__CMP_SWAP_QUAD_ACQ

 Performs a conditional atomic compare and exchange operation with
 acquire semantics on a quadword.  The quadword pointed to by source
 is read and compared with the quadword old_value.  If they are
 equal, the quadword new_value is written into the quadword pointed
 to by source.  The read and write is performed atomically, with no
 intervening access to the same memory region.

 Acquire memory ordering guarantees that the memory read/write is
 made visible before all subsequent memory data accesses to the same
 memory location by other processors.

 The function returns 1 if the write occurs, and 0 otherwise.

 Syntax:

      int __CMP_SWAP_QUAD_ACQ (volatile void *source, int old_value,
      int new_value);