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
[email protected]
