Get member of __m128 by index?

Question

I\'ve got some code, originally given to me by someone working with MSVC, and I\'m trying to get it to work on Clang. Here\'s the function that I\'m having trouble with:

Answer 1

A union is probably the most portable way to do this:

union {
    __m128 v;    // SSE 4 x float vector
    float a[4];  // scalar array of 4 floats
} U;

float vectorGetByIndex(__m128 V, unsigned int i)
{
    U u;

    assert(i <= 3);
    u.v = V;
    return u.a[i];
}

Answer 2

As a modification to hirschhornsalz's solution, if i is a compile-time constant, you could avoid the union path entirely by using a shuffle/store:

template<unsigned i>
float vectorGetByIndex( __m128 V)
{
#ifdef __SSE4_1__
    return _mm_extract_epi32(V, i);
#else
    float ret;
    // shuffle V so that the element that you want is moved to the least-
    // significant element of the vector (V[0])
    V = _mm_shuffle_ps(V, V, _MM_SHUFFLE(i, i, i, i));
    // return the value in V[0]
    return _mm_cvtss_f32(V);
#endif
}

Answer 3

Use

template<unsigned i>
float vectorGetByIndex( __m128 V) {
    union {
        __m128 v;    
        float a[4];  
    } converter;
    converter.v = V;
    return converter.a[i];
}

which will work regardless of the available instruction set.

Note: Even if SSE4.1 is available and i is a compile time constant, you can't use pextract etc. this way, because these instructions extract a 32-bit integer, not a float:

// broken code starts here
template<unsigned i>
float vectorGetByIndex( __m128 V) {
    return _mm_extract_epi32(V, i);
}
// broken code ends here

I don't delete it because it is a useful reminder how to not do things.

Answer 4

The way I use is

union vec { __m128 sse, float f[4] };

float accessmember(__m128 v, int index)
{
    vec v.sse = v;
    return v.f[index];
}

Seems to work out pretty well for me.