SSE additions of chars - c

Question

I have got a vector of 16 char that contains either 0 or 1 and I would like to add each 4 non-overlapping elements using SSE.
A simplified version of the code without vectorization looks like this
char a[16]={1,0,0,1 ,0,0,1,0, 0,1,0,0, 0,0,0,1};
char sum1 = a[0] + a[1] + a[2] + a[3];
char sum2 = a[4] + a[5] + a[6] + a[7];
char sum3 = a[8] + a[9] + a[10] + a[11];
char sum4 = a[12] + a[13] + a[14] + a[15];
In my application, the length of the vector is much larger than 16 but it is always a multiple of 16. I get this vector using other SSE logical operations that provide me with a good speedup so I would like to know how I could vectorise those additions. Below is the full code where vec1, vec2 and vec3 have the same length n (multiple of 16) and vector counts is n/4.
void myfunc( const char *vec1, const char *vec2, char *vec3, int *counts, int n){
__m128i *r1 = (__m128i*)vec1;
__m128i *r2 = (__m128i*)vec2;
char *a = vec3;
char temp[16] __attribute__ ((aligned (16)));
for ( int i = 0; i < n; i+=16, r1++, r2++, a+=16 ) {
_mm_store_si128((__m128i*)a, _mm_and_si128(*r1, *r2));
_mm_store_si128((__m128i*)temp, _mm_or_si128(*r1, *r2));
char size = a[0]+a[1]+a[2]+a[3];
if( size == 0 ){
memcpy(a, temp, 4*sizeof(char));
counts[k]++;
}
k++;
size = a[4]+a[5]+a[6]+a[7];
if( size == 0 ){
memcpy(a+4, temp+4, 4*sizeof(char));
counts[k]++;
}
k++;
size = a[8]+a[9]+a[10]+a[11];
if( size == 0 ){
memcpy(a+8, temp+8, 4*sizeof(char));
counts[k]++;
}
k++;
size = a[12]+a[13]+a[14]+a[15];
if( size == 0 ){
memcpy(a+12, temp+12, 4*sizeof(char));
counts[k]++;
}
k++;
}
}
Any help would be greatly appreciated.

Answer 1

Instead of comparing bytes you can compare integers. Load four integers from a, temp and counts into SSE registers ( call them a4, tmp4, and counts4 in the code below). Then you can process four integers at once with SSE. This assumes that counts is a int32 array.
For example let's assume a4 = {0,3,0,4}, counts4 = {1,2,3,4}, and tmp4 = {5,6,7,8}. In the code below
test will be {-1, 0, -1, 0}. Subtracting that from counts gives counts = {2,2,4,4}. Logical AND of test with tmp4 is {5,0,7,0}. Adding that to a4 gives a4 = {5,3,7,4}. This should do what you want.
for ( int i = 0; i < n; i+=16, r1++, r2++, a+=16, k+=4 ) {
_mm_store_si128((__m128i*)a, _mm_and_si128(*r1, *r2));
_mm_store_si128((__m128i*)temp, _mm_or_si128(*r1, *r2));
__m128i a4 = _mm_load_si128((__m128i*)a);
__m128i tmp4 = _mm_load_si128((__m128i*)tmp);
__m128i counts4 = _mm_load_si128((__m128i*)&counts[k]);
__m128i test = _mm_cmpeq_epi32(_mm_set1_epi32(0), a4);
a4 = _mm_add_epi32(a4, _mm_and_si128(tmp4,test));
counts4 = _mm_sub_epi32(counts4, test);
_mm_store_si128((__m128i*)a, a4);
_mm_store_si128((__m128i*)counts, counts4);
}