Speed and code space optimziation:

- Cache variables.lc_messages->errmsgs->errmsgs in variables.errmsgs

This gives us 15 byte less code space and 2 memory references for any access to language dependent message,
of which there are 500 in the server..

sql/mysqld.cc

@@ -4376,9 +4376,11 @@ static int init_common_variables()
     sql_print_error("Unknown locale: '%s'", lc_messages);
     return 1;
   }
-  global_system_variables.lc_messages= my_default_lc_messages;
+
   if (init_errmessage())	/* Read error messages from file */
     return 1;
+  global_system_variables.lc_messages= my_default_lc_messages;
+  global_system_variables.errmsgs= my_default_lc_messages->errmsgs->errmsgs;
   init_client_errs();
   mysql_library_init(unused,unused,unused); /* for replication */
   lex_init();

sql/sql_class.h

@@ -644,6 +644,8 @@ typedef struct system_variables
 
   /* Error messages */
   MY_LOCALE *lc_messages;
+  const char **errmsgs;             /* lc_messages->errmsg->errmsgs */
+
   /* Locale Support */
   MY_LOCALE *lc_time_names;
 

sql/sys_vars.cc

@@ -4587,11 +4587,22 @@ static bool check_locale(sys_var *self, THD *thd, set_var *var)
   return false;
 }
 
+static bool update_locale(sys_var *self, THD* thd, enum_var_type type)
+{
+  /* Cache pointer to error messages */
+  if (type == OPT_SESSION)
+    thd->variables.errmsgs= thd->variables.lc_messages->errmsgs->errmsgs;
+  else
+    global_system_variables.errmsgs=
+      global_system_variables.lc_messages->errmsgs->errmsgs;
+  return false;
+}
+  
 static Sys_var_struct Sys_lc_messages(
        "lc_messages", "Set the language used for the error messages",
        SESSION_VAR(lc_messages), NO_CMD_LINE,
        my_offsetof(MY_LOCALE, name), DEFAULT(&my_default_lc_messages),
-       NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(check_locale));
+       NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(check_locale), ON_UPDATE(update_locale));
 
 static Sys_var_struct Sys_lc_time_names(
        "lc_time_names", "Set the language used for the month "

sql/unireg.h

@@ -43,15 +43,14 @@
 #define PLUGINDIR	"lib/plugin"
 #endif
 
-#define CURRENT_THD_ERRMSGS current_thd->variables.lc_messages->errmsgs->errmsgs
+#define CURRENT_THD_ERRMSGS current_thd->variables.errmsgs
 #define DEFAULT_ERRMSGS     my_default_lc_messages->errmsgs->errmsgs
 
 #define ER(X)         CURRENT_THD_ERRMSGS[(X) - ER_ERROR_FIRST]
 #define ER_DEFAULT(X) DEFAULT_ERRMSGS[(X) - ER_ERROR_FIRST]
 #define ER_SAFE(X) (((X) >= ER_ERROR_FIRST && (X) <= ER_ERROR_LAST) ? ER(X) : "Invalid error code")
 #define ER_SAFE_THD(T,X) (((X) >= ER_ERROR_FIRST && (X) <= ER_ERROR_LAST) ? ER_THD(T,X) : "Invalid error code")
-#define ER_THD(thd,X) ((thd)->variables.lc_messages->errmsgs->errmsgs[(X) - \
-                       ER_ERROR_FIRST])
+#define ER_THD(thd,X) ((thd)->variables.errmsgs[(X) - ER_ERROR_FIRST])
 #define ER_THD_OR_DEFAULT(thd,X) ((thd) ? ER_THD(thd, X) : ER_DEFAULT(X))
 
 #define ME_INFO (ME_HOLDTANG+ME_OLDWIN+ME_NOREFRESH)