Messages in this thread | | | Date | Sun, 17 Aug 2008 20:45:10 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions |
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* 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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