[PATCH] Define 3-argument overloads of std::hypot for C++17 (P0030R1)

[Jonathan Wakely <jwakely at redhat dot com>]

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.

The test might not be very good, but tests some small integer values
and some other values where accuracy is lost for one or other of the
alternative implementations mentioned above. If this FAILs for some
32-bit targets we might need to adjust the tolerances or the
dg-options.

	* doc/xml/manual/status_cxx2017.xml: Update status.
	* include/c_global/cmath (hypot): Add three-dimensional overloads.
	* testsuite/26_numerics/headers/cmath/hypot.cc: New.

Tested powerpc64le-linux and x86_64-linux, committed to trunk.

Attachment: patch.txt
Description: Text document