From c186591e312fb90c4c2d847be01255fb7dba0c8b Mon Sep 17 00:00:00 2001
From: Max Rottenkolber <max@mr.gy>
Date: Mon, 25 Apr 2016 16:25:06 +0200
Subject: [PATCH] lib.protocol.ethernet: Add n_mcast, branch-free Multicast
 predicate.

---
 src/apps/vhost/vhost_user.lua | 16 ++++++----------
 src/lib/protocol/README.md    |  9 +++++++++
 src/lib/protocol/ethernet.lua |  7 ++++++-
 3 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/src/apps/vhost/vhost_user.lua b/src/apps/vhost/vhost_user.lua
index 96e82780eb..16936709bf 100644
--- a/src/apps/vhost/vhost_user.lua
+++ b/src/apps/vhost/vhost_user.lua
@@ -102,20 +102,16 @@ end
 
 function VhostUser:tx_callback (p)
    counter.add(self.counters['out-octets'], packet.length(p))
-   if ethernet:is_mcast(packet.data(p)) then
-      counter.add(self.counters['out-multicast'])
-   else
-      counter.add(self.counters['out-unicast'])
-   end
+   local mcast = ethernet:n_mcast(packet.data(p))
+   counter.add(self.counters['out-multicast'], mcast)
+   counter.add(self.counters['out-unicast'], 1 - mcast)
 end
 
 function VhostUser:rx_callback (p)
    counter.add(self.counters['in-octets'], packet.length(p))
-   if ethernet:is_mcast(packet.data(p)) then
-      counter.add(self.counters['in-multicast'])
-   else
-      counter.add(self.counters['in-unicast'])
-   end
+   local mcast = ethernet:n_mcast(packet.data(p))
+   counter.add(self.counters['in-multicast'], mcast)
+   counter.add(self.counters['in-unicast'], 1 - mcast)
 end
 
 function VhostUser:rxdrop_callback (p)
diff --git a/src/lib/protocol/README.md b/src/lib/protocol/README.md
index 6f239687d7..4a13483a4b 100644
--- a/src/lib/protocol/README.md
+++ b/src/lib/protocol/README.md
@@ -98,6 +98,15 @@ Returns the binary representation of MAC address denoted by *string*.
 
 Returns the string representation of *mac* address.
 
+— Function **ethernet:is_mcast** *mac*
+
+Returns a true value if *mac* address denotes a [Multicast address](https://en.wikipedia.org/wiki/Multicast_address#Ethernet).
+
+— Function **ethernet:n_mcast** *mac*
+
+Returns 1 if *mac* address denotes a [Multicast address](https://en.wikipedia.org/wiki/Multicast_address#Ethernet)
+and 0 otherwise.
+
 — Function **ethernet:ipv6_mcast** *ip*
 
 Returns the MAC address for IPv6 multicast *ip* as defined by RFC2464,
diff --git a/src/lib/protocol/ethernet.lua b/src/lib/protocol/ethernet.lua
index 0a55c20f1b..b5abed8d31 100644
--- a/src/lib/protocol/ethernet.lua
+++ b/src/lib/protocol/ethernet.lua
@@ -76,9 +76,14 @@ function ethernet:ipv6_mcast(ip)
    return result
 end
 
+-- Return 1 if MAC address has its group bit set and 0 otherwise
+function ethernet:n_mcast (addr)
+   return band(addr[0], 0x01)
+end
+
 -- Check whether a MAC address has its group bit set
 function ethernet:is_mcast (addr)
-   return band(addr[0], 0x01) ~= 0
+   return ethernet:n_mcast(addr) ~= 0
 end
 
 -- Instance methods