SIMD minmag and maxmag

Question

I want to implement SIMD minmag and maxmag functions. As far as I understand these functions are

minmag(a,b) = |a|<|b| ? a : b
maxmag(a,b) = |a|>|b| ?         


        

Answer 1

Here's an alternate implementation which uses fewer instructions:

static inline void maxminmag_test(__m128d & a, __m128d & b) {
    __m128d cmp     = _mm_add_pd(a, b); // test for mean(a, b) >= 0
    __m128d amin    = _mm_min_pd(a, b);
    __m128d amax    = _mm_max_pd(a, b);
    __m128d minmag  = _mm_blendv_pd(amin, amax, cmp);
    __m128d maxmag  = _mm_blendv_pd(amax, amin, cmp);
    a = maxmag, b = minmag;
}

It uses a somewhat subtle algorithm (see below), combined with the fact that we can use the sign bit as a selection mask.

It also uses @EOF's suggestion of using only one mask and switching the operand order, which saves an instruction.

I've tested it with a small number of cases and it seems to match your original implementation.

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Algorithm:

 if (mean(a, b) >= 0)       // this can just be reduced to (a + b) >= 0
 {
     minmag = min(a, b);
     maxmag = max(a, b);
 }
 else
 {
     minmag = max(a, b);
     maxmag = min(a, b);
 }