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

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Date:   Sun, 2 Aug 2020 22:46:31 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@....com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
cc:     Jiaxun Yang <jiaxun.yang@...goat.com>,
        Serge Semin <fancer.lancer@...il.com>,
        linux-mips@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Huacai Chen <chenhc@...ote.com>,
        Zhou Yanjie <zhouyanjie@...o.com>,
        周琰杰 (Zhou Yanjie) 
        <zhouyanjie@...yeetech.com>,
        Liangliang Huang <huanglllzu@...il.com>,
        linux-kernel@...r.kernel.org,
        "Maciej W. Rozycki" <macro@...ux-mips.org>
Subject: Re: [PATCH] MIPS: Provide Kconfig option for default IEEE754
 conformance mode

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 
(in which case you don't need the relaxed setting in the first place).

  Maciej