linux-kernel - Re: [PATCH] slab.h: Avoid using & for logical and of booleans

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Date:   Mon, 19 Nov 2018 13:51:46 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Pavel Machek <pavel@....cz>, Bart Van Assche <bvanassche@....org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Christoph Lameter <cl@...ux.com>, Roman Gushchin <guro@...com>
Subject: Re: [PATCH] slab.h: Avoid using & for logical and of booleans

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@...e.cz>
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Cc: Christoph Lameter <cl@...ux.com>
>> Cc: Roman Gushchin <guro@...com>
>> Signed-off-by: Bart Van Assche <bvanassche@....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
>