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

[Bart Van Assche]

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:
> > > ÂFrom 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:
>  ÂÂ http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2218.htm
>
> That is in C++20 and on the agenda for C2x:
>  ÂÂ 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.