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

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Message-ID: <b4e29e48-a97c-67e5-a284-6ddc13222c5b@arm.com>
Date:   Tue, 8 Oct 2019 15:34:04 +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 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...

I suppose when you really just want a division you should ask for division
semantics - i.e. use '/'. I'd expect compilers to be smart enough to turn
that into a shift if a power of 2 is involved, and to do something else
if negative values can be involved.