linux-kernel - Re: [PATCH v2 net-next 4/5] sunvnet: count multicast packets

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Date:   Wed, 15 Mar 2017 17:17:44 -0700
From:   Shannon Nelson <shannon.nelson@...cle.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Cc:     "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 net-next 4/5] sunvnet: count multicast packets

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