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    SubjectRe: [RFC] div64_64 support
    On Tue, 27 Feb 2007 00:02:50 +0100 (MET)
    Jan Engelhardt <jengelh@linux01.gwdg.de> wrote:

    >
    > On Feb 26 2007 13:28, Stephen Hemminger wrote:
    > >>
    > >> ./arch/arm26/lib/udivdi3.c
    > >> ./arch/sh/lib/udivdi3.c
    > >> ./arch/sparc/lib/udivdi3.S
    > >>
    > >> should not this be consolidated too?
    > >
    > >Hmm. Those are the GCC internal versions, that are picked up but
    > >doing divide in place. Do we want to allow general 64 bit in kernel to
    > >be easily used? It could cause sloppy slow code, but it would look
    > >cleaner.
    >
    > Then our reviewers should catch it, and if not, the janitors will
    > (/me winks at R.P.J.Day and trivial@).
    >
    > >@@ -134,7 +112,7 @@
    > > */
    > > do {
    > > x1 = x;
    > >- x = (2 * x + (uint32_t) div64_64(a, x*x)) / 3;
    > >+ x = (2 * x + (u32) (a / x*x)) / 3;
    >
    > Eye see a bug.
    >
    > Previously there was div64_64(a, x*x) which is equivalent to
    > (a)/(x*x), or just: a/(x^2). But now you do a/x*x, which is
    > equivalent to a*x/x (in the domain of real numbers). Furthermore,
    > a/x*x is a-(a%x), which does not even remotely match a/(x^2).
    >
    > Please keep the math intact, thank you ;-)

    Been there, done that, don't want to repeat it...

    --
    Stephen Hemminger <shemminger@linux-foundation.org>
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