Assuming this is enough precision for layout, I'd like to see neon versions before we landed anything like this. Will the API a user sees look the same? In other words, is the change localized to MaybeAuto's implementation?
Also, speaking of these SIMD optimizations, does rustc have a compiler flag available to switch on the optimized versions? Will we have to do configure-style feature detection? jack. On Mon, Apr 14, 2014 at 11:00 PM, Patrick Walton <pcwal...@mozilla.com> wrote: > Hi everyone, > > I'm considering using 32-bit fixed-point (16 bits for the fraction, 16 bits > for the integer portion) for percentages in CSS. The reason is that we can > use the following four SSE4/AVX instructions on x86 to compute two sides at > once: > > ; xmm0 contains the style values (percentage or fixed length) > ; xmm1 contains ~0 if the value is a percentage or 0 if the value > ; is fixed > ; xmm2 contains the value the percentages are relative to > 0x0000000100000b68 <+8>: vpshufd xmm2,xmm2,0x0 > 0x0000000100000b6d <+13>: vpmulld xmm2,xmm2,xmm0 > 0x0000000100000b72 <+18>: vpsrad xmm2,xmm2,0x10 > 0x0000000100000b77 <+23>: vblendvps xmm0,xmm0,xmm2,xmm1 > ; result in xmm0 > > This is done in many places in layout: grep for `MaybeAuto` to find them. > Almost all places where one side is computed, another side is computed too, > and in some places, all four sides are computed, which can be done with only > one or two more instructions in the above sequence (as XMM registers are 128 > bit). > > I have done a simple microbenchmark and found that using SSE4/AVX to compute > two sides at once is 66% faster than doing it sequentially using branches. > This is true even if percentages are not used and all branches are predicted > correctly. > > Thoughts? > > Patrick > _______________________________________________ > dev-servo mailing list > dev-servo@lists.mozilla.org > https://lists.mozilla.org/listinfo/dev-servo _______________________________________________ dev-servo mailing list dev-servo@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-servo
