Re: Devise macros to encapsulate (x & -x)

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 2023-11-17 12:39, Jan Beulich wrote:
> On 17.11.2023 11:17, Nicola Vetrini wrote:
> > Hi all,
> >
> > As discussed in this thread [1], which is about complying with MISRA C
> > Rule 10.1,
> > a macro was introduced to encapsulate a well-known construct:
> >
> > /*
> >   * Given an unsigned integer argument, expands to a mask where just 
> > the
> > least
> >   * significant nonzero bit of the argument is set, or 0 if no bits 
> > are
> > set.
> >   */
> > #define ISOLATE_LSB(x) ((x) & -(x))
> >
> > This macro has a gained some calls in the subsequent patches in that
> > thread, but concerns were raised around the fact that it would be 
> > better
> > to devise a macro that evaluates its argument only once. A proposed
> > solution is this (thanks to Jan Beulich):
> >
> > #define ISOLATE_LSB(x) ({ \
> >       typeof(x) x_ = (x); \
> >       x_ & -x_; \
> > })
> >
> > However, it can't be used in all call sites that the previous macro
> > would have once that series is committed, as can be seen in [2].
> > Therefore, while the implementation looks ok,
> > a case has been made to have separate macros, of which the latter form
> > is preferred.
> >
> > The following points require some thought:
> >
> > - where the single evaluation macro should be placed?
> >    One proposed location is xen/include/xen/bitops.h
> Or next to the existing one in macros.h. I can see pros and cons for 
> either.
> > - is the proposed form actually the best, or maybe it could be an 
> > inline
> > function?
> How would you make such a function type generic?
Ah, yes indeed this wouldn't work.

--
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)