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    SubjectRe: [PATCH] Fix get_order()

    On Tue, 6 Mar 2007, David Howells wrote:
    > /**
    > + * ilog2_up - rounded up log of base 2 of 32-bit or a 64-bit unsigned value
    > + * @n - parameter
    > + *
    > + * constant-capable log of base 2 calculation
    > + * - this can be used to initialise global variables from constant data, hence
    > + * the massive ternary operator construction
    > + * - the result is rounded up
    > + * - the result is undefined when n < 1
    > + *
    > + * selects the appropriately-sized optimised version depending on sizeof(n)
    > + */
    > +#define ilog2_up(n) ((n) == 1 ? 0 : ilog2((n) - 1) + 1)

    This is wrong. It uses "n" twice, which makes it unsafe as a macro.

    It would need to be an inline function, but then the global initializer
    comment is wrong.

    Or it could use a "__builtin_constant_p()" (which gcc defines to not have
    side effects) to allow the multiple use for constant data.

    Or we could require that "ilog2(0)" returns -1, and then we could just say

    #define ilog2_up(n) (ilog2((n)-1)+1)

    Or.. ?

    The whole "get_order()" macro also has some serious lack of parenthesis.
    In general, commit 39d61db0edb34d60b83c5e0d62d0e906578cc707 just was
    pretty damn bad!

    I'm becoming a bit disgruntled about this whole thing, I have to admit.
    I'm just not sure the bugs here are worth it. Especially considering that
    __get_order() has apparently never even tested these things to begin with,
    since nobody but FRV has ever #defined the ARCH_HAS_ILOG2_U?? macros.

    The whole *reason* for that mess seems to be bogus too, since at least
    ia64 still has its own inline "get_order()", which means that nobody can
    use get_order() for constant initializers *anyway*, quite unlike the
    comments say.

    So the whole thing is:
    - buggy
    - untested
    - has untrue comments
    - makes no real sense

    and I'm inclined to just revert 39d61db0 instead of adding more and more
    breakage to it, since it's simply not going to help with the fundamental

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      Last update: 2007-03-07 04:31    [W:0.035 / U:31.336 seconds]
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