[lng-odp] [PATCH v2 2/5] example: l3fwd: optimize ip checksum update

[Matias Elo]

When decrementing TTL the IPv4 checksum can be incrementally updated.

Signed-off-by: Matias Elo <matias....@nokia.com>
---
 example/l3fwd/odp_l3fwd.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/example/l3fwd/odp_l3fwd.c b/example/l3fwd/odp_l3fwd.c
index 44778b0..95c3d85 100644
--- a/example/l3fwd/odp_l3fwd.c
+++ b/example/l3fwd/odp_l3fwd.c
@@ -158,6 +158,22 @@ static void setup_fwd_db(void)
        }
 }
 
+/**
+ * Decrement TTL and incrementally update checksum
+ *
+ * @param ip  IPv4 header
+ */
+static inline void ipv4_dec_ttl_csum_update(odph_ipv4hdr_t *ip)
+{
+       uint16_t a = ~odp_cpu_to_be_16(1 << 8);
+
+       ip->ttl--;
+       if (ip->chksum >= a)
+               ip->chksum -= a;
+       else
+               ip->chksum += odp_cpu_to_be_16(1 << 8);
+}
+
 static int l3fwd_pkt_hash(odp_packet_t pkt, int sif)
 {
        fwd_db_entry_t *entry;
@@ -186,8 +202,7 @@ static int l3fwd_pkt_hash(odp_packet_t pkt, int sif)
                key.dst_port = 0;
        }
        entry = find_fwd_db_entry(&key);
-       ip->ttl--;
-       ip->chksum = odph_ipv4_csum_update(pkt);
+       ipv4_dec_ttl_csum_update(ip);
        eth = odp_packet_l2_ptr(pkt, NULL);
        if (entry) {
                eth->src = entry->src_mac;
@@ -211,8 +226,7 @@ static int l3fwd_pkt_lpm(odp_packet_t pkt, int sif)
        int ret;
 
        ip = odp_packet_l3_ptr(pkt, &len);
-       ip->ttl--;
-       ip->chksum = odph_ipv4_csum_update(pkt);
+       ipv4_dec_ttl_csum_update(ip);
        eth = odp_packet_l2_ptr(pkt, NULL);
 
        /* network byte order maybe different from host */
-- 
2.7.4