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

* Boaz Harrosh <bharrosh@panasas.com> wrote:

> > Link time warnings are easy enough to miss.
> >
> > So unless there's a better way of doing it all at compile time (i'd
> > really prefer that!) i'd prefer the link time error about botched
> > BUILD_BUG_ON() conditions - as my commits introduce.
> >
> > Ingo
> > --
>
> #define BUILD_BUG_ON(condition) \
> do {
> enum { bad = !!(condition)}; \
> static struct { char arr[1 - 2*bad]; } x __maybe_unused; \
> } while(0)
>
> the enum definition will not let in anything not compile-time constant.

nice trick!

> But then I fail on: (include/linux/virtio_config.h:99)
>
> if (__builtin_constant_p(fbit))
> BUILD_BUG_ON(fbit >= 32);
>
> is that code broken?

hmm ... that's a bit sad, gcc ought to have been able to figure this
out. Can this be fixed somehow, without losing the strength of the
checking here? I think we should not change BUILD_BUG_ON() to make it
less useful.

Ingo


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