_and(object, operand) \
+    InterlockedAnd64(object, operand)
+#else
+#define atomic_fetch_add(object, operand) \
+    InterlockedExchangeAdd(object, operand)
+
+#define atomic_fetch_sub(object, operand) \
+    InterlockedExchangeAdd(object, -(operand))
+
+#define atomic_fetch_or(object, operand) \
+    InterlockedOr(object, operand)
+
+#define atomic_fetch_xor(object, operand) \
+    InterlockedXor(object, operand)
+
+#define atomic_fetch_and(object, operand) \
+    InterlockedAnd(object, operand)
+#endif /* _WIN64 */
+
+#define atomic_fetch_add_explicit(object, operand, order) \
+    atomic_fetch_add(object, operand)
+
+#define atomic_fetch_sub_explicit(object, operand, order) \
+    atomic_fetch_sub(object, operand)
+
+#define atomic_fetch_or_explicit(object, operand, order) \
+    atomic_fetch_or(object, operand)
+
+#define atomic_fetch_xor_explicit(object, operand, order) \
+    atomic_fetch_sub(object, operand)
+
+#define atomic_fetch_and_explicit(object, operand, order) \
+    atomic_fetch_and(object, operand)
+
+#define atomic_flag_test_and_set(object) \
+    atomic_exchange(object, 1)
+
+#define atomic_flag_test_and_set_explicit(object, order) \
+    atomic_flag_test_and_set(object)
+
+#define atomic_flag_clear(object) \
+    atomic_store(object, 0)
+
+#define atomic_flag_clear_explicit(object, order) \
+    atomic_flag_clear(object)
+
+#endif /* FFMPEG_COMPAT_ATOMICS_WIN32_STDATOMIC_H */

configure

@@ -6384,6 +6384,8 @@ done
 if disabled stdatomic_h; then
     if enabled atomics_gcc; then
         add_cppflags '-I\$(SRC_PATH)/compat/atomics/gcc'
+    elif enabled atomics_win32; then
+        add_cppflags '-I\$(SRC_PATH)/compat/atomics/win32'
     fi
 fi