Subject: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness

From: [email-masked] (Joe Perches)
Date: Wed, 01 Feb 2017 11:04:54 -0800

On Wed, 2017-02-01 at 18:19 +0000, Ard Biesheuvel wrote:
> On 1 February 2017 at 17:36, Ard Biesheuvel <[email-masked]> 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?