Performs an interlocked addition in which the first operand is a 64-bit value.
This intrinsic is not atomic because it is implemented as two separate locked instructions. An atomic 64-bit read that occurs on another thread during the execution of this intrinsic could result in an inconsistent value being read.
In Visual C++ 2005, this function behaves as a read-write barrier. For more information, see _ReadWriteBarrier.