buildin 字节交换

template<typename T> T EndianScalar(T t) {
  #if FLATBUFFERS_LITTLEENDIAN
    return t;
  #else
    #if defined(_MSC_VER)
      #pragma push_macro("__builtin_bswap16")
      #pragma push_macro("__builtin_bswap32")
      #pragma push_macro("__builtin_bswap64")
      #define __builtin_bswap16 _byteswap_ushort
      #define __builtin_bswap32 _byteswap_ulong
      #define __builtin_bswap64 _byteswap_uint64
    #endif
    // If you're on the few remaining big endian platforms, we make the bold
    // assumption you're also on gcc/clang, and thus have bswap intrinsics:
    if (sizeof(T) == 1) {   // Compile-time if-then's.
      return t;
    } else if (sizeof(T) == 2) {
      auto r = __builtin_bswap16(*reinterpret_cast<uint16_t *>(&t));
      return *reinterpret_cast<T *>(&r);
    } else if (sizeof(T) == 4) {
      auto r = __builtin_bswap32(*reinterpret_cast<uint32_t *>(&t));
      return *reinterpret_cast<T *>(&r);
    } else if (sizeof(T) == 8) {
      auto r = __builtin_bswap64(*reinterpret_cast<uint64_t *>(&t));
      return *reinterpret_cast<T *>(&r);
    } else {
      assert(0);
    }
    #if defined(_MSC_VER)
      #pragma pop_macro("__builtin_bswap16")
      #pragma pop_macro("__builtin_bswap32")
      #pragma pop_macro("__builtin_bswap64")
    #endif
  #endif
}