Message-ID: <4B0D77CC.3020807@cs.helsinki.fi>
Date: Wed, 25 Nov 2009 20:30:36 +0200
From: Pekka Enberg <penberg@cs.helsinki.fi>
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To: Joe Perches <joe@perches.com>
CC: linux-kernel@vger.kernel.org, Christoph Lameter <cl@linux-foundation.org>
Subject: Re: [PATCH] SLUB: Fix __GFP_ZERO unlikely() annotation
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Joe Perches wrote:
> On Wed, 2009-11-25 at 20:21 +0200, Pekka Enberg wrote:
>> The unlikely() annotation in slab_alloc() covers too much of the expression.
>> It's actually very likely that the object is not NULL so use unlikely() only
>> for the __GFP_ZERO expression like SLAB does.
> []
>> +++ b/mm/slub.c
>> @@ -1735,7 +1735,7 @@ static __always_inline void *slab_alloc(struct kmem_cache *s,
>> -	if (unlikely((gfpflags & __GFP_ZERO) && object))
>> +	if (unlikely(gfpflags & __GFP_ZERO) && object)
>>  		memset(object, 0, objsize);
> 
> so why not use
> 
> 	if (unlikely(gfpflags & __GFP_ZERO) && likely(object))

Because that has no effect on the generated code.
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