3 ms·

Thank you for sharing this. How would you emulate this kind of operation on avx2?

• variadix 2mo ago
  Once you have a mask of the positions you want to compress, you can generate a shuffle index vector from that mask to place the desired elements in the low part of the vector. You can expand the mask into nibble-sized indices using pext/pdep and some magic constants, then expand those nibble-sized indices into a vector of indices to use as the shuffle indices.
  • anematode 2mo ago
    Yes, that's one approach. Another reasonable approach is to get out a mask from the comparison using `vmovmskpd` and use that to look up a shuffle constant, since there are only 16 possibilities. This also works well on NEON, although I wonder there whether it'd make more sense to find the shuffle dynamically rather than loading it.