Hi! On Fri, Oct 22, 2021 at 12:28:49PM -0500, Paul A. Clarke wrote: > Power9 ISA added `vabsdub` instruction which is realized in the > `vec_absd` instrinsic. > > Use `vec_absd` for `_mm_sad_epu8` compatibility intrinsic, when > `_ARCH_PWR9`. > > Also, the realization of `vec_sum2s` on little-endian includes > two shifts in order to position the input and output to match > the semantics of `vec_sum2s`: > - Shift the second input vector left 12 bytes. In the current usage, > that vector is `{0}`, so this shift is unnecessary, but is currently > not eliminated under optimization.
The vsum2sws implementation uses an unspec, so there is almost no chance of anything with it being optimised :-( It rotates it right by 4 bytes btw, it's not a shift. > - Shift the vector produced by the `vsum2sws` instruction left 4 bytes. > The two words within each doubleword of this (shifted) result must then > be explicitly swapped to match the semantics of `_mm_sad_epu8`, > effectively reversing this shift. So, this shift (and a susequent swap) > are unnecessary, but not currently removed under optimization. Rotate left by 4 -- same thing once you consider word 0 and 2 are set to zeroes by the sum2sws. Not sure why it is not optimised, what do the dump files say? -dap and I'd start looking at the combine dump. > Using `__builtin_altivec_vsum2sws` retains both shifts, so is not an > option for removing the shifts. > > For little-endian, use the `vsum2sws` instruction directly, and > eliminate the explicit shift (swap). > > 2021-10-22 Paul A. Clarke <p...@us.ibm.com> > > gcc > * config/rs6000/emmintrin.h (_mm_sad_epu8): Use vec_absd > when _ARCH_PWR9, optimize vec_sum2s when LE. Please don't break changelog lines early. > - vmin = vec_min (a, b); > - vmax = vec_max (a, b); > +#ifndef _ARCH_PWR9 > + __v16qu vmin = vec_min (a, b); > + __v16qu vmax = vec_max (a, b); > vabsdiff = vec_sub (vmax, vmin); > +#else > + vabsdiff = vec_absd (a, b); > +#endif So hrm, maybe we should have the vec_absd macro (or the builtin) always, just expanding to three insns if necessary. Okay for trunk with approproate changelog and commit message changes. Thanks! Segher