[cfe-dev] signbit returns false for negative NaN

[Chandler Carruth]

On Fri, Jun 6, 2014 at 12:52 PM, Stephen Canon <scanon at apple.com> wrote:

> 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.


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. =[
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