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

[Vlastimil Babka]

On 11/9/18 8:00 PM, Andrew Morton wrote:
> On Fri, 9 Nov 2018 09:12:09 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote:
> 
>> Multiple people have reported the following sparse warning:
>>
>> ./include/linux/slab.h:332:43: warning: dubious: x & !y
>>
>> The minimal fix would be to change the logical & to boolean &&, which emits the
>> same code, but Andrew has suggested that the branch-avoiding tricks are maybe
>> not worthwile. David Laight provided a nice comparison of disassembly of
>> multiple variants, which shows that the current version produces a 4 deep
>> dependency chain, and fixing the sparse warning by changing logical and to
>> multiplication emits an IMUL, making it even more expensive.
>>
>> The code as rewritten by this patch yielded the best disassembly, with a single
>> predictable branch for the most common case, and a ternary operator for the
>> rest, which gcc seems to compile without a branch or cmov by itself.
>>
>> The result should be more readable, without a sparse warning and probably also
>> faster for the common case.
>>
>> Reported-by: Bart Van Assche <bvanassche@xxxxxxx>
>> Reported-by: Darryl T. Agostinelli <dagostinelli@xxxxxxxxx>
>> Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Suggested-by: David Laight <David.Laight@xxxxxxxxxx>
>> Fixes: 1291523f2c1d ("mm, slab/slub: introduce kmalloc-reclaimable caches")
>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>> ---
>>  include/linux/slab.h | 24 ++++++++++++------------
>>  1 file changed, 12 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index 918f374e7156..18c6920c2803 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -304,6 +304,8 @@ enum kmalloc_cache_type {
>>  	KMALLOC_RECLAIM,
>>  #ifdef CONFIG_ZONE_DMA
>>  	KMALLOC_DMA,
>> +#else
>> +	KMALLOC_DMA = KMALLOC_NORMAL,
>>  #endif
>>  	NR_KMALLOC_TYPES
>>  };
> 
> I don't think this works correctly.  Resetting KMALLOC_DMA to 0 will
> cause NR_KMALLOC_TYPES to have value 1.

Doh, right! Thanks for catching this.

This? Not terribly elegant, but I don't see a nicer way right now...

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