Re: [PATCH] c++: Implement P1467R9 - Extended floating-point types and standard names compiler part except for bfloat16 [PR106652]

[Joseph Myers <joseph@codesourcery.com>]

On Mon, 12 Sep 2022, Jakub Jelinek via Gcc-patches wrote:

> Now, I guess for the fixincludes it could also use
> #  if !__GNUC_PREREQ (7, 0) || (defined __cplusplus && !__GNUC_PREREQ (13, 0))
> where earlier GCC 13 snapshots would not be doing the fixincludes,
> but the question is what to use for upstream glibc, because
> there will be 13.0 snapshots where C++ doesn't support _Float{16,32,64,128}
> and where it is essential to use what glibc has been doing previously
> and using the #else would fail miserably, and then 13.0 snapshots where it
> does support it and where using the if would fail miserably.

We don't claim in glibc to support old snapshots from master, so checking 
for __GNUC_PREREQ (13, 0) and failing for such older GCC 13 versions is 
fine.

> Conversion from BFmode to SFmode is easy, left shift by 16 and ought to be
> implemented inline, SFmode -> BFmode conversion is harder,

Properly the right way for converting from BFmode to SFmode in the 
presence of -fsignaling-nans should depend on how the result is used.  If 
it's used for arithmetic, it's OK to have converted a BFmode signaling NaN 
to an SFmode signaling NaN, but if e.g. the result is examined with 
issignaling or otherwise stored so it may be significant later whether the 
result is a quiet or signaling NaN, IEEE semantics would mean a signaling 
NaN should be a converted to a quiet NaN with "invalid" raised.  Though I 
don't know how far hardware instructions for BFmode attempt to follow IEEE 
semantics.

(Cf. powerpc single-precision load instructions whose effect is defined as 
a purely bitwise conversion from single to double precision, so that 
single-precision load and store of a signaling NaN never end up converting 
it to a quiet NaN even though the in-register format is double precision.)

> (untested) and the question is if it should be implemented in libgcc
> (and using soft-fp or not), or inline, or both depending on -Os.

Also if you try to do a direct conversion between BFmode and HFmode, 
soft-fp's current support for conversions may not handle that case (where 
one type has wider exponent range and other type has higher precision).

-- 
Joseph S. Myers
joseph@codesourcery.com