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SubjectRe: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness
On 1 February 2017 at 19:04, Joe Perches <joe@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@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, ->),
so its behavior for negative inputs is best left undefined.

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