How can I exchange the low 128 bits and high 128 bits in a 256 bit AVX (YMM) register

Question

I am porting SSE SIMD code to use the 256 bit AVX extensions and cannot seem to find any instruction that will blend/shuffle/move the high 128 bits and the low 128 bits.

The backing story:

What I really want is VHADDPS/_mm256_hadd_ps to act like HADDPS/_mm_hadd_ps, only with 256 bit words. Unfortunately, it acts like two calls to HADDPS acting independently on the low and high words.

Answer 1

Using VPERM2F128, one can swap the low 128 and high 128 bits ( as well as other permutations). The instrinsic function usage looks like

x = _mm256_permute2f128_ps( x , x , 1) 

The third argument is a control word which gives the user a lot of flexibility. See the Intel Instrinsic Guide for details.

Answer 2

x = _mm256_permute4x64_epi64(x, 0b01'00'11'10); 

Read about it here. And Try it online!

Note: This instruction needs AVX2 (not just AVX1).

As commented by @PeterCordes speed-wise on Zen2 / Zen3 CPUs _mm256_permute2x128_si256(x, x, i) is the best option, even though it has 3 arguments compared to function _mm256_permute4x64_epi64(x, i) suggested by me having 2 arguments. On Zen1 and KNL/KNM (and Bulldozer-family Excavator), _mm256_permute4x64_epi64(x, i) suggested by me is more efficient. On other CPUs (including mainstream Intel), both choices are equal.

As already said both _mm256_permute2x128_si256(x, y, i) and _mm256_permute4x64_epi64(x, i) need AVX2, while _mm256_permute2f128_si256(x, i) needs just AVX1.