From: David Miller <davem@davemloft.net>
To: brouer@redhat.com
Cc: netdev@vger.kernel.org
Subject: Re: [net-next PATCH] net: help compiler generate better code in eth_get_headlen
Date: Mon, 28 Sep 2015 22:51:38 -0700 (PDT) [thread overview]
Message-ID: <20150928.225138.363029814550861853.davem@davemloft.net> (raw)
In-Reply-To: <20150928104713.12016.65491.stgit@canyon>
From: Jesper Dangaard Brouer <brouer@redhat.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@redhat.com>
Applied, thanks Jesper.
prev parent reply other threads:[~2015-09-29 5:51 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-28 10:47 [net-next PATCH] net: help compiler generate better code in eth_get_headlen Jesper Dangaard Brouer
2015-09-29 5:51 ` David Miller [this message]
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