[committed] libstdc++: Support VAX floats in std::strong_order

[Koning, Paul]

> On Mar 10, 2022, at 9:27 AM, Jonathan Wakely via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
> On Thu, 10 Mar 2022 at 12:16, Jonathan Wakely wrote:
>> 
>> On Thu, 10 Mar 2022 at 11:53, Jonathan Wakely via Libstdc++
>> <libstdc++@gcc.gnu.org> wrote:
>>> 
>>> Tested x86_64-linux, and basic soundness check on vax-dec-netbsdelf.
>> 
>> But apparently not enough of a soundness check, because
>> isnan(__builtin_nan("")) is true for VAX, so GCC seems to have a NaN
>> pattern, despite what I read online about the format.

VAX float has signalling NaN, but not a non-signalling NaN nor an Inf.  See the VAX architecture manual.  Signalling NaN (called "reserved operand") is encoded as sign=1 and exponent=0.

	paul