netdev - Re: [RFC PATCH v1] tun: Cleanup error handling in tun_set

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Message-ID: <20090807000021.GA1566@gondor.apana.org.au>
Date:	Fri, 7 Aug 2009 10:00:21 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Paul Moore <paul.moore@...com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH v1] tun: Cleanup error handling in tun_set_iff()

On Thu, Aug 06, 2009 at 02:20:20PM -0400, Paul Moore wrote:
>
> The code currently looks something like this:
> 
> 	err = -ENOMEM;
> 	buf = alloc(...);
> 	if (!buf)
> 		goto label;
> 
> This means that in the common case where 'alloc()' completes without error we 
> are doing an extra, unnecessary assignment where we set the value in 'err'.  
> Now, if we change this slightly to match what I proposed in the patch:
> 
> 	buf = alloc(...);
> 	if (!buf) {
> 		err = -ENOMEM;
> 		goto label;
> 	}
> 
> We eliminate that extra assignment in the case where 'alloc()' completes 
> without error, which should result in more efficient code (less instructions 
> in the common case).  Am I wrong?  If that is the case I would appreciate an 
> explanation ...

Your style potentially introduces a second jump which may end
up being worse compared to the extra work on a modern CPU.


Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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