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Date: Mon, 28 Sep 2015 12:47:14 +0200 From: Jesper Dangaard Brouer <brouer@...hat.com> To: netdev@...r.kernel.org Cc: Jesper Dangaard Brouer <brouer@...hat.com> Subject: [net-next PATCH] net: help compiler generate better code in eth_get_headlen 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> --- net/ethernet/eth.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c index d850fdc828f9..9e63f252a89e 100644 --- a/net/ethernet/eth.c +++ b/net/ethernet/eth.c @@ -127,7 +127,7 @@ u32 eth_get_headlen(void *data, unsigned int len) struct flow_keys keys; /* this should never happen, but better safe than sorry */ - if (len < sizeof(*eth)) + if (unlikely(len < sizeof(*eth))) return len; /* parse any remaining L2/L3 headers, check for L4 */ -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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