lib.protocol.ethernet: Add n_mcast, branch-free Multicast predicate.

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)

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,

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