linux-kernel - Re: [PATCH] blkdev: use an efficient way to check merge flags

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Message-ID: <20140402181354.GB13479@quack.suse.cz>
Date:	Wed, 2 Apr 2014 20:13:54 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jianyu Zhan <nasa4836@...il.com>
Cc:	axboe@...nel.dk, hch@....de, jack@...e.cz, viro@...iv.linux.org.uk,
	kmo@...erainc.com, namjae.jeon@...sung.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blkdev: use an efficient way to check merge flags

On Wed 02-04-14 23:19:06, Jianyu Zhan wrote:
> A bitwise flag comparison could be done using a more efficient bit-ops way.
  OK, but have you checked the generated code is actually any better? This
is something I'd expect a compiler might be able to optimize anyway. And the
original code looks more readable to me.

								Honza

> Signed-off-by: Jianyu Zhan <nasa4836@...il.com>
> ---
>  include/linux/blkdev.h | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 1e1fa3f..adfa40a 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -655,16 +655,9 @@ static inline bool rq_mergeable(struct request *rq)
>  static inline bool blk_check_merge_flags(unsigned int flags1,
>  					 unsigned int flags2)
>  {
> -	if ((flags1 & REQ_DISCARD) != (flags2 & REQ_DISCARD))
> -		return false;
> -
> -	if ((flags1 & REQ_SECURE) != (flags2 & REQ_SECURE))
> -		return false;
> -
> -	if ((flags1 & REQ_WRITE_SAME) != (flags2 & REQ_WRITE_SAME))
> -		return false;
> -
> -	return true;
> +	return (flags1 & (REQ_DISCARD | REQ_SECURE | REQ_WRITE_SAME)) ^
> +	       (flags2 & (REQ_DISCARD | REQ_SECURE | REQ_WRITE_SAME))
> +		== 0;
>  }
>  
>  static inline bool blk_write_same_mergeable(struct bio *a, struct bio *b)
> -- 
> 1.9.0.GIT
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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