linux-kernel - Re: [PATCH] md/raid5: eliminate if-statements in cmp

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Date:   Mon, 4 Sep 2023 04:10:24 +0800
From:   Kuan-Wei Chiu <visitorckw@...il.com>
To:     Roman Mamedov <rm@...anrm.net>
Cc:     song@...nel.org, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] md/raid5: eliminate if-statements in cmp_stripe()

On Sun, Sep 03, 2023 at 06:30:58PM +0500, Roman Mamedov wrote:
> On Sun,  3 Sep 2023 17:50:59 +0800
> Kuan-Wei Chiu <visitorckw@...il.com> wrote:
> 
> > Replace the conditional statements in the cmp_stripe() function with a
> > branchless version to improve code readability and potentially enhance
> > performance.
> 
> The new code will always do two comparisons and a subtraction (3
> instructions in total), whereas the old version could return after just 1
> comparison, or after 2 comparisons. So depending on the data values it is 3x
> to 1.5x as much operations performed than before, there unlikely to be any
> enhancement of performance.
> 
> Also IMO the previous version is more easily readable.
>
The reason behind my proposed changes was to eliminate conditional
branches in the code. While the original code could occasionally achieve
early returns, many compilers, such as x86-64 gcc 13.2 compiling with
-O2 flag, still generate branch instructions. Processors typically have
deep pipelines, and a branch prediction miss can result in a high
penalty. Therefore, even though early return might not be possible, the
new branchless version of code could still offer efficiency
improvements.
> > The new code calculates the result using a subtraction of
> > comparison results, making it more concise and avoiding conditional
> > branches. This change does not alter the functionality of the code.
> > 
> > Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> > ---
> >  drivers/md/raid5.c | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> > 
> > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> > index 4cb9c608ee19..b14d7ba38f0f 100644
> > --- a/drivers/md/raid5.c
> > +++ b/drivers/md/raid5.c
> > @@ -1035,11 +1035,7 @@ static int cmp_stripe(void *priv, const struct list_head *a,
> >  				struct r5pending_data, sibling);
> >  	const struct r5pending_data *db = list_entry(b,
> >  				struct r5pending_data, sibling);
> > -	if (da->sector > db->sector)
> > -		return 1;
> > -	if (da->sector < db->sector)
> > -		return -1;
> > -	return 0;
> > +	return (da->sector > db->sector) - (da->sector < db->sector);
> >  }
> >  
> >  static void dispatch_defer_bios(struct r5conf *conf, int target,
> 
> 
> -- 
> With respect,
> Roman

--
Best regards,
Kuan-Wei Chiu