atomic_int64_xor

Performs an atomic bitwise 'xor' of the value of atomic and val, storing the result back in atomic.

Think of this operation as an atomic version of:

  { tmp = *atomic; *atomic ^= val; return tmp; }

This call acts as a full compiler and hardware memory barrier.

Parameters:

atomic	a pointer to a int64 or uint64
val	the value to 'xor'

Returns:

the value of atomic before the operation, unsigned