ref: 6ed4c253a97e7b2950ad42c82ee4d50299fcc1f9
dir: /vpx_util/vpx_atomics.h/
/*
* Copyright (c) 2017 The WebM project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#ifndef VPX_UTIL_VPX_ATOMICS_H_
#define VPX_UTIL_VPX_ATOMICS_H_
#include "./vpx_config.h"
#ifdef __cplusplus
extern "C" {
#endif // __cplusplus
#if CONFIG_OS_SUPPORT && CONFIG_MULTITHREAD
// Look for built-in atomic support. We cannot use <stdatomic.h> or <atomic>
// since neither is guaranteed to exist on both C and C++ platforms, and we need
// to back the atomic type with the same type (g++ needs to be able to use
// gcc-built code). g++ 6 doesn't support _Atomic as a keyword and can't use the
// stdatomic.h header. Even if both <stdatomic.h> and <atomic> existed it's not
// guaranteed that atomic_int is the same type as std::atomic_int.
// See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60932#c13.
#if !defined(__has_builtin)
#define __has_builtin(x) 0 // Compatibility with non-clang compilers.
#endif // !defined(__has_builtin)
#if (__has_builtin(__atomic_load_n)) || \
(defined(__GNUC__) && \
(__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)))
// For GCC >= 4.7 and Clang versions that support __atomic builtins, use those.
#define VPX_USE_ATOMIC_BUILTINS
#else
// Use platform-specific asm barriers.
#if defined(_MSC_VER)
// TODO(pbos): This assumes that newer versions of MSVC are building with the
// default /volatile:ms (or older, where this is always true. Consider adding
// support for using <atomic> instead of stdatomic.h when building C++11 under
// MSVC. It's unclear what to do for plain C under /volatile:iso (inline asm?),
// there're no explicit Interlocked* functions for only storing or loading
// (presumably because volatile has historically implied that on MSVC).
//
// For earlier versions of MSVC or the default /volatile:ms volatile int are
// acquire/release and require no barrier.
#define vpx_atomic_memory_barrier() \
do { \
} while (0)
#else
#if ARCH_X86 || ARCH_X86_64
// Use a compiler barrier on x86, no runtime penalty.
#define vpx_atomic_memory_barrier() __asm__ __volatile__("" ::: "memory")
#elif ARCH_ARM
#define vpx_atomic_memory_barrier() __asm__ __volatile__("dmb ish" ::: "memory")
#elif ARCH_MIPS
#define vpx_atomic_memory_barrier() __asm__ __volatile__("sync" ::: "memory")
#else
#error Unsupported architecture!
#endif // ARCH_X86 || ARCH_X86_64
#endif // defined(_MSC_VER)
#endif // atomic builtin availability check
// These are wrapped in a struct so that they are not easily accessed directly
// on any platform (to discourage programmer errors by setting values directly).
// This primitive MUST be initialized using vpx_atomic_init or VPX_ATOMIC_INIT
// (NOT memset) and accessed through vpx_atomic_ functions.
typedef struct vpx_atomic_int {
volatile int value;
} vpx_atomic_int;
#define VPX_ATOMIC_INIT(num) \
{ num }
// Initialization of an atomic int, not thread safe.
static INLINE void vpx_atomic_init(vpx_atomic_int *atomic, int value) {
atomic->value = value;
}
static INLINE void vpx_atomic_store_release(vpx_atomic_int *atomic, int value) {
#if defined(VPX_USE_ATOMIC_BUILTINS)
__atomic_store_n(&atomic->value, value, __ATOMIC_RELEASE);
#else
vpx_atomic_memory_barrier();
atomic->value = value;
#endif // defined(VPX_USE_ATOMIC_BUILTINS)
}
static INLINE int vpx_atomic_load_acquire(const vpx_atomic_int *atomic) {
#if defined(VPX_USE_ATOMIC_BUILTINS)
return __atomic_load_n(&atomic->value, __ATOMIC_ACQUIRE);
#else
int v = atomic->value;
vpx_atomic_memory_barrier();
return v;
#endif // defined(VPX_USE_ATOMIC_BUILTINS)
}
#undef VPX_USE_ATOMIC_BUILTINS
#undef vpx_atomic_memory_barrier
#endif /* CONFIG_OS_SUPPORT && CONFIG_MULTITHREAD */
#ifdef __cplusplus
} // extern "C"
#endif // __cplusplus
#endif // VPX_UTIL_VPX_ATOMICS_H_