lkml.org 
[lkml]   [2017]   [Mar]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v2 net-next 4/5] sunvnet: count multicast packets
From
Date
On 3/15/2017 1:50 AM, David Laight wrote:
> From: Shannon Nelson
>> Sent: 14 March 2017 17:25
> ...
>> + if (unlikely(is_multicast_ether_addr(eth_hdr(skb)->h_dest)))
>> + dev->stats.multicast++;
>
> I'd guess that:
> dev->stats.multicast += is_multicast_ether_addr(eth_hdr(skb)->h_dest);
> generates faster code.
> Especially if is_multicast_ether_addr(x) is (*x >> 7).
>
> David

Hi David, thanks for the comment. My local instruction level
performance guru is on vacation this week so I can't do a quick check
with him today on this. However, I"m not too worried here since the
inline code for is_multicast_ether_addr() is simply

return 0x01 & addr[0];

and objdump tells me that on sparc it compiles down to a simple single
byte load and compare:

325c: c2 08 80 03 ldub [ %g2 + %g3 ], %g1
3260: 80 88 60 01 btst 1, %g1
3264: 32 60 00 b3 bne,a,pn %xcc, 3530 <vnet_rx_one+0x430>
3268: c2 5c 61 68 ldx [ %l1 + 0x168 ], %g1
dev->stats.multicast++;

I don't think this driver will ever be used on anything bug sparc, so
I'm not worried about how x86 might compile this.

sln

\
 
 \ /
  Last update: 2017-03-16 01:20    [W:0.104 / U:0.024 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site