linux-kernel - Re: [PATCH v3 04/10] sched/fair: rework load

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Message-ID: <5de71c9f-cd6b-0284-f4a0-0d1fe4059099@arm.com>
Date:   Tue, 8 Oct 2019 17:39:28 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Phil Auld <pauld@...hat.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v3 04/10] sched/fair: rework load_balance

On 08/10/2019 17:33, Peter Zijlstra wrote:
> On Tue, Oct 08, 2019 at 03:34:04PM +0100, Valentin Schneider wrote:
>> On 08/10/2019 15:16, Peter Zijlstra wrote:
>>> On Wed, Oct 02, 2019 at 11:47:59AM +0100, Valentin Schneider wrote:
>>>
>>>> Yeah, right shift on signed negative values are implementation defined.
>>>
>>> Seriously? Even under -fno-strict-overflow? There is a perfectly
>>> sensible operation for signed shift right, this stuff should not be
>>> undefined.
>>>
>>
>> Mmm good point. I didn't see anything relevant in the description of that
>> flag. All my copy of the C99 standard (draft) says at 6.5.7.5 is:
>>
>> """
>> The result of E1 >> E2 [...] If E1 has a signed type and a negative value,
>> the resulting value is implementation-defined.
>> """
>>
>> Arithmetic shift would make sense, but I think this stems from twos'
>> complement not being imposed: 6.2.6.2.2 says sign can be done with
>> sign + magnitude, twos complement or ones' complement...
> 
> But -fno-strict-overflow mandates 2s complement for all such signed
> issues.
> 

So then there really shouldn't be any ambiguity. I have no idea if
-fno-strict-overflow then also lifts the undefinedness of the right shifts,
gotta get my spade and dig some more.