Reduce the size of Packet structure

Share memory space for IPV4Vars and (IPV6Vars, IPV6ExtHdrs), since a
packet can only be either IPv4 or IPv6, but not both.

Share memory for TCPVars, UDPVars, ICMPV4Vars and ICMPV6Vars, since a
packet can only be only of these.

Then move other structure members around to remove holes reported by pahole.

This reduces the size of the Packet structure from 2944 bytes (46 cachelines)
down to 1976 (31 cachelines), a 33% reduction.

src/decode.h

@@ -375,14 +375,12 @@ typedef struct Packet_
     uint16_t vlan_id[2];
     uint8_t vlan_idx;
 
-    /* Pkt Flags */
-    uint32_t flags;
-
     /* flow */
     uint8_t flowflags;
     /* coccinelle: Packet:flowflags:FLOW_PKT_ */
 
-    uint8_t pkt_src;
+    /* Pkt Flags */
+    uint32_t flags;
 
     struct Flow_ *flow;
 
@@ -408,16 +406,6 @@ typedef struct Packet_
         PcapPacketVars pcap_v;
     };
 
-    /** data linktype in host order */
-    int datalink;
-
-    /* IPS action to take */
-    uint8_t action;
-
-    /* used to hold flowbits only if debuglog is enabled */
-    int debuglog_flowbits_names_len;
-    const char **debuglog_flowbits_names;
-
     /** The release function for packet structure and data */
     void (*ReleasePacket)(struct Packet_ *);
 
@@ -433,25 +421,34 @@ typedef struct Packet_
     int32_t level4_comp_csum;
 
     IPV4Hdr *ip4h;
-    IPV4Vars ip4vars;
 
     IPV6Hdr *ip6h;
-    IPV6Vars ip6vars;
-    IPV6ExtHdrs ip6eh;
+
+    /* IPv4 and IPv6 are mutually exclusive */
+    union {
+        IPV4Vars ip4vars;
+        struct {
+            IPV6Vars ip6vars;
+            IPV6ExtHdrs ip6eh;
+        };
+    };
+    /* Can only be one of TCP, UDP, ICMP at any given time */
+    union {
+        TCPVars tcpvars;
+        UDPVars udpvars;
+        ICMPV4Vars icmpv4vars;
+        ICMPV6Vars icmpv6vars;
+    };
 
     TCPHdr *tcph;
-    TCPVars tcpvars;
 
     UDPHdr *udph;
-    UDPVars udpvars;
 
     SCTPHdr *sctph;
 
     ICMPV4Hdr *icmpv4h;
-    ICMPV4Vars icmpv4vars;
 
     ICMPV6Hdr *icmpv6h;
-    ICMPV6Vars icmpv6vars;
 
     PPPHdr *ppph;
     PPPOESessionHdr *pppoesh;
@@ -466,10 +463,15 @@ typedef struct Packet_
     uint8_t *payload;
     uint16_t payload_len;
 
+    /* IPS action to take */
+    uint8_t action;
+
+    uint8_t pkt_src;
+
     /* storage: set to pointer to heap and extended via allocation if necessary */
+    uint32_t pktlen;
     uint8_t *pkt;
     uint8_t *ext_pkt;
-    uint32_t pktlen;
 
     /* Incoming interface */
     struct LiveDevice_ *livedev;
@@ -482,15 +484,6 @@ typedef struct Packet_
     /** packet number in the pcap file, matches wireshark */
     uint64_t pcap_cnt;
 
-    /** mutex to protect access to:
-     *  - tunnel_rtv_cnt
-     *  - tunnel_tpr_cnt
-     */
-    SCMutex tunnel_mutex;
-    /* ready to set verdict counter, only set in root */
-    uint16_t tunnel_rtv_cnt;
-    /* tunnel packet ref count */
-    uint16_t tunnel_tpr_cnt;
 
     /* engine events */
     PacketEngineEvents events;
@@ -501,6 +494,13 @@ typedef struct Packet_
     struct Packet_ *next;
     struct Packet_ *prev;
 
+    /** data linktype in host order */
+    int datalink;
+
+    /* used to hold flowbits only if debuglog is enabled */
+    int debuglog_flowbits_names_len;
+    const char **debuglog_flowbits_names;
+
     /* tunnel/encapsulation handling */
     struct Packet_ *root; /* in case of tunnel this is a ptr
                            * to the 'real' packet, the one we
@@ -508,6 +508,17 @@ typedef struct Packet_
                            * It should always point to the lowest
                            * packet in a encapsulated packet */
 
+    /** mutex to protect access to:
+     *  - tunnel_rtv_cnt
+     *  - tunnel_tpr_cnt
+     */
+    SCMutex tunnel_mutex;
+    /* ready to set verdict counter, only set in root */
+    uint16_t tunnel_rtv_cnt;
+    /* tunnel packet ref count */
+    uint16_t tunnel_tpr_cnt;
+
+
 #ifdef PROFILING
     PktProfiling profile;
 #endif