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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@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Jesper Dangaard Brouer 2015-09-28 12:47:14 +02:00 committed by David S. Miller
parent d2e1339f40
commit 8a4683a5e0
1 changed files with 1 additions and 1 deletions
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2
net/ethernet/eth.c
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@ -127,7 +127,7 @@ u32 eth_get_headlen(void *data, unsigned int len)
struct flow_keys keys; struct flow_keys keys;
/* this should never happen, but better safe than sorry */ /* this should never happen, but better safe than sorry */
if (len < sizeof(*eth)) if (unlikely(len < sizeof(*eth)))
return len; return len;
/* parse any remaining L2/L3 headers, check for L4 */ /* parse any remaining L2/L3 headers, check for L4 */