[cfe-commits] [libcxx] r154508 - /libcxx/trunk/include/atomic

[David Chisnall]

On 11 Apr 2012, at 18:32, Howard Hinnant wrote:

>> -    /*constexpr*/ __atomic_base(_Tp __d) { __atomic_store(&__a_, __d, memory_order_seq_cst); }
>> +    /*constexpr*/ __atomic_base(_Tp __d) : __a_(__d) {}
> 
> What is the reason that __atomic_init is not needed here?  Is this now not an atomic store?
>> -    atomic_flag(bool __b) { __atomic_store(&__a_, __b, memory_order_seq_cst); }
>> +    atomic_flag(bool __b) : __a_(__b) {}
> 
> and here?

Initialisation of an _Atomic() value is not an atomic operation in C11.  Clang now understands that the C++ syntax for initialisation  should have the same semantics.  Short version:

_Atomic(int) i = j; // Non-atomic store
i = k; // Sequentially consistent atomic store
__atomic_store(&i, k, whatever) // Atomic store with whatever ordering
__atomic_init(&i, j); // Non-atomic store

David