count number of unique values in a 128bit avx vector, or detecting if all elements are equal?

Question

I'm optimizing a hot path in my codebase and i have turned to vectorization. Keep in mind, I'm still quite new to all of this SIMD stuff. Here is the problem I'm trying to solve, implemented using non-SIMD

inline int count_unique(int c1, int c2, int c3, int c4)
{
    return 4 - (c2 == c1)
             - ((c3 == c1) || (c3 == c2))
             - ((c4 == c1) || (c4 == c2) || (c4 == c3));
}

the assembly output after compiling with -O3:

count_unique:
        xor     eax, eax
        cmp     esi, edi
        mov     r8d, edx
        setne   al
        add     eax, 3
        cmp     edi, edx
        sete    dl
        cmp     esi, r8d
        sete    r9b
        or      edx, r9d
        movzx   edx, dl
        sub     eax, edx
        cmp     edi, ecx
        sete    dl
        cmp     r8d, ecx
        sete    dil
        or      edx, edi
        cmp     esi, ecx
        sete    cl
        or      edx, ecx
        movzx   edx, dl
        sub     eax, edx
        ret

How would something like this be done when storing c1,c2,c3,c4 as a 16byte integer vector?

Answer 1

For your simplified problem (test all 4 lanes for equality), I would do it slightly differently, here’s how. This way it only takes 3 instructions for the complete test.

// True when the input vector has the same value in all 32-bit lanes
inline bool isSameValue( __m128i v )
{
    // Rotate vector by 4 bytes
    __m128i v2 = _mm_shuffle_epi32( v, _MM_SHUFFLE( 0, 3, 2, 1 ) );
    // The XOR outputs zero for equal bits, 1 for different bits
    __m128i xx = _mm_xor_si128( v, v2 );
    // Use PTEST instruction from SSE 4.1 set to test the complete vector for all zeros
    return (bool)_mm_testz_si128( xx, xx );
}