On 25 February 2017 at 11:09, Markus Trippelsdorf <mar...@trippelsdorf.de> wrote: > 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.) >
Well, given that it is really dead code that is being emitted, and that log2(0) is really undefined, perhaps we should simply replace ilog2_NaN() with __builtin_unreachable()?
