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

On 3/15/2017 1:50 AM, David Laight wrote:

From: Shannon Nelson

quoted

Sent: 14 March 2017 17:25

...

quoted

+	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