Messages in this thread Patch in this message |  | | From | Ard Biesheuvel <> | Date | Wed, 1 Feb 2017 17:36:53 +0000 | Subject | Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness |
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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 */
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