linux-kernel - Re: [PATCH] slab: Remove unnecessary __builtin_constant

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Date:	Wed, 17 Apr 2013 17:03:21 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Pekka Enberg <penberg@...nel.org>
cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux.com>,
	Behan Webster <behanw@...verseincode.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] slab: Remove unnecessary __builtin_constant_p()

On Wed, 17 Apr 2013, Steven Rostedt wrote:

> The slab.c code has a size check macro that checks the size of the
> following structs:
> 
> struct arraycache_init
> struct kmem_list3
> 
> The index_of() function that takes the sizeof() of the above two structs
> and does an unnecessary __builtin_constant_p() on that. As sizeof() will
> always end up being a constant making this always be true. The code is
> not incorrect, but it just adds added complexity, and confuses users and
> wastes the time of reviewers of the code, who spends time trying to
> figure out why the builtin_constant_p() was used.
> 
> This patch is just a clean up that makes the index_of() code a little
> bit less complex.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Acked-by: David Rientjes <rientjes@...gle.com>

Adding Pekka to the cc.

> 
> diff --git a/mm/slab.c b/mm/slab.c
> index 856e4a1..6047900 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -325,9 +325,7 @@ static void cache_reap(struct work_struct *unused);
>  static __always_inline int index_of(const size_t size)
>  {
>  	extern void __bad_size(void);
> -
> -	if (__builtin_constant_p(size)) {
> -		int i = 0;
> +	int i = 0;
>  
>  #define CACHE(x) \
>  	if (size <=x) \
> @@ -336,9 +334,7 @@ static __always_inline int index_of(const size_t size)
>  		i++;
>  #include <linux/kmalloc_sizes.h>
>  #undef CACHE
> -		__bad_size();
> -	} else
> -		__bad_size();
> +	__bad_size();
>  	return 0;
>  }
>  
--
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