Re: [PATCH 2/3] mm/sparse: expand the CONFIG_SPARSEMEM_EXTREME range in __nr_to_section()

[Wei Yang <richard.weiyang@gmail.com>]

On Thu, Aug 23, 2018 at 05:09:12PM -0700, Dave Hansen wrote:
>On 08/23/2018 06:21 AM, Michal Hocko wrote:
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -1155,9 +1155,9 @@ static inline struct mem_section *__nr_to_section(unsigned long nr)
>>  #ifdef CONFIG_SPARSEMEM_EXTREME
>>  	if (!mem_section)
>>  		return NULL;
>> -#endif
>>  	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
>>  		return NULL;
>> +#endif
>>  	return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
>>  }
>
>This patch has no practical effect and only adds unnecessary churn.
>
>#ifdef CONFIG_SPARSEMEM_EXTREME
>...
>#else
>struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
>#endif
>
>The compiler knows that NR_SECTION_ROOTS==1 and that
>!mem_section[SECTION_NR_TO_ROOT(nr) is always false.  It doesn't need
>our help.
>

I didn't know the compile would optimize the code when this is a one dimension
array. Just wrote a code and their assembly looks the same.

Thanks for pointing out.

>My goal with the sparsemem code, and code in general is t avoid #ifdefs
>whenever possible and limit their scope to the smallest possible area
>whenever possible.

-- 
Wei Yang
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