Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 2 Feb 2017 09:17:29 +0000 | Subject | Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness |
| |
On 1 February 2017 at 21:50, Laura Abbott <labbott@redhat.com> wrote: > On 02/01/2017 09:36 AM, Ard Biesheuvel wrote: >> On 1 February 2017 at 16:58, Laura Abbott <labbott@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@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?
> Is it worth fixing this for ilog2 as well just to avoid > the link nonsense there as well? >
ilog2() is truly undefined for input 0, so the link nonsense is justified there imo
| |