_mm_cvtsd_f64 analogon for higher order floating point

Question

I\'m playing around with SIMD and wonder why there is no analogon to _mm_cvtsd_f64 to extrat the higher order floating point from a __m128d.

GCC 4.6+ has an extensio

Answer 1

I suggest that you use the following code:

inline static _mm_cvtsd_f64_h(__m128d x) {
    return _mm_cvtsd_f64(_mm_unpackhi_pd(x, x));
}

This is likely the fastest way to get get the upper half of xmm register, and it is compatible with MSVC/icc/gcc/clang.

Answer 2

You can just use a union:

union {
    __m128d v;
    double a[2];
} U;

Assign your __m128d to U.v and read back U.a[0] or U.a[1]. Any decent compiler will optimise away redundant stores and loads.