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/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index c4efaa97280c..79e33e02f11a 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1369,9 +1369,9 @@ static int tcp_v6_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;
@@ -1379,7 +1379,7 @@ static int tcp_v6_rcv(struct sk_buff *skb)
         if (skb_checksum_init(skb, IPPROTO_TCP, ip6_compute_pseudo))
                 goto csum_error;
 
-        th = tcp_hdr(skb);
+        th = (const struct tcphdr *)skb->data;
         hdr = ipv6_hdr(skb);
 
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