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?
Jan
