net: calculate dns header size only once per query/response

net/dns/dns_query.cc

Index: net/dns/dns_query.cc
diff --git a/net/dns/dns_query.cc b/net/dns/dns_query.cc
index aa9bc70454e7f15f0660864f357093572ba91ca9..ac74ca479615a31abd407cd12ca2572b126637b0 100644
--- a/net/dns/dns_query.cc
+++ b/net/dns/dns_query.cc
@@ -13,14 +13,19 @@
 
 namespace net {
 
+namespace {
+
+const size_t kHeaderSize = sizeof(dns_protocol::Header);
+
+}  // namespace
+
 // DNS query consists of a 12-byte header followed by a question section.
 // For details, see RFC 1035 section 4.1.1.  This header template sets RD
 // bit, which directs the name server to pursue query recursively, and sets
 // the QDCOUNT to 1, meaning the question section has a single entry.
 DnsQuery::DnsQuery(uint16_t id, const base::StringPiece& qname, uint16_t qtype)
     : qname_size_(qname.size()),
-      io_buffer_(
-          new IOBufferWithSize(sizeof(dns_protocol::Header) + question_size())),
+      io_buffer_(new IOBufferWithSize(kHeaderSize + question_size())),
       header_(reinterpret_cast<dns_protocol::Header*>(io_buffer_->data())) {
   DCHECK(!DNSDomainToString(qname).empty());
   *header_ = {};
@@ -29,8 +34,8 @@ DnsQuery::DnsQuery(uint16_t id, const base::StringPiece& qname, uint16_t qtype)
   header_->qdcount = base::HostToNet16(1);
 
   // Write question section after the header.
-  base::BigEndianWriter writer(
-      io_buffer_->data() + sizeof(dns_protocol::Header), question_size());
+  base::BigEndianWriter writer(io_buffer_->data() + kHeaderSize,
+                               question_size());
   writer.WriteBytes(qname.data(), qname.size());
   writer.WriteU16(qtype);
   writer.WriteU16(dns_protocol::kClassIN);
@@ -48,20 +53,18 @@ uint16_t DnsQuery::id() const {
 }
 
 base::StringPiece DnsQuery::qname() const {
-  return base::StringPiece(io_buffer_->data() + sizeof(dns_protocol::Header),
-                           qname_size_);
+  return base::StringPiece(io_buffer_->data() + kHeaderSize, qname_size_);
 }
 
 uint16_t DnsQuery::qtype() const {
   uint16_t type;
-  base::ReadBigEndian<uint16_t>(
-      io_buffer_->data() + sizeof(dns_protocol::Header) + qname_size_, &type);
+  base::ReadBigEndian<uint16_t>(io_buffer_->data() + kHeaderSize + qname_size_,
+                                &type);
   return type;
 }
 
 base::StringPiece DnsQuery::question() const {
-  return base::StringPiece(io_buffer_->data() + sizeof(dns_protocol::Header),
-                           question_size());
+  return base::StringPiece(io_buffer_->data() + kHeaderSize, question_size());
 }
 
 void DnsQuery::set_flags(uint16_t flags) {