linux-kernel - Re: [PATCH 4/9] drivers/iio: Sign extend without triggering implementation-defined behavior

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Date:   Thu, 31 Oct 2019 10:55:09 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     dgilbert@...erlog.com, Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>
Subject: Re: [PATCH 4/9] drivers/iio: Sign extend without triggering
 implementation-defined behavior

On 10/30/19 3:13 PM, Douglas Gilbert wrote:
> On 2019-10-30 4:02 p.m., Peter Zijlstra wrote:
>> On Mon, Oct 28, 2019 at 01:06:55PM -0700, Bart Van Assche wrote:
>>>  From the C standard: "The result of E1 >> E2 is E1 right-shifted E2 bit
>>> positions. If E1 has an unsigned type or if E1 has a signed type and a
>>> nonnegative value, the value of the result is the integral part of the
>>> quotient of E1 / 2E2 . If E1 has a signed type and a negative value, the
>>> resulting value is implementation-defined."
>>
>> FWIW, we actually hard rely on this implementation defined behaviour all
>> over the kernel. See for example the generic sign_extend{32,64}()
>> functions.
>>
>> AFAIR the only reason the C standard says this is implementation defined
>> is because it wants to support daft things like 1s complement and
>> saturating integers.
> 
> See:
>     http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2218.htm
> 
> That is in C++20 and on the agenda for C2x:
>     https://gustedt.wordpress.com/2018/11/12/c2x/

Thanks Peter and Doug. This is very useful feedback. I will drop the 
sign_extend_24_to_32() function and use sign_extend32() instead.

Bart.