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SubjectRe: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness
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On Wed, 2017-02-01 at 18:19 +0000, Ard Biesheuvel wrote:
> On 1 February 2017 at 17:36, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > I still think order_base_2() is broken, since it may invoke
> > roundup_pow_of_two() with an input value that is documented as
> > producing undefined output. I would argue that the below is the
> > correct fix.
> >
> > diff --git a/include/linux/log2.h b/include/linux/log2.h
> > index fd7ff3d91e6a..46523731bec0 100644
> > --- a/include/linux/log2.h
> > +++ b/include/linux/log2.h
> > @@ -203,6 +203,18 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
> > * ... and so on.
> > */
> >
> > -#define order_base_2(n) ilog2(roundup_pow_of_two(n))
> > +static inline __attribute__((__const__))
> > +unsigned long __order_base_2(unsigned long n)
> > +{
> > + return n ? 1UL << fls_long(n - 1) : 1;
> > +}
> > +
> > +#define order_base_2(n) \
> > +( \
> > + __builtin_constant_p(n) ? ( \
> > + ((n) < 2) ? (n) : \
> > + ilog2((n) - 1) + 1) : \
> > + ilog2(__order_base_2(n)) \
> > + )
> >
> > #endif /* _LINUX_LOG2_H */
>
> Actually, there is a still a redundant shift/fls() in there, this is
> even simpler:
>
> diff --git a/include/linux/log2.h b/include/linux/log2.h
> index fd7ff3d91e6a..4741534bd7af 100644
> --- a/include/linux/log2.h
> +++ b/include/linux/log2.h
> @@ -203,6 +203,18 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
> * ... and so on.
> */
>
> -#define order_base_2(n) ilog2(roundup_pow_of_two(n))
> +static inline __attribute__((__const__))

commonly __attribute_const__

> +unsigned long __order_base_2(unsigned long n)
> +{
> + return n > 1 ? ilog2(n - 1) + 1 : 0;
> +}
> +
> +#define order_base_2(n) \
> +( \
> + __builtin_constant_p(n) ? ( \
> + ((n) < 2) ? (n) : \
> + ilog2((n) - 1) + 1) : \
> + __order_base_2(n) \
> + )

Does this work properly when n is a signed negative value?

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