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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 17 Nov 2023, Andrew Cooper wrote:
> On 17/11/2023 10:17 am, 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_; \
> > })
> 
> Of course this was going to explode.
> 
> This isn't even the first time an unwise attempt to do single-evaluation
> has needed to be reverted because it doesn't work with Integer Constant
> Expressions.
> 
> Switch it back to the first form.  It's obviously a macro to begin with,
> and not likely to be used in cases that have side effects.

+1