[mysql-5.6][PR] m_auto_incr_map: reduce a redundant search operation

Summary:
This PR reduced redundant searchs on `m_auto_incr_map`.

Pull Request resolved: facebook#1259
GitHub Author: leipeng <peng@topling.cn>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Reviewers: luqun, rpan, mung, chni

Reviewed By: mung

Subscribers: webscalesql-eng@fb.com

Differential Revision: https://phabricator.intern.facebook.com/D42928166

Tags: accept2ship

Author: hermanlee

storage/rocksdb/ha_rocksdb.cc

@@ -4249,14 +4249,15 @@ class Rdb_transaction {
   }
 
   void set_auto_incr(const GL_INDEX_ID &gl_index_id, ulonglong curr_id) {
-    m_auto_incr_map[gl_index_id] =
-        std::max(m_auto_incr_map[gl_index_id], curr_id);
+    auto &existing = m_auto_incr_map[gl_index_id];
+    existing = std::max(existing, curr_id);
   }
 
 #ifndef NDEBUG
   ulonglong get_auto_incr(const GL_INDEX_ID &gl_index_id) {
-    if (m_auto_incr_map.count(gl_index_id) > 0) {
-      return m_auto_incr_map[gl_index_id];
+    auto iter = m_auto_incr_map.find(gl_index_id);
+    if (m_auto_incr_map.end() != iter) {
+      return iter->second;
     }
     return 0;
   }