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

On 11 Feb 2025, at 03:30, Zhenlei Huang <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.

Jess