638 // Most platforms do not support atomic add on a 2-byte value. However,

   639 // if the value occupies the most significant 16 bits of an aligned 32-bit

   640 // word, then we can do this with an atomic add of (add_value << 16)

   641 // to the 32-bit word.

   642 //

   643 // The least significant parts of this 32-bit word will never be affected, even

   644 // in case of overflow/underflow.

   645 //

   646 // Use the ATOMIC_SHORT_PAIR macro (see macros.hpp) to get the desired alignment.

   647 template<>

   648 struct Atomic::AddImpl<short, short> {

   649   short operator()(short add_value, short volatile* dest, atomic_memory_order order) const {

   650 #ifdef VM_LITTLE_ENDIAN

   651     assert((intx(dest) & 0x03) == 0x02, "wrong alignment");

   652     int new_value = Atomic::add(add_value << 16, (volatile int*)(dest-1), order);

   653 #else

   654     assert((intx(dest) & 0x03) == 0x00, "wrong alignment");

   655     int new_value = Atomic::add(add_value << 16, (volatile int*)(dest), order);

   656 #endif

   657     return (short)(new_value >> 16); // preserves sign

   658   }

   659 };

   660