Re: [PATCH] MIPS: Provide Kconfig option for default IEEE754 conformance mode

[Zhou Yanjie]

在 2020/8/3 下午5:01, Jiaxun Yang 写道:
> 在 2020/8/3 上午5:46, Maciej W. Rozycki 写道:
> > On Fri, 31 Jul 2020, Serge Semin wrote:
> >
> > > > Requested by downstream distros, a Kconfig option for default
> > > > IEEE754 conformance mode allows them to set their mode to
> > > > relaxed by default.
> > > That's what should have been here in the first place. Thanks!
> >   Well, originally plans were there to have NaN interlinking implemented
> > and no such mess or desire for hacks like one here would result.  Cf.:
> >
> > <https://gcc.gnu.org/ml/gcc/2015-11/msg00068.html>,
> > <https://gcc.gnu.org/ml/gcc/2016-05/msg00137.html>,
> >
> > and then:
> >
> > <https://lkml.org/lkml/2015/11/16/386>,
> > <https://sourceware.org/ml/libc-alpha/2015-11/msg00485.html>,
> > <https://sourceware.org/ml/binutils/2015-11/msg00170.html>,
> > <https://gcc.gnu.org/ml/gcc-patches/2015-11/msg03241.html>.
> >
> > You could well pick this work up and complete it if you like. Final
> > conclusions for further work were made here:
> >
> > <https://gcc.gnu.org/ml/gcc/2016-11/msg00027.html>,
> > <https://gcc.gnu.org/ml/gcc/2017-08/msg00260.html>,
> > <https://gcc.gnu.org/ml/gcc/2017-10/msg00142.html>.
> >
> >   In the relaxed mode math programs may produce wrong results unless you
> > rebuild all your software for the correct NaN mode for the hardware used
>
> Unfortunately most of the hardware guys didn't understood the 
> difficulty here.
> They decided to implement their hardware (P5600 & LS3A4000) as NaN2008 
> only.

All SoCs based on Ingenic XBurst2 CPU core are also NaN2008 only.


> I was thinking about let Kernel drop SIGFPE exception was caused by 
> mismatched NaN,
> as most applications don't rely on signaling NaN, but it is still a 
> dirty hack. Not a good
> idea in general.
>
> Thanks.
>
> - Jiaxun
>
> > (in which case you don't need the relaxed setting in the first place).
>
> >    Maciej