Computing Hamming distances to several strings with SSE - c

Question

I have n (8 bit) character strings all of them of the same length (say m), and another string s of the same length. I need to compute Hamming distances from s to each of the others strings. In plain C, something like:
unsigned char strings[n][m];
unsigned char s[m];
int distances[n];
for(i=0; i<n; i++) {
int distances[i] = 0;
for(j=0; j<m; j++) {
if(strings[i][j] != s[j])
distances[i]++;
}
}
I would like to use SIMD instructions with gcc to perform such computations more efficiently. I have read that PcmpIstrI in SSE 4.2 can be useful and my target computer supports that instruction set, so I would prefer a solution using SSE 4.2.
EDIT:
I wrote following function to compute Hamming distance between two strings:
static inline int popcnt128(__m128i n) {
const __m128i n_hi = _mm_unpackhi_epi64(n, n);
return _mm_popcnt_u64(_mm_cvtsi128_si64(n)) + _mm_popcnt_u64(_mm_cvtsi128_si64(n_hi));
}
int HammingDist(const unsigned char *p1, unsigned const char *p2, const int len) {
#define MODE (_SIDD_UBYTE_OPS | _SIDD_CMP_EQUAL_EACH | _SIDD_BIT_MASK | _SIDD_NEGATIVE_POLARITY)
__m128i smm1 = _mm_loadu_si128 ((__m128i*) p1);
__m128i smm2 = _mm_loadu_si128 ((__m128i*) p2);
__m128i ResultMask;
int iters = len / 16;
int diffs = 0;
int i;
for(i=0; i<iters; i++) {
ResultMask = _mm_cmpestrm (smm1,16,smm2,16,MODE);
diffs += popcnt128(ResultMask);
p1 = p1+16;
p2 = p2+16;
smm1 = _mm_loadu_si128 ((__m128i*)p1);
smm2 =_mm_loadu_si128 ((__m128i*)p2);
}
int mod = len % 16;
if(mod>0) {
ResultMask = _mm_cmpestrm (smm1,mod,smm2,mod,MODE);
diffs += popcnt128(ResultMask);
}
return diffs;
}
So I can solve my problem by means of:
for(i=0; i<n; i++) {
int distances[i] = HammingDist(s, strings[i], m);
}
Is this the best I can do or can I use the fact that one of the strings compared is always the same? In addition, should I do some alignment on my arrays to improve performance?
ANOTHER ATTEMPT
Following Harold's recomendation, I have written following code:
void _SSE_hammingDistances(const ByteP str, const ByteP strings, int *ds, const int n, const int m) {
int iters = m / 16;
__m128i *smm1, *smm2, diffs;
for(int j=0; j<n; j++) {
smm1 = (__m128i*) str;
smm2 = (__m128i*) &strings[j*(m+1)]; // m+1, as strings are '\0' terminated
diffs = _mm_setzero_si128();
for (int i = 0; i < iters; i++) {
diffs = _mm_add_epi8(diffs, _mm_cmpeq_epi8(*smm1, *smm2));
smm1 += 1;
smm2 += 1;
}
int s = m;
signed char *ptr = (signed char *) &diffs;
for(int p=0; p<16; p++) {
s += *ptr;
ptr++;
}
*ds = s;
ds++;
}
}
but I am not able to do the final addition of bytes in __m128i by using psadbw. Can anyone please help me with that?

Answer 1

Here's an improved version of your latest routine, which uses PSADBW (_mm_sad_epu8) to eliminate the scalar code:
void hammingDistances_SSE(const uint8_t * str, const uint8_t * strings, int * const ds, const int n, const int m)
{
const int iters = m / 16;
const __m128i smm1 = _mm_loadu_si128((__m128i*)str);
assert((m & 15) == 0); // m must be a multiple of 16
for (int j = 0; j < n; j++)
{
__m128i smm2 = _mm_loadu_si128((__m128i*)&strings[j*(m+1)]); // m+1, as strings are '\0' terminated
__m128i diffs = _mm_setzero_si128();
for (int i = 0; i < iters; i++)
{
diffs = _mm_sub_epi8(diffs, _mm_cmpeq_epi8(smm1, smm2));
}
diffs = _mm_sad_epu8(diffs, _mm_setzero_si128());
ds[j] = m - (_mm_extract_epi16(diffs, 0) + _mm_extract_epi16(diffs, 4));
}
}