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

[Laura Abbott]

On 02/02/2017 01:17 AM, Ard Biesheuvel wrote:
> On 1 February 2017 at 21:50, Laura Abbott <labbott at redhat.com> wrote:
>> On 02/01/2017 09:36 AM, Ard Biesheuvel wrote:
>>> On 1 February 2017 at 16:58, Laura Abbott <labbott at redhat.com> wrote:
>>>> On 10/19/2016 09:22 AM, Will Deacon wrote:
>>>>> On Wed, Oct 19, 2016 at 09:01:33AM -0700, Linus Torvalds wrote:
>>>>>> On Wed, Oct 19, 2016 at 8:56 AM, Markus Trippelsdorf
>>>>>> <markus at trippelsdorf.de> wrote:
>>>>>>> On 2016.10.19 at 08:55 -0700, Linus Torvalds wrote:
>>>>>>>>
>>>>>>>> Well, in the meantime we apparently have to live with it. Unless Will
>>>>>>>> is using some unreleased gcc version that nobody else is using and we
>>>>>>>> can just ignore it?
>>>>>>>
>>>>>>> Yes, he is using gcc-7 that is unreleased. (It will be released April
>>>>>>> next year.)
>>>>>>
>>>>>> Ahh, self-built? So it's not part of some experimental ARM distro
>>>>>> setup and this will be annoying lots of people?
>>>>>
>>>>> Our friendly compiler guys built it, but it's just a snapshot of trunk,
>>>>> so it's all heading towards GCC 7.0. AFAIU, the problematic optimisation
>>>>> is also a mid-end pass, so it would affect other architectures too.
>>>>>
>>>>>> If so, still think that we could just get rid of the ____ilog2_NaN()
>>>>>> thing as it's not _that_ important, but it's certainly not very
>>>>>> high-priority. Will can do it in his tree too for testing, and it can
>>>>>> remind people to get the gcc problem fixed.
>>>>>
>>>>> I'm carrying the diff below, which fixes arm64 defconfig, but I'm worried
>>>>> that we might be relying on this trick elsewhere. The arm __bad_cmpxchg
>>>>> function, for example.
>>>>>
>>>>> Will
>>>>>
>>>>> --->8
>>>>>
>>>>> diff --git a/include/linux/log2.h b/include/linux/log2.h
>>>>> index fd7ff3d91e6a..9cf5ad69065d 100644
>>>>> --- a/include/linux/log2.h
>>>>> +++ b/include/linux/log2.h
>>>>> @@ -16,12 +16,6 @@
>>>>>  #include <linux/bitops.h>
>>>>>
>>>>>  /*
>>>>> - * deal with unrepresentable constant logarithms
>>>>> - */
>>>>> -extern __attribute__((const, noreturn))
>>>>> -int ____ilog2_NaN(void);
>>>>> -
>>>>> -/*
>>>>>   * non-constant log of base 2 calculators
>>>>>   * - the arch may override these in asm/bitops.h if they can be implemented
>>>>>   *   more efficiently than using fls() and fls64()
>>>>> @@ -85,7 +79,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
>>>>>  #define ilog2(n)                             \
>>>>>  (                                            \
>>>>>       __builtin_constant_p(n) ? (             \
>>>>> -             (n) < 1 ? ____ilog2_NaN() :     \
>>>>> +             (n) < 1 ? 0 :                   \
>>>>>               (n) & (1ULL << 63) ? 63 :       \
>>>>>               (n) & (1ULL << 62) ? 62 :       \
>>>>>               (n) & (1ULL << 61) ? 61 :       \
>>>>> @@ -149,9 +143,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
>>>>>               (n) & (1ULL <<  3) ?  3 :       \
>>>>>               (n) & (1ULL <<  2) ?  2 :       \
>>>>>               (n) & (1ULL <<  1) ?  1 :       \
>>>>> -             (n) & (1ULL <<  0) ?  0 :       \
>>>>> -             ____ilog2_NaN()                 \
>>>>> -                                ) :          \
>>>>> +             0) :                            \
>>>>>       (sizeof(n) <= 4) ?                      \
>>>>>       __ilog2_u32(n) :                        \
>>>>>       __ilog2_u64(n)                          \
>>>>> @@ -194,7 +186,6 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
>>>>>   * @n: parameter
>>>>>   *
>>>>>   * The first few values calculated by this routine:
>>>>> - *  ob2(0) = 0
>>>>>   *  ob2(1) = 0
>>>>>   *  ob2(2) = 1
>>>>>   *  ob2(3) = 2
>>>>>
>>>>
>>>> Reviving this thread as gcc 7 has now hit Fedora rawhide and has this
>>>> same issue. I pulled in the above patch from Will as a temporary work
>>>> around for building. It didn't look like there was consensus on a
>>>> permanent solution though from the thread.
>>>>
>>>
>>> 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 */
>>>
>>
>> This fixes the problem although the comments should be updated
>> as well.
> 
> This brings order_base_2() in line with the comments, so I am not sure
> what you'd want to update here?
> 

ob2(1) = 1  for the __builtin_constant_p case which doesn't
match the comment of ob2(1) = 0. So my statement should actually
be is this correct?

Thanks,
Laura