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

[Ard Biesheuvel]

On 1 February 2017 at 19:49, Joe Perches <joe at perches.com> wrote:
> On Wed, 2017-02-01 at 19:31 +0000, Ard Biesheuvel wrote:
>> On 1 February 2017 at 19:04, Joe Perches <joe at perches.com> wrote:
>> > On Wed, 2017-02-01 at 18:19 +0000, Ard Biesheuvel wrote:
>> > > On 1 February 2017 at 17:36, Ard Biesheuvel <ard.biesheuvel at 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__
>> >
>>
>> ... except in <linux/ilog2.h>, which probably predates that.
>>
>> > > +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?
>> >
>>
>> No, but order_base_2() is explicitly only defined for inputs [0, ->),
>
> where?
>

The comment describes it as follows

 /**
  * order_base_2 - calculate the (rounded up) base 2 order of the argument
  * @n: parameter
  *
  * The first few values calculated by this routine:
  *  ob2(0) = 0
  *  ob2(1) = 0
  *  ob2(2) = 1
  *  ob2(3) = 2
  *  ob2(4) = 2
  *  ob2(5) = 3
  *  ... and so on.
  */

i.e., it defines the output for inputs 0, 1, 2, 3, 4, 5, ..., and not
for negative inputs, hence undefined.

>> so its behavior for negative inputs is best left undefined.
>
> Or maybe add a BUILD_BUG_ON something like:
>
> #define order_base_2(n)                                                 \
> ({                                                                      \
>         typeof(n) _n = n;                                               \
>         BUILD_BUG_ON(__builtin_constant_p(_n) && _n < 0);               \
>         __builtin_constant_p(_n) ? (_n < 2 ? _n : ilog2((_n) - 1) + 1)) \
>                                  : __order_base_2(_n);                  \
> })
>

This would interfere with the ability to use order_base_2() in
initializers for global variables.