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

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Date:   Thu, 6 Aug 2020 04:49:52 +0800
From:   Zhou Yanjie <zhouyu@...yeetech.com>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>,
        "Maciej W. Rozycki" <macro@....com>,
        Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     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


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