resolved: rearrange DnsResourceRecord for better layout

The structure is rearranged to have less holes. Also fields in the union
are rearranged not to have holes (though most variants of the union still
have some padding at the end).

The full size does not decrease a lot, but the compiler should be able to
copy less bytes when it knows the specific type of the union.

Bitfields are dropped because they don't give any memory savings due to
alignment requirements (but would still require more complex to access).

The change from the this and previous commit:
/* size: 128, cachelines: 2, members: 13 */
/* sum members: 112, holes: 3, sum holes: 15 */
/* sum bitfield members: 2 bits, bit holes: 1, sum bit holes: 6 bits */
     ↓
/* size: 112, cachelines: 2, members: 13 */
/* sum members: 108, holes: 1, sum holes: 4 */

1 files changed, 32 insertions, 23 deletions

diff --git a/src/resolve/resolved-dns-rr.h b/src/resolve/resolved-dns-rr.h
index 10c05dd9c3..66aa10b82c 100644
--- a/src/resolve/resolved-dns-rr.h
+++ b/src/resolve/resolved-dns-rr.h
@@ -91,21 +91,21 @@ struct DnsTxtItem {
 
 struct DnsResourceRecord {
         unsigned n_ref;
+        uint32_t ttl;
+        usec_t expiry; /* RRSIG signature expiry */
+
         DnsResourceKey *key;
 
         char *to_string;
 
-        uint32_t ttl;
-        usec_t expiry; /* RRSIG signature expiry */
-
         /* How many labels to strip to determine "signer" of the RRSIG (aka, the zone). -1 if not signed. */
         uint8_t n_skip_labels_signer;
         /* How many labels to strip to determine "synthesizing source" of this RR, i.e. the wildcard's immediate parent. -1 if not signed. */
         uint8_t n_skip_labels_source;
 
-        bool unparsable:1;
+        bool unparsable;
+        bool wire_format_canonical;
 
-        bool wire_format_canonical:1;
         void *wire_format;
         size_t wire_format_size;
         size_t wire_format_rdata_offset;
@@ -117,10 +117,10 @@ struct DnsResourceRecord {
                 } generic, opt;
 
                 struct {
+                        char *name;
                         uint16_t priority;
                         uint16_t weight;
                         uint16_t port;
-                        char *name;
                 } srv;
 
                 struct {
@@ -155,8 +155,8 @@ struct DnsResourceRecord {
                 } soa;
 
                 struct {
-                        uint16_t priority;
                         char *exchange;
+                        uint16_t priority;
                 } mx;
 
                 /* https://tools.ietf.org/html/rfc1876 */
@@ -172,23 +172,29 @@ struct DnsResourceRecord {
 
                 /* https://tools.ietf.org/html/rfc4255#section-3.1 */
                 struct {
-                        uint8_t algorithm;
-                        uint8_t fptype;
                         void *fingerprint;
                         size_t fingerprint_size;
+
+                        uint8_t algorithm;
+                        uint8_t fptype;
                 } sshfp;
 
                 /* http://tools.ietf.org/html/rfc4034#section-2.1 */
                 struct {
+                        void* key;
+                        size_t key_size;
+
                         uint16_t flags;
                         uint8_t protocol;
                         uint8_t algorithm;
-                        void* key;
-                        size_t key_size;
                 } dnskey;
 
                 /* http://tools.ietf.org/html/rfc4034#section-3.1 */
                 struct {
+                        char *signer;
+                        void *signature;
+                        size_t signature_size;
+
                         uint16_t type_covered;
                         uint8_t algorithm;
                         uint8_t labels;
@@ -196,9 +202,6 @@ struct DnsResourceRecord {
                         uint32_t expiration;
                         uint32_t inception;
                         uint16_t key_tag;
-                        char *signer;
-                        void *signature;
-                        size_t signature_size;
                 } rrsig;
 
                 /* https://tools.ietf.org/html/rfc4034#section-4.1 */
@@ -209,41 +212,47 @@ struct DnsResourceRecord {
 
                 /* https://tools.ietf.org/html/rfc4034#section-5.1 */
                 struct {
+                        void *digest;
+                        size_t digest_size;
+
                         uint16_t key_tag;
                         uint8_t algorithm;
                         uint8_t digest_type;
-                        void *digest;
-                        size_t digest_size;
                 } ds;
 
                 struct {
-                        uint8_t algorithm;
-                        uint8_t flags;
-                        uint16_t iterations;
+                        Bitmap *types;
                         void *salt;
                         size_t salt_size;
                         void *next_hashed_name;
                         size_t next_hashed_name_size;
-                        Bitmap *types;
+
+                        uint8_t algorithm;
+                        uint8_t flags;
+                        uint16_t iterations;
                 } nsec3;
 
                 /* https://tools.ietf.org/html/draft-ietf-dane-protocol-23 */
                 struct {
+                        void *data;
+                        size_t data_size;
+
                         uint8_t cert_usage;
                         uint8_t selector;
                         uint8_t matching_type;
-                        void *data;
-                        size_t data_size;
                 } tlsa;
 
                 /* https://tools.ietf.org/html/rfc6844 */
                 struct {
-                        uint8_t flags;
                         char *tag;
                         void *value;
                         size_t value_size;
+
+                        uint8_t flags;
                 } caa;
         };
+
+        /* Note: fields should be ordered to minimize alignment gaps. Use pahole! */
 };
 
 /* We use uint8_t for label counts above, and UINT8_MAX/-1 has special meaning. */