improve HSW 16->8 bit pack

__builtin_convertvector(..., U8x4) is producing a fairly long
sequence of code to convert U16x4 to U8x4 on HSW:
    vextracti128  $0x1,%ymm2,%xmm3
    vmovdqa       0x1848(%rip),%xmm4
    vpshufb       %xmm4,%xmm3,%xmm3
    vpshufb       %xmm4,%xmm2,%xmm2
    vpunpcklqdq   %xmm3,%xmm2,%xmm2
    vextracti128  $0x1,%ymm0,%xmm3
    vpshufb       %xmm4,%xmm3,%xmm3
    vpshufb       %xmm4,%xmm0,%xmm0
    vpunpcklqdq   %xmm3,%xmm0,%xmm0
    vinserti128   $0x1,%xmm2,%ymm0,%ymm0

We can do much better with _mm256_packus_epi16:
    vinserti128   $0x1,%xmm0,%ymm2,%ymm3
    vperm2i128    $0x31,%ymm0,%ymm2,%ymm0
    vpackuswb     %ymm0,%ymm3,%ymm0

vpackuswb packs the values in a somewhat surprising order,
which the first two instructions get us lined up for.

This is a pretty noticeable speedup, 7-8% on some benchmarks.

The same sort of change could be made for SSE2 and SSE4.1 also
using _mm_packus_epi16, but the difference for that change is
much less dramatic.  Might as well stick to focusing on HSW.

Change-Id: I0d6765bd67e0d024d658a61d19e6f6826b4d392c
Reviewed-on: https://skia-review.googlesource.com/30420
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

0 files changed, 0 insertions, 0 deletions