[cfe-dev] signbit returns false for negative NaN

[Stephen Canon]

This appears to have nothing to do with signbit, but rather that we are somehow “optimizing” –NaN —> NaN.

int main(int argc, char *argv[]) {
    double x = -__builtin_nan("");
    printf("%d\t%d\n", signbit(x));
    return 0;
}

this prints zero on recent clang (should print 1).  Stepping through, x is being set to a positive NaN.
– Steve

> On Jun 6, 2014, at 12:21 PM, victor.zverovich at gmail.com wrote:
> 
> I've checked and the same happens with the signbit macro.
> 
> Victor
> 
> 
> On Fri, Jun 6, 2014 at 11:56 AM, Aaron Ballman <aaron at aaronballman.com> wrote:
> On Fri, Jun 6, 2014 at 2:51 PM, victor.zverovich at gmail.com
> <victor.zverovich at gmail.com> wrote:
> > Hello,
> >
> > When compiling with a recent version of Clang, the following code
> >
> >   std::signbit(-std::numeric_limits<double>::quiet_NaN())
> >
> > returns true with optimizations disabled and false with optimizations
> > enabled. Other compilers like GCC and older versions of Clang always return
> > true as expected. Is it a bug or a feature?
> >
> > I'm using Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn).
> 
> I believe this is likely a bug. std::signbit is defined to behave the
> same as the C macro of the same name, which has the footnote, "The
> signbit macro reports the sign of all values, including infinities,
> zeros, and NaNs. If zero is unsigned, it is treated as positive."
> 
> ~Aaron
> 
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