如何256字节对齐呢

template <typename T>
constexpr UINT CalcConstantBufferByteSize()
{
    // Constant buffers must be a multiple of the minimum hardware
    // allocation size (usually 256 bytes).  So round up to nearest
    // multiple of 256.  We do this by adding 255 and then masking off
    // the lower 2 bytes which store all bits < 256.
    // Example: Suppose byteSize = 300.
    // (300 + 255) & ~255
    // 555 & ~255
    // 0x022B & ~0x00ff
    // 0x022B & 0xff00
    // 0x0200
    // 512
    UINT byteSize = sizeof(T);
    return (byteSize + 255) & ~255;
}