tcp: minor optimizations around tcp_hdr() usage

tcp_hdr() is slightly more expensive than using skb->data in contexts
where we know they point to the same byte.

In receive path, tcp_v4_rcv() and tcp_v6_rcv() are in this situation,
as tcp header has not been pulled yet.

In output path, the same can be said when we just pushed the tcp header
in the skb, in tcp_transmit_skb() and tcp_make_synack()

Also factorize the two checks for tcb->tcp_flags & TCPHDR_SYN in
tcp_transmit_skb() and pass tcp header pointer to tcp_ecn_send(),
so that compiler can further optimize and avoid a reload.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 3 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 8219d0d8dc83..3708de2a6683 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1556,9 +1556,9 @@ int tcp_v4_rcv(struct sk_buff *skb)
         if (!pskb_may_pull(skb, sizeof(struct tcphdr)))
                 goto discard_it;
 
-        th = tcp_hdr(skb);
+        th = (const struct tcphdr *)skb->data;
 
-        if (th->doff < sizeof(struct tcphdr) / 4)
+        if (unlikely(th->doff < sizeof(struct tcphdr) / 4))
                 goto bad_packet;
         if (!pskb_may_pull(skb, th->doff * 4))
                 goto discard_it;
@@ -1571,7 +1571,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
         if (skb_checksum_init(skb, IPPROTO_TCP, inet_compute_pseudo))
                 goto csum_error;
 
-        th = tcp_hdr(skb);
+        th = (const struct tcphdr *)skb->data;
         iph = ip_hdr(skb);
         /* This is tricky : We move IPCB at its correct location into TCP_SKB_CB()
          * barrier() makes sure compiler wont play fool^Waliasing games.