最近尝试用C实现原子引用计数器,所以参考了STL中std::shared_ptr的实现,对weak_ptr::lock的实现很困惑。
执行compare_and_exchange时,clang指定了memory_order_seq_cst,g++指定了memory_order_acq_rel,MSVC指定了memory_order_relaxed。
我认为 memory_order_relaxed 已经足够了,因为如果 user_count 非零,则不需要同步数据。
我不是这方面的专家,有人可以提供一些建议吗?
以下是代码片段:
MSVC https://github.com/microsoft/STL/blob/main/stl/inc/memory#L1084
bool _Incref_nz() noexcept { // increment use count if not zero, return true if successful
auto& _Volatile_uses = reinterpret_cast<volatile long&>(_Uses);
#ifdef _M_CEE_PURE
long _Count = *_Atomic_address_as<const long>(&_Volatile_uses);
#else
long _Count = __iso_volatile_load32(reinterpret_cast<volatile int*>(&_Volatile_uses));
#endif
while (_Count != 0) {
const long _Old_value = _INTRIN_RELAXED(_InterlockedCompareExchange)(&_Volatile_uses, _Count + 1, _Count);
if (_Old_value == _Count) {
return true;
}
_Count = _Old_value;
}
return false;
}
铿锵/libcxx https://github.com/llvm/llvm-project/blob/main/libcxx/src/memory.cpp#L119
__shared_weak_count*
__shared_weak_count::lock() noexcept
{
long object_owners = __libcpp_atomic_load(&__shared_owners_);
while (object_owners != -1)
{
if (__libcpp_atomic_compare_exchange(&__shared_owners_,
&object_owners,
object_owners+1))
return this;
}
return nullptr;
}
海湾合作委员会/libstdc++ https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/shared_ptr_base.h#L277
template<>
inline bool
_Sp_counted_base<_S_atomic>::
_M_add_ref_lock_nothrow() noexcept
{
// Perform lock-free add-if-not-zero operation.
_Atomic_word __count = _M_get_use_count();
do
{
if (__count == 0)
return false;
// Replace the current counter value with the old value + 1, as
// long as it's not changed meanwhile.
}
while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
true, __ATOMIC_ACQ_REL,
__ATOMIC_RELAXED));
return true;
}