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

[Jan Beulich <jbeulich@xxxxxxxx>]

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