• Austin Clements's avatar
    runtime/internal/sys: implement Ctz and Bswap in assembly for 386 · 9c8809f8
    Austin Clements authored
    Ctz is a hot-spot in the Go 1.7 memory manager. In SSA it's
    implemented as an intrinsic that compiles to a few instructions, but
    on the old backend (all architectures other than amd64), it's
    implemented as a fairly complex Go function. As a result, switching to
    bitmap-based allocation was a significant hit to allocation-heavy
    workloads like BinaryTree17 on non-SSA platforms.
    
    For unknown reasons, this hit 386 particularly hard. We can regain a
    lot of the lost performance by implementing Ctz in assembly on the
    386. This isn't as good as an intrinsic, since it still generates a
    function call and prevents useful inlining, but it's much better than
    the pure Go implementation:
    
    name                      old time/op    new time/op    delta
    BinaryTree17-12              3.59s ± 1%     3.06s ± 1%  -14.74%  (p=0.000 n=19+20)
    Fannkuch11-12                3.72s ± 1%     3.64s ± 1%   -2.09%  (p=0.000 n=17+19)
    FmtFprintfEmpty-12          52.3ns ± 3%    52.3ns ± 3%     ~     (p=0.829 n=20+19)
    FmtFprintfString-12          156ns ± 1%     148ns ± 3%   -5.20%  (p=0.000 n=18+19)
    FmtFprintfInt-12             137ns ± 1%     136ns ± 1%   -0.56%  (p=0.000 n=19+13)
    FmtFprintfIntInt-12          227ns ± 2%     225ns ± 2%   -0.93%  (p=0.000 n=19+17)
    FmtFprintfPrefixedInt-12     210ns ± 1%     208ns ± 1%   -0.91%  (p=0.000 n=19+17)
    FmtFprintfFloat-12           375ns ± 1%     371ns ± 1%   -1.06%  (p=0.000 n=19+18)
    FmtManyArgs-12               995ns ± 2%     978ns ± 1%   -1.63%  (p=0.000 n=17+17)
    GobDecode-12                9.33ms ± 1%    9.19ms ± 0%   -1.59%  (p=0.000 n=20+17)
    GobEncode-12                7.73ms ± 1%    7.73ms ± 1%     ~     (p=0.771 n=19+20)
    Gzip-12                      375ms ± 1%     374ms ± 1%     ~     (p=0.141 n=20+18)
    Gunzip-12                   61.8ms ± 1%    61.8ms ± 1%     ~     (p=0.602 n=20+20)
    HTTPClientServer-12         87.7µs ± 2%    86.9µs ± 3%   -0.87%  (p=0.024 n=19+20)
    JSONEncode-12               20.2ms ± 1%    20.4ms ± 0%   +0.53%  (p=0.000 n=18+19)
    JSONDecode-12               65.3ms ± 0%    65.4ms ± 1%     ~     (p=0.385 n=16+19)
    Mandelbrot200-12            4.11ms ± 1%    4.12ms ± 0%   +0.29%  (p=0.020 n=19+19)
    GoParse-12                  3.75ms ± 1%    3.61ms ± 2%   -3.90%  (p=0.000 n=20+20)
    RegexpMatchEasy0_32-12       104ns ± 0%     103ns ± 0%   -0.96%  (p=0.000 n=13+16)
    RegexpMatchEasy0_1K-12       805ns ± 1%     803ns ± 1%     ~     (p=0.189 n=18+18)
    RegexpMatchEasy1_32-12       111ns ± 0%     111ns ± 3%     ~     (p=1.000 n=14+19)
    RegexpMatchEasy1_1K-12      1.00µs ± 1%    1.00µs ± 1%   +0.50%  (p=0.003 n=19+19)
    RegexpMatchMedium_32-12      133ns ± 2%     133ns ± 2%     ~     (p=0.218 n=20+20)
    RegexpMatchMedium_1K-12     41.2µs ± 1%    42.2µs ± 1%   +2.52%  (p=0.000 n=18+16)
    RegexpMatchHard_32-12       2.35µs ± 1%    2.38µs ± 1%   +1.53%  (p=0.000 n=18+18)
    RegexpMatchHard_1K-12       70.9µs ± 2%    72.0µs ± 1%   +1.42%  (p=0.000 n=19+17)
    Revcomp-12                   1.06s ± 0%     1.05s ± 0%   -1.36%  (p=0.000 n=20+18)
    Template-12                 86.2ms ± 1%    84.6ms ± 0%   -1.89%  (p=0.000 n=20+18)
    TimeParse-12                 425ns ± 2%     428ns ± 1%   +0.77%  (p=0.000 n=18+19)
    TimeFormat-12                517ns ± 1%     519ns ± 1%   +0.43%  (p=0.001 n=20+19)
    [Geo mean]                  74.3µs         73.5µs        -1.05%
    
    Prior to this commit, BinaryTree17-12 on 386 was 33% slower than at
    the go1.6 tag. With this commit, it's 13% slower.
    
    On arm and arm64, BinaryTree17-12 is only ~5% slower than it was at
    go1.6. It may be worth implementing Ctz for them as well.
    
    I consider this change low risk, since the functions it replaces are
    simple, very well specified, and well tested.
    
    For #16117.
    
    Change-Id: Ic39d851d5aca91330134596effd2dab9689ba066
    Reviewed-on: https://go-review.googlesource.com/24640Reviewed-by: 's avatarRick Hudson <rlh@golang.org>
    Reviewed-by: 's avatarKeith Randall <khr@golang.org>
    Run-TryBot: Austin Clements <austin@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    9c8809f8
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