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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew