Re: [PATCH 1/4] Remove architecture specific sched_cpucount optimizations

[Paul Eggert <eggert@cs.ucla.edu>]

On 5/5/21 10:28 AM, Florian Weimer via Libc-alpha wrote:
>> diff --git a/posix/sched_cpucount.c b/posix/sched_cpucount.c
>> index b0ca4ea7bc..529286e777 100644
>> --- a/posix/sched_cpucount.c
>> +++ b/posix/sched_cpucount.c
>> @@ -22,31 +22,11 @@ int
>>   __sched_cpucount (size_t setsize, const cpu_set_t *setp)
>>   {
>>     int s = 0;
>> +  for (int i = 0; i < setsize / sizeof (__cpu_mask); i++)
>>       {
>> +      __cpu_mask si = setp->__bits[i];
>> +      /* Clear the least significant bit set.  */
>> +      for (; si != 0; si &= si - 1, s++);
>>       }
>> -
>>     return s;
>>   }
> Why “si”?  It think si &= si - 1 clears the*most*  significant bit in
> si.  If you agree, please update the comment.

Better yet, define a static function 'popcount' that uses Kernighan's 
trick and call that function. As things stand it's not obvious what the 
code is doing, regardless of which bit it's clearing. The function's 
comment should explain why it's not using __builtin_popcount.