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SubjectRe: [PATCH] slab.h: Avoid using & for logical and of booleans
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On 11/19/18 12:04 PM, Pavel Machek wrote:
> On Mon 2018-11-05 12:40:00, Bart Van Assche wrote:
>> This patch suppresses the following sparse warning:
>>
>> ./include/linux/slab.h:332:43: warning: dubious: x & !y
>>
>> Fixes: 1291523f2c1d ("mm, slab/slub: introduce kmalloc-reclaimable caches")
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Christoph Lameter <cl@linux.com>
>> Cc: Roman Gushchin <guro@fb.com>
>> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
>> ---
>> include/linux/slab.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index 918f374e7156..97d0599ddb7b 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -329,7 +329,7 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
>> * If an allocation is both __GFP_DMA and __GFP_RECLAIMABLE, return
>> * KMALLOC_DMA and effectively ignore __GFP_RECLAIMABLE
>> */
>> - return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
>> + return type_dma + is_reclaimable * !is_dma * KMALLOC_RECLAIM;
>> }
>>
>
> What is wrong with && ?

Nothing, it would work and generate the same assembly as '&'. But Andrew
noted that this code is probably too clever for its own good, and he has
a point. The single predictable branch is also likely faster than the
chain of arithmetic calculations anyway. Nobody has actually measured
it, so I'd go with the easier-to-read variant.

> If logical and is better done by multiply,
> that's compiler job, and compiler should be fixed to do it...

Multiply was just another way (equivalent to '&&' semantically) to shut
up sparse warning. But gcc actually emits IMUL in that case, which is
wasteful, so yeah there's a bug report now:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87954

>
> Pavel
>

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