linux-kernel - Re: Mark bitrevX() functions as const

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Date:	Mon, 11 Dec 2006 07:57:00 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	David Howells <dhowells@...hat.com>
CC:	Akinobu Mita <akinobu.mita@...il.com>, torvalds@...l.org,
	akpm@...l.org, linux-kernel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Mark bitrevX() functions as const

David Howells wrote:
> Mark the bit reversal functions as being const as they always return the same
> output for any given input.
> 
> Signed-Off-By: David Howells <dhowells@...hat.com>
> ---
> 
>  include/linux/bitrev.h |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/bitrev.h b/include/linux/bitrev.h
> index 05e540d..032056b 100644
> --- a/include/linux/bitrev.h
> +++ b/include/linux/bitrev.h
> @@ -5,11 +5,11 @@ #include <linux/types.h>
>  
>  extern u8 const byte_rev_table[256];
>  
> -static inline u8 bitrev8(u8 byte)
> +static inline __attribute__((const)) u8 bitrev8(u8 byte)
>  {
>  	return byte_rev_table[byte];
>  }
>  
> -extern u32 bitrev32(u32 in);
> +extern __attribute__((const)) u32 bitrev32(u32 in);


Comments:

* overall, I agree with this type of change.  several Linux lib 
functions could use this sort of annotation.

* I question its usefulness on static [inline] functions, because the 
compiler should be able to figure out side effects.  have you examined 
before-and-after asm to see if the code generation changes for the 
inlined area?

* naked __attribute__ is ugly.  define something short and memorable in 
include/linux/compiler.h.

* another annotation to consider is C99 keyword 'restrict'.

	Jeff



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