uint16_t AtomicExchange(uint16_t *a, uint16_t new_val) {
  // InterlockedExchange16 seems unavailable on some MSVS installations.
  // Everybody stand back, I pretend to know inline assembly!
  // FIXME: I assume VC is smart enough to save/restore eax/ecx?
#ifdef _MSC_VER
  __asm {
    mov eax, a
    mov cx, new_val
    xchg [eax], cx  ; NOLINT
    mov new_val, cx
  }
#else
  __asm__("xchgw %[new_val], %[a]" : [new_val] "+r" (new_val) : [a] "m" (*a) : "memory");
#endif
  return new_val;
}