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



On 2023-11-17 12:04, Andrew Cooper wrote:
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

Actually no usages of either forms are yet committed, just the definition of the first form, so nothing needs to be reverted. I should have clarified that, sorry. If the other patches in the series go in unmodified (modulo renaming, but that's trivial), they would use the first form.

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



 


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