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

[Ard Biesheuvel]

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 */