Devise macros to encapsulate (x & -x)

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

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
- is the proposed form actually the best, or maybe it could be an inline 
function?

Then testing can happen and a subsequent version of those uncommitted 
patches introducing uses of either construct will be submitted, to 
modify all the call sites only once in the commit history.

Let me know what you think of this.
Regards,

[1] 
https://lore.kernel.org/xen-devel/8a1313b3ab5ba6dd556cf37409e3b703@bugseng.com/T/#mdeb510325e1acacb6477a88de8577e9e87351ba5
[2] https://gitlab.com/xen-project/people/bugseng/xen/-/jobs/5423693947

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