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 >> <[email protected]> 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. Thanks, Laura
