SSE Alignment with class

Question

Having some really weird problem and as beginner with c++ I don't know why.

struct DeviceSettings
{
public:
....somevariables
    DXSize BackbufferSize;

....somemethods
};

struct DXPoint;
typedef DXPoint DXSize;

__declspec(align(16)) struct DXPoint
{
public:
    union
    {
        struct
        {
            int x;
            int y;
        };
        struct
        {
            int width;
            int height;
        };
        int dataint[2];
        __m128i m;
    };

    DXPoint(void);
    DXPoint(int x, int y);
    ~DXPoint(void);

    void operator = (const DXPoint& v);
};

For some reason when i declare a DeviceSettings the app crash cause the DXSize var is not aligned correctly.

But this only if compiled on 32 bit mode. Works fine in 64 bit mode...

Any clues? Am i missing something obvious?

Answer 1

The align declspec only guarantees that the __m128i is aligned relative to the start of the data structure. If your memory allocator creates objects that aren't 16-byte aligned in the first place, the __m128i will be carefully misaligned. Many modern memory allocators give only 8-byte alignment.

You'll need to overload operator new for DXPoint to use an allocator with better alignment control, or use statically allocated and correctly aligned __m128is, or find some other solution.

--

Sorry, overlooked the "C++ beginner" part of your question. operator new overloading and custom memory allocators aren't really C++ beginner topics. If your application is such that you can allocate your DXPoint/DXSize objects statically (i.e. as globals instead of with 'new'), then that might also work. Otherwise you're diving in the pool at the deep end.