如何对齐16位整数以与SSE内在函数一起使用

Question

I am working with two-dimensional arrays of 16-bit integers defined as

int16_t e[MAX_SIZE*MAX_NODE][MAX_SIZE];
int16_t C[MAX_SIZE][MAX_SIZE];

Where Max_SIZE and MAX_NODE are constant values. I'm not a professional programmer, but somehow with the help of people in StackOverflow I managed to write a piece of code that deploys SSE instruction on my data and achieved a significant speed-up. Currently, I am using the intrinsics that do not require data alignment (mainly _mm_loadu_si128 and _mm_storeu_si128 ).

for (b=0; b<n; b+=8){
    v1 = _mm_loadu_si128((__m128i*)&C[level][b]); // level defined elsewhere.
    v2 = _mm_loadu_si128((__m128i*)&e1[node][b]); // node defined elsewhere.
    v3 = _mm_and_si128(v1,v2);
    _mm_storeu_si128((__m128i*)&C[level+1][b],v3);
}

When I change the intrinsics to their counterparts for aligned data (ie _mm_load_si128 and _mm_store_si128 ), I get run-time errors, which leads me to the assumption that my data is not aligned properly.

My question is now, if my data is not aligned properly, how can I align it to be able to use the corresponding intrinsics? I'd think since the integers are 16 bits, they're automatically aligned. But I seem to be wrong!

Any insight on this will be highly appreciated.

Thanks!

Answer 1

SSE needs data to be aligned on 16 bytes boundary, not 16 bits , that's your problem.

What you're looking for to align your static arrays is compiler dependent.

If you're using MSVC, you'll have to use __declspec(align(16)) , or with GCC, this would be __attribute__((aligned (16))) .