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    SubjectRe: [PATCH 04/14] lib/vsprintf.c: expand field_width to 24 bits
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    On Tue, Nov 24 2015, Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

    > On Mon, Nov 23, 2015 at 11:29 PM, Rasmus Villemoes
    > <linux@rasmusvillemoes.dk> wrote:
    >> Maurizio Lombardi reported a problem [1] with the %pb extension: It
    >> doesn't work for sufficiently large bitmaps, since the size is stashed
    >> in the field_width field of the struct printf_spec, which is currently
    >> an s16. Concretely, this manifested itself in
    >> /sys/bus/pseudo/drivers/scsi_debug/map being empty, since the bitmap
    >> printer got a size of 0, which is the 16 bit truncation of the actual
    >> bitmap size.
    >>
    >> We do want to keep struct printf_spec at 8 bytes so that it can
    >> cheaply be passed by value. The qualifier field is only used for
    >> internal bookkeeping in format_decode, so we might as well use a local
    >> variable for that. This gives us an additional 8 bits, which we can
    >> then use for the field width.
    >>
    >> To stay in 8 bytes, we need to do a little rearranging and make the
    >> type member a bitfield as well. For consistency, change all the
    >> members to bit fields. gcc doesn't generate much worse code with these
    >> changes (in fact, bloat-o-meter says we save 300 bytes - which I think
    >> is a little surprising).
    >>
    >> I didn't find a BUILD_BUG/compiletime_assertion/... which would work
    >> outside function context, so for now I just open-coded it.
    >
    > And any objections to put it into vsnprintf() ?

    I'd like to keep it close to the type definition. And I was hoping
    someone would come forward and say "yeah, that's been bugging me too,
    here's a patch I've been sitting on to fix that". Almost every compiler
    released this decade has _Static_assert, it's about time we start using
    that instead of the current mess of homegrown workarounds...

    Rasmus


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