Minor speedup ip_output_if when writing TTL/proto to ip header

src/core/ipv4/ip.c

@@ -553,8 +553,7 @@ err_t ip_output_if_opt(struct pbuf *p, struct ip_addr *src, struct ip_addr *dest
     LWIP_ASSERT("check that first pbuf can hold struct ip_hdr",
                (p->len >= sizeof(struct ip_hdr)));
 
-    IPH_TTL_SET(iphdr, ttl);
-    IPH_PROTO_SET(iphdr, proto);
+    IPH_TTL_PROTO_SET(iphdr, ttl, proto);
 
     ip_addr_set(&(iphdr->dest), dest);
 

src/include/ipv4/lwip/ip.h

@@ -149,6 +149,7 @@ PACK_STRUCT_END
 #define IPH_LEN_SET(hdr, len) (hdr)->_len = (len)
 #define IPH_ID_SET(hdr, id) (hdr)->_id = (id)
 #define IPH_OFFSET_SET(hdr, off) (hdr)->_offset = (off)
+#define IPH_TTL_PROTO_SET(hdr, ttl, proto) (hdr)->_ttl_proto = (htons((u8_t)(proto) | ((u16_t)(ttl) << 8)))
 #define IPH_TTL_SET(hdr, ttl) (hdr)->_ttl_proto = (htons(IPH_PROTO(hdr) | ((u16_t)(ttl) << 8)))
 #define IPH_PROTO_SET(hdr, proto) (hdr)->_ttl_proto = (htons((proto) | (IPH_TTL(hdr) << 8)))
 #define IPH_CHKSUM_SET(hdr, chksum) (hdr)->_chksum = (chksum)