On 2017.02.25 at 09:11 +0000, Ard Biesheuvel wrote: > On 25 February 2017 at 08:18, Markus Trippelsdorf <mar...@trippelsdorf.de> > wrote: > > > > Why not simply get rid of the ____ilog2_NaN thing altogether? > > > > That would remove the issue, sure. But we lose an opportunity to spot > incorrect code at compile time.
In the case of kernel/time/timekeeping.c it is clearly a false positive. Was ever incorrect code spotted by ____ilog2_NaN in the past? > My concern is that it by not pushing back on changes to the semantics > of __builtin_constant_p() such as this one, we may start seeing other > issues where we can no longer use it, and we lose a very useful tool. We had a long discussion in: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785 As you can see there is no real consensus. But ilog2 seems to be the only place where this ever popped up. (There were several distro-wide mass rebuilds with gcc-7 and no other __builtin_constant_p() issue was found yet.) -- Markus
