This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH] Define 3-argument overloads of std::hypot for C++17 (P0030R1)
On Wed, 28 Sep 2016, Jonathan Wakely wrote:
> On 27/09/16 23:28 +0000, Joseph Myers wrote:
> > On Tue, 27 Sep 2016, Jonathan Wakely wrote:
> >
> > > This adds the new 3D std::hypot() functions. This implementation seems
> > > to be faster than the naïve sqrt(x*x + y*y + z*z) implementation, or
> > > hypot(hypot(x, y), z), and should be a bit more accurate at very large
> > > or very small values due to reducing the arguments by the largest one.
> > > Improvements welcome though, as this is not my forte.
> >
> > Should I take it from this implementation that C++ is not concerned by
> > certain considerations that would arise for C: spurious underflow
> > exceptions from the scaling when some arguments much larger than others;
> > spurious "invalid" exceptions from the comparisons when any argument is
> > (quiet) NaN; handling of mixed (quiet) NaN and Inf arguments (where ISO C
> > Annex F has Inf returned, not NaN)?
>
> The entire spec from the C++ draft is: Returns: √ x^2 + y^2 + z^2
As a further issue: even if you ignore exceptions and Annex F issues, and
don't have NaN arguments, if you have an Inf argument it will be the
largest, and so your code will do Inf/Inf, and so return NaN, if any
argument is Inf (when obviously the correct answer in that case is Inf).
--
Joseph S. Myers
joseph@codesourcery.com