Gentle ping...
On Fri, Oct 22, 2021 at 12:28:49PM -0500, Paul A. Clarke via Gcc-patches 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.
> - 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.
>
> 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 <[email protected]>
>
> gcc
> * config/rs6000/emmintrin.h (_mm_sad_epu8): Use vec_absd
> when _ARCH_PWR9, optimize vec_sum2s when LE.
> ---
> Tested on powerpc64le-linux on Power9, with and without `-mcpu=power9`,
> and on powerpc/powerpc64-linux on Power8.
>
> OK for trunk?
>
> gcc/config/rs6000/emmintrin.h | 24 +++++++++++++++++-------
> 1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/gcc/config/rs6000/emmintrin.h b/gcc/config/rs6000/emmintrin.h
> index ab16c13c379e..c4758be0e777 100644
> --- a/gcc/config/rs6000/emmintrin.h
> +++ b/gcc/config/rs6000/emmintrin.h
> @@ -2197,27 +2197,37 @@ extern __inline __m128i
> __attribute__((__gnu_inline__, __always_inline__, __arti
> _mm_sad_epu8 (__m128i __A, __m128i __B)
> {
> __v16qu a, b;
> - __v16qu vmin, vmax, vabsdiff;
> + __v16qu vabsdiff;
> __v4si vsum;
> const __v4su zero = { 0, 0, 0, 0 };
> __v4si result;
>
> a = (__v16qu) __A;
> b = (__v16qu) __B;
> - 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
> /* Sum four groups of bytes into integers. */
> vsum = (__vector signed int) vec_sum4s (vabsdiff, zero);
> +#ifdef __LITTLE_ENDIAN__
> + /* Sum across four integers with two integer results. */
> + asm ("vsum2sws %0,%1,%2" : "=v" (result) : "v" (vsum), "v" (zero));
> + /* Note: vec_sum2s could be used here, but on little-endian, vector
> + shifts are added that are not needed for this use-case.
> + A vector shift to correctly position the 32-bit integer results
> + (currently at [0] and [2]) to [1] and [3] would then need to be
> + swapped back again since the desired results are two 64-bit
> + integers ([1]|[0] and [3]|[2]). Thus, no shift is performed. */
> +#else
> /* Sum across four integers with two integer results. */
> result = vec_sum2s (vsum, (__vector signed int) zero);
> /* Rotate the sums into the correct position. */
> -#ifdef __LITTLE_ENDIAN__
> - result = vec_sld (result, result, 4);
> -#else
> result = vec_sld (result, result, 6);
> #endif
> - /* Rotate the sums into the correct position. */
> return (__m128i) result;
> }
>
> --
> 2.27.0
>
