Re: [PATCHv2 8/10] xfs: avoid repeated pointer dereferences

From: Dave Chinner <david@fromorbit.com>
To: Alex Elder <aelder@sgi.com>
Cc: xfs@oss.sgi.com
Subject: Re: [PATCHv2 8/10] xfs: avoid repeated pointer dereferences
Date: Mon, 12 Apr 2010 16:56:44 +1000	[thread overview]
Message-ID: <20100412065644.GJ2493@dastard> (raw)
In-Reply-To: <1270852242.7840.158.camel@doink>

On Fri, Apr 09, 2010 at 05:30:42PM -0500, Alex Elder wrote:
> In xlog_find_cycle_start() use a local variable for some repeated
> operations rather than constantly updating the memory location
> whose address is passed in.

Won't the compiler optimise that out for us? i.e. does the dissassembly
of the function look any better before and after this change?

> Signed-off-by: Alex Elder <aelder@sgi.com>
> 
> ---
> fs/xfs/xfs_log_recover.c |   83 ++++++++++++++++++++++++-----------------------
>  fs/xfs/xfs_log_recover.c |   26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
> 
> Index: b/fs/xfs/xfs_log_recover.c
> ===================================================================
> --- a/fs/xfs/xfs_log_recover.c
> +++ b/fs/xfs/xfs_log_recover.c
> @@ -354,26 +354,28 @@ xlog_find_cycle_start(
>  {
>  	xfs_caddr_t	offset;
>  	xfs_daddr_t	mid_blk;
> +	xfs_daddr_t	end_blk;
>  	uint		mid_cycle;
>  	int		error;
>  
> -	mid_blk = BLK_AVG(first_blk, *last_blk);
> -	while (mid_blk != first_blk && mid_blk != *last_blk) {
> +	ASSERT(last_blk != NULL);
> +	end_blk = *last_blk;

FWIW, there is no need for that ASSERT there - the machine will
panic on the very next line, anyway....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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