SSE code for sum bytes. Where's the bug?

algorithm, c, sse, visual-studio-2005

Solution

As requested, here's what I had in mind when I said "4 accumulators in the loop and summing them after the loop".

__int64 Sum (const unsigned char* pData, const int& nLength)
{
    __int64 nSum (0);

    __m128i* pp = (__m128i*)pData;


    __m128i zero = _mm_setzero_si128(),
        a = _mm_setzero_si128(),
        b = _mm_setzero_si128(),
        c = _mm_setzero_si128(),
        d = _mm_setzero_si128(), tmp;

    int i (0);

    for ( ; i < (nLength - 63); i+=64)
    {
        a = _mm_add_epi64( _mm_sad_epu8( *(pp++), zero ), a );
        b = _mm_add_epi64( _mm_sad_epu8( *(pp++), zero ), b );
        c = _mm_add_epi64( _mm_sad_epu8( *(pp++), zero ), c );
        d = _mm_add_epi64( _mm_sad_epu8( *(pp++), zero ), d );
    }

    tmp = _mm_add_epi64( _mm_add_epi64( a, b ), _mm_add_epi64( c, d ));
    tmp = _mm_add_epi64( _mm_srli_si128( tmp, 8 ), tmp );
    nSum = (_mm_cvtsi128_si32( tmp ) & 0xFFFFFFFFULL) + 
               (((__int64)_mm_cvtsi128_si32( _mm_srli_si128( tmp, 4 ) )) << 32);

    // ... the rest
    for (; i < nLength; i++)
        nSum += pData [i];

    return nSum;
}

Problem

I wrote SSE code for summing up byte values. (VS2005.) As it is simple enough it works quite well (and fast). Only there are crashes with some sizes of the array. And it crashes only in release mode - in debug never. Maybe someone sees the "obvious" bug? Any help appreciated. ``` __int64 Sum (const unsigned char* pData, const unsigned int& nLength) { __int64 nSum (0); __m128i* pp = (__m128i*)pData; ATLASSERT( ( (DWORD)pp & 15 ) == 0 ); // pointer must point to address multiple of 16 (cache line) __m128i zero = _mm_setzero_si128(), a, b, c, d, tmp; unsigned int i (0); for ( ; i < nLength; i+=64) // 4-fach loop-unroll (x 16) { a = _mm_sad_epu8( *(pp++), zero); b = _mm_sad_epu8( *(pp++), zero); // It crashes here. c = _mm_sad_epu8( *(pp++), zero); d = _mm_sad_epu8( *(pp++), zero); // commenting the following line prevents the crash (???) tmp = _mm_add_epi64( _mm_add_epi64( _mm_add_epi64( a, b ), c ), d); a = _mm_srli_si128 ( tmp, 8 ); nSum += _mm_cvtsi128_si32( a ) + _mm_cvtsi128_si32( tmp ); } // ... the rest if (nLength % 64) for (i -= 64; i < nLength; i++) nSum += pData [i]; return nSum; } ``` The function is called like this: ``` unsigned int nLength = 3571653; // One of the values that causes crash unsigned char *pData = (unsigned char*) _aligned_malloc(nLength, 16); Sum (pData, nLength); ```

Original source