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    SubjectRe: [PATCH 1/8] add lib/gcd.c
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    >>>>> "|" == James Cloos <cloos@jhcloos.com> writes:
    >>>>> "Alan" == Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

    |> Would the binary gcd algorithm not be a better fit for the kernel?

    Alan> Could well be the shift based one is better for some processors only.

    |> Very likely, I suspect.

    |> In any case, I do not have the hardware to do any statistically
    |> significant testing;

    I take that back. Just in case speed is a relevant issue, I ran a test
    on my MX, which is a small xen domU running on a:
    ,----
    | EFamily: 0 EModel: 0 Family: 6 Model: 15 Stepping: 11
    | CPU Model: Core 2 Quad
    | Processor name string: Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz
    `----
    I got, compiling with gcc-4.4 -march=native -O3:

    binary
    408.39user 0.05system 6:52.75elapsed 98%CPU

    quick (the code in the kernel)
    600.96user 0.16system 10:19.06elapsed 97%CPU

    contfrac (the typical euclid algo)
    569.19user 0.12system 9:35.50elapsed 98%CPU

    extended euclid (calculates g=ia+jb=gcd(a,b))
    684.53user 0.13system 11:32.77elapsed 98%CPU

    I also tried on an old Alpha at freeshell; it had gcc-3.3; gcc's -S
    output looks like it uses hardware div there, just like it does on
    x86 and amd64. The bgcd, though, was 10-16 times faster than either
    version of euclid's algo.

    On my laptop's P3M, binary gcd was about twice as fast as euclid.

    So, although modern processors are *much* better at int div, the
    binary gcd algo is still faster.

    The timings on the alpha and the laptop were of:

    for (a=0xFFF; a > 0; a--)
    for (b=a; b > 0; b--)
    g=gcd(a,b);

    For the core2 times quoted above, I started with a=0xFFFF.

    And I forgot to mention: the bgcd code I posted was based on
    some old notes of mine which most likely trace to TAoCP.

    -JimC
    --
    James Cloos <cloos@jhcloos.com> OpenPGP: 1024D/ED7DAEA6


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