> On 25 Feb 2017, at 11:23, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote: > > 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()?
... or perhaps it is better to just pass the constant == 0 to the runtime implementation? The second ilog2_NaN is really unreachable, given that it deals with unsigned values >0 without a single bit set.
