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    SubjectRe: buggy ia64_fls() ? (was Re: /dev/random problem on 2.6.12-rc1)
    On Fri, Apr 08, 2005 at 02:12:04PM +0200, Simon Derr wrote:
    > I enabled the debug messages in random.c and I think I found the problem
    > lying in the IA64 version of fls().

    Good catch.

    > It turns out that the generic and IA64 versions of fls() disagree:
    > (output from a small test program)
    > x ia64_fls(x) generic_fls(x)
    > i=-1, t=0, ia64: -65535 et generic:0
    > i=0, t=1, ia64: 0 et generic:1
    > i=1, t=2, ia64: 1 et generic:2
    > i=2, t=4, ia64: 2 et generic:3
    > i=3, t=8, ia64: 3 et generic:4

    Well PPC at least sez:

    * fls: find last (most-significant) bit set.
    * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.

    And that agrees with the generic code (used by x86). So I think IA64
    is probably wrong here indeed. It's amazing that the other users of
    fls don't blow up spectacularly.

    > I tried to fix it with an ia64 version that would give the same result as
    > the generic version, but the kernel did not boot, I guess some functions
    > rely on the ""broken"" ia64_fls() behaviour.
    > So I just changed fls() to use generic_fls() instead of ia64_fls().

    If the "fixed" version didn't boot, how did the "alternate fixed"
    version boot?

    Mathematics is the supreme nostalgia of our time.
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      Last update: 2005-04-08 18:32    [W:0.021 / U:2.136 seconds]
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