Re: [PATCH 0/5] IEEE 128-bit built-in overload support.

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Aug 11, 2022 at 08:44:17PM +0000, Joseph Myers wrote:
> On Thu, 11 Aug 2022, Michael Meissner via Gcc-patches wrote:
> 
> > In looking at it, I now believe that the type for _Float128 and __float128
> > should always be the same within the compiler.  Whether we would continue to
> > use the same type for long double and _Float128/__float128 remains to be seen.
> 
> long double and _Float128 must always be different types; that's how it's 
> defined in C23.

And when we implement C++23 P1467R9, if std::float128_t will be
_Float128 under the hood, then long double and _Float128 have to remain
distinct types and mangle differently, long double (and __float128 if
long double is IEEE quad and __float128 exists?) need to mangle the way
they currently do and _Float128 should mangle as  DF128_ .
             ::= DF <number> _ # ISO/IEC TS 18661 binary floating point type _FloatN (N bits)
Wonder how shall we mangle the underlying type of std::bfloat16_t though.

I assume e.g. for libstdc++ implementation purposes we need to have
__ibm128 and __float128 types mangling as long double mangles when the
-mabi={ibm,ieee}longdouble option is used, because otherwise it would be
really hard to implement it.

	Jakub