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)
