Re: git: 7bcc7a0b88cc - stable/14 - libkern: avoid local var in order_base_2()

> On Feb 10, 2025, at 7:15 PM, Doug Moore <dougm@FreeBSD.org> wrote:
> 
> The branch stable/14 has been updated by dougm:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=7bcc7a0b88ccb5e1fe31de88ab9a17e46859318b
> 
> commit 7bcc7a0b88ccb5e1fe31de88ab9a17e46859318b
> Author:     Doug Moore <dougm@FreeBSD.org>
> AuthorDate: 2024-09-27 23:43:07 +0000
> Commit:     Doug Moore <dougm@FreeBSD.org>
> CommitDate: 2025-02-10 10:30:05 +0000
> 
>    libkern: avoid local var in order_base_2()
> 
>    order_base_2(n) is implemented with a variable, which keeps it from
>    being used at file scope. Implement it instead as ilog2(2*n-1), which
>    produces a different result when 2*n overflows, which appears unlikely
>    in practice.
> 
>    Reviewed by:    bz
>    Differential Revision:  https://reviews.freebsd.org/D46826
> 
>    (cherry picked from commit b7cbf741d55468ba34305a14ac3acc1c286af034)
> ---
> sys/sys/libkern.h | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/sys/sys/libkern.h b/sys/sys/libkern.h
> index a10289d72dc7..835e5ffaf0b7 100644
> --- a/sys/sys/libkern.h
> +++ b/sys/sys/libkern.h
> @@ -229,10 +229,7 @@ ilog2_long_long(long long n)
> 
> #define	ilog2(n) (__builtin_constant_p(n) ? ilog2_const(n) : ilog2_var(n))
> #define	rounddown_pow_of_two(n)	((__typeof(n))1 << ilog2(n))
> -#define	order_base_2(n) ({			\
> -	__typeof(n) _n = (n);			\

This local var `_n` is within the scope of the macro `order_base_2`, it is surrounded with
{} and is harmless, so it will not pollute the file scoped variables.

Am I reading the commit message wrong ?

Best regards,
Zhenlei

> -	_n == 1 ? 0 : 1 + ilog2(_n - 1);	\
> -})
> +#define order_base_2(n) ilog2(2*(n)-1)
> #define	roundup_pow_of_two(n)	((__typeof(n))1 << order_base_2(n))
> 
> #define	bitcount64(x)	__bitcount64((uint64_t)(x))