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

* Linus Torvalds <torvalds@linux-foundation.org> 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.

Ingo


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