[cfe-dev] signbit returns false for negative NaN

[victor.zverovich at gmail.com]

Thanks for fixing this.

- Victor


On Sat, Jun 7, 2014 at 11:11 PM, Steve (Numerics) Canon <scanon at apple.com>
wrote:

> On Jun 6, 2014, at 2:36 PM, Stephen Canon <scanon at apple.com> wrote:
>
> >> On Jun 6, 2014, at 2:18 PM, Philip Reames <listmail at philipreames.com>
> wrote:
> >>
> >> On 06/06/2014 01:02 PM, Chandler Carruth wrote:
> >>>
> >>> The standards for C and C++, and consequentially Clang and LLVM, are
> terribly underspecified w.r.t. things like NaN behavior. =/ We have seen
> this before, and even if we fix it in Clang/LLVM I worry that it will
> regress as there are many places where we model NaNs in a manner slightly
> too aggressive to be strictly IEEE-754 conforming. =[
> >> Is being IEEE-754 conforming a worthwhile goal?  If so, we could talk
> about how to improve testing to prevent such regressions and find other
> latent bugs.
> >>
> >> I would generally argue in favour of conformance, but am not familiar
> with exactly what the C/C++ rules are.
> >
> > C and C++ recommend but do not require IEEE-754 conformance (they define
> a means for a platform to claim conformance, but do not require that it do
> so).  At least within Apple, we’ve historically put a fair bit of effort
> into maintaining IEEE-754 conformance, at least when it comes to the
> semantics of basic arithmetic in default rounding (like this case does).
> >
> >> To say this differently, are there important optimizations we don't
> want to loose which aren't conforming?
> >
> > We have relaxed modes (-ffast-math and friends) that license those
> optimizations.  The default mode should conform as much as reasonably
> possible.
>
> This turns out to be a bug in APFloat.  I'll post a patch to llvm-commits
> in a minute.
>
> – Steve
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