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    SubjectRe: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

    * Linus Torvalds <> wrote:

    > On Sun, 17 Aug 2008, Ingo Molnar wrote:
    > >
    > > try the patch below - it only gives this error during build:
    > Well, you didn't do it right:
    > > +#define __BBO(c) sizeof(const char[1 - 2*!!(c)])
    > > +#define __BBONC(c) __BBO(!__builtin_constant_p(c))
    > > +#define BUILD_BUG_ON_ZERO2(c) (__BBO(c) - __BBONC(c))
    > > +#define BUILD_BUG_ON2(c) (void)BUILD_BUG_ON_ZERO(c)
    > Look at the #define of BUILD_BUG_ON2 a bit more.
    > Hint: you're using the _wrong_ BUILD_BUG_ON_ZERO. The old one, not the v2
    > one!

    yeah, i already tried various variants earlier today so i really didnt try
    that hard with yours. (and i pointed out this mistake in the previous mail)

    > That said, with that fixed, there's still something wrong. It does seem
    > like gcc has some very odd interaction there with __builtin_constant_p.
    > Odd.

    yeah. I tried various integer arithmetic expressions (which the array trick
    relies on) and it didnt work as expected - it's always zero. It only makes
    a difference when used in comparisons. (and that's where the kernel uses
    __builtin_constant_p quite heavily, and it works fine there.)

    Odd indeed.


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      Last update: 2008-08-17 20:49    [W:0.023 / U:18.264 seconds]
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