C++ Adding 2 arrays together quickly

Question

Given the arrays:

int canvas[10][10];
int addon[10][10];

Where all the values range from 0 - 100, what is the fastest way in C++ to add

Answer 1

Here is an SSE4 implementation that should perform pretty well on Nehalem (Core i7):

#include 
#include 
#include 

static inline int canvas_add(int canvas[10][10], int addon[10][10])
{
    __m128i * cp = (__m128i *)&canvas[0][0];
    const __m128i * ap = (__m128i *)&addon[0][0];
    const __m128i vlimit = _mm_set1_epi32(100);
    __m128i vmax = _mm_set1_epi32(INT_MIN);
    __m128i vcmp;
    int cmp;
    int i;

    for (i = 0; i < 10 * 10; i += 4)
    {
        __m128i vc = _mm_loadu_si128(cp);
        __m128i va = _mm_loadu_si128(ap);

        vc = _mm_add_epi32(vc, va);
        vmax = _mm_max_epi32(vmax, vc);   // SSE4 *

        _mm_storeu_si128(cp, vc);

        cp++;
        ap++;
    }
    vcmp = _mm_cmpgt_epi32(vmax, vlimit); // SSE4 *
    cmp = _mm_testz_si128(vcmp, vcmp);    // SSE4 *
    return cmp == 0;
}

Compile with gcc -msse4.1 ... or equivalent for your particular development environment.

For older CPUs without SSE4 (and with much more expensive misaligned loads/stores) you'll need to (a) use a suitable combination of SSE2/SSE3 intrinsics to replace the SSE4 operations (marked with an * above) and ideally (b) make sure your data is 16-byte aligned and use aligned loads/stores (_mm_load_si128/_mm_store_si128) in place of _mm_loadu_si128/_mm_storeu_si128.