netdev - Re: [net-next PATCH] net: help compiler generate better code in eth_get

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Message-Id: <20150928.225138.363029814550861853.davem@davemloft.net>
Date:	Mon, 28 Sep 2015 22:51:38 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	brouer@...hat.com
Cc:	netdev@...r.kernel.org
Subject: Re: [net-next PATCH] net: help compiler generate better code in
 eth_get_headlen

 From: Jesper Dangaard Brouer <brouer@...hat.com>
Date: Mon, 28 Sep 2015 12:47:14 +0200

> Noticed that the compiler (gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC))
> generated suboptimal assembler code in eth_get_headlen().
> 
> This early return coding style is usually not an issue, on super scalar CPUs,
> but the compiler choose to put the return statement after this very unlikely
> branch, thus creating larger jump down to the likely code path.
> 
> Performance wise, I could measure slightly less L1-icache-load-misses
> and less branch-misses, and an improvement of 1 nanosec with an IP-forwarding
> use-case with 257 bytes packets with ixgbe (CPU i7-4790K @ 4.00GHz).
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>

Applied, thanks Jesper.
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