CAS

对32位or64位变量做compare_and_swap操作（以32位为例）。

• x86平台

  static inline bool
  pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
                                                                          uint32 *expected, uint32 newval)
  {
          char    ret;
   
          /*
           * Perform cmpxchg and use the zero flag which it implicitly sets when
           * equal to measure the success.
           */
          __asm__ __volatile__(
                  "       lock                            \n"
                  "       cmpxchgl        %4,%5   \n"
                  "   setz                %2              \n"
  :               "=a" (*expected), "=m"(ptr->value), "=q" (ret)
  :               "a" (*expected), "r" (newval), "m"(ptr->value)
  :               "memory", "cc");
          return (bool) ret;
  }

• AArch64平台

  static inline bool
  pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
                                                                          uint32 *expected, uint32 newval)
  {
          /* FIXME: we can probably use a lower consistency model */
          return __atomic_compare_exchange_n(&ptr->value, expected, newval, false,
                                                                             __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
  }
   
  static inline bool
  pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
                                                                          uint32 *expected, uint32 newval)
  {
          bool    ret;
          uint32  current;
          current = __sync_val_compare_and_swap(&ptr->value, *expected, newval);
          ret = current == *expected;
          *expected = current;
          return ret;
  }