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    SubjectRe: [patch] CFS scheduler, -v8


    On Sat, 5 May 2007, Linus Torvalds wrote:

    >
    >
    > On Sat, 5 May 2007, Esben Nielsen wrote:
    >>
    >> I have been wondering why you use usigned for timers anyway. It is also like
    >> that in hrtimers. Why not use signed and avoid (almost) all worries about wrap
    >> around issues. The trick is that when all
    >> a < b
    >> is be replaced by
    >> a - b < 0
    >> the code will work on all 2-complement machines even if the (signed!) integers
    >> a and b wrap around.
    >
    > No. BOTH of the above are buggy.
    >
    > The C language definition doesn't allow signed integers to wrap (ie it's
    > undefined behaviour), so "a-b < 0" can be rewritten by the compiler as a
    > simple signed "a < b".
    >
    > And the unsigned (or signed) "a < b" is just broken wrt any kind of
    > wrap-around (whether wrapping around zero or the sign bit).
    >
    > So the _only_ valid way to handle timers is to
    > - either not allow wrapping at all (in which case "unsigned" is better,
    > since it is bigger)
    > - or use wrapping explicitly, and use unsigned arithmetic (which is
    > well-defined in C) and do something like "(long)(a-b) > 0".
    >
    > Notice? The signed variant is basically _never_ correct.
    >

    What is (long)(a-b) ? I have tried to look it up in the C99 standeard but
    I can't find it. Maybe it is in the referred LIA-1 standeard, which I
    can't find with google.

    I think the best would be to use "a-b > ULONG_MAX/2" when you mean "a<b"
    as that should be completely portable.

    According to C99 Appendix H2.2
    (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf) an
    implementation can choose to do modulo signed integers as it is
    mandatory for unsigned integers. If an implementation have choosen
    to do that it must be a bug to to do the "a-b < 0" -> "a<b" optimization.

    I have never experienced a compiler/architecture combination _not_ doing
    wrapped signed integers.

    Esben


    > Linus
    >
    -
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