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

Use

template
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
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.