Question

Quick Question guys… Are these code spinets have the same alignment ?

struct sse_t {
     float sse_data[4];
};

// the array "cacheline" will be aligned to 64-byte boundary
struct sse_t alignas(64) cacheline[1000000];

Or

// every object of type sse_t will be aligned to 64-byte boundary
struct sse_t {
     float sse_data[4];
} __attribute((aligned(64)));

struct sse_t cacheline[1000000];

Answer 1

Are these code spinets have the same alignment ?

Not quite. Your two examples are actually very different.

In your first example, you will get an array of sse_t objects. A sse_t object is only guaranteed 4-byte alignment. But since the entire array is aligned to 64-bytes, each sse_t object will be properly aligned for SSE access.

In your second example, you are forcing each sse_t object to be aligned to 64-bytes. But each sse_t object is only 16 bytes. So the array will be 4x larger. (You will have 48 bytes of padding at the end of each sse_t object).

---

struct objA {
     float sse_data[4];
};
struct objB {
     float sse_data[4];
} __attribute((aligned(64)));

int main(){
    cout << sizeof(objA) << endl;
    cout << sizeof(objB) << endl;
}

Output:

16
64

I’m pretty sure that the second case is not what you want.