alignSize为什么有的是16字节，有的是4字节？

[PANCHENG91]

detail | 详细描述 | 詳細な説明

void Mat::create(int _w, int _h, int _c, size_t _elemsize, Allocator* _allocator)
{
    if (dims == 3 && w == _w && h == _h && c == _c && elemsize == _elemsize && elempack == 1 && allocator == _allocator)
        return;

    release();

    elemsize = _elemsize;
    elempack = 1;
    allocator = _allocator;

    dims = 3;
    w = _w;
    h = _h;
    d = 1;
    c = _c;

    cstep = alignSize((size_t)w * h * elemsize, 16) / elemsize;

    size_t totalsize = alignSize(total() * elemsize, 4);
    if (totalsize > 0)
    {
        if (allocator)
            data = allocator->fastMalloc(totalsize + (int)sizeof(*refcount));
        else
            data = fastMalloc(totalsize + (int)sizeof(*refcount));
    }

    if (data)
    {
        refcount = (int*)(((unsigned char*)data) + totalsize);
        *refcount = 1;
    }
}

各位大佬，请教下在ncnn创建mat的时候，alignSize对16字节和对4字节的对齐有什么特别的考虑吗？没理解为什么是16字节对齐，后面又变成4字节对齐。

    cstep = alignSize((size_t)w * h * elemsize, 16) / elemsize;

    size_t totalsize = alignSize(total() * elemsize, 4);