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

> On Feb 11, 2025, at 1:23 PM, Jessica Clarke <jrtc27@freebsd.org> wrote:
> 
> On 11 Feb 2025, at 03:30, Zhenlei Huang <zlei@FreeBSD.org <mailto:zlei@FreeBSD.org>> wrote:
>>> 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 ?
> 
> It’s not about pollution. GNU statement expressions are just not valid
> at file scope, they must be used within a function (since otherwise the
> implementation would have to have a full blown interpreter to constant
> evaluate the whole thing, just like C++ and now C to some extent have
> ended up with these days with constexpr).
> 
> That is, you cannot write:
> 
>  int x = ({ ... });
> 
> at file scope.

Ahh, I see. Thanks for the explaining!

> 
> Jess