Bug#45288: pb2 returns a lot of compilation warnings

Fix assorted warnings that are generated in optimized builds.
Most of it is silencing variables that are set but unused.

This patch also introduces the MY_ASSERT_UNREACHABLE macro
which helps the compiler to deduce that a certain piece of
code is unreachable.

include/my_compiler.h:
  Use GCC's __builtin_unreachable if available. It allows
  GCC to deduce the unreachability of certain code paths,
  thus avoiding warnings that, for example, accused that a
  variable could be used without being initialized (due to
  unreachable code paths).

7 files changed, 9 insertions, 20 deletions

diff --git a/sql/lock.cc b/sql/lock.cc
index 93d8b868688..e2216876635 100644
--- a/sql/lock.cc
+++ b/sql/lock.cc
@@ -693,7 +693,6 @@ TABLE_LIST *mysql_lock_have_duplicate(THD *thd, TABLE_LIST *needle,
                                       TABLE_LIST *haystack)
 {
   MYSQL_LOCK            *mylock;
-  TABLE                 **lock_tables;
   TABLE                 *table;
   TABLE                 *table2;
   THR_LOCK_DATA         **lock_locks;
@@ -722,12 +721,11 @@ TABLE_LIST *mysql_lock_have_duplicate(THD *thd, TABLE_LIST *needle,
   if (mylock->table_count < 2)
     goto end;
 
-  lock_locks=  mylock->locks;
-  lock_tables= mylock->table;
+  lock_locks= mylock->locks;
 
   /* Prepare table related variables that don't change in loop. */
   DBUG_ASSERT((table->lock_position < mylock->table_count) &&
-              (table == lock_tables[table->lock_position]));
+              (table == mylock->table[table->lock_position]));
   table_lock_data= lock_locks + table->lock_data_start;
   end_data= table_lock_data + table->lock_count;
 
@@ -741,7 +739,7 @@ TABLE_LIST *mysql_lock_have_duplicate(THD *thd, TABLE_LIST *needle,
 
     /* All tables in list must be in lock. */
     DBUG_ASSERT((table2->lock_position < mylock->table_count) &&
-                (table2 == lock_tables[table2->lock_position]));
+                (table2 == mylock->table[table2->lock_position]));
 
     for (lock_data2=  lock_locks + table2->lock_data_start,
            end_data2= lock_data2 + table2->lock_count;
diff --git a/sql/log.cc b/sql/log.cc
index 2d5b62a5b2b..38f4677f06f 100644
--- a/sql/log.cc
+++ b/sql/log.cc
@@ -1217,7 +1217,7 @@ void LOGGER::deactivate_log_handler(THD *thd, uint log_type)
     file_log= file_log_handler->get_mysql_log();
     break;
   default:
-    assert(0);                                  // Impossible
+    MY_ASSERT_UNREACHABLE();
   }
 
   if (!(*tmp_opt))
diff --git a/sql/set_var.cc b/sql/set_var.cc
index c5517da92f8..9fbce870dc4 100644
--- a/sql/set_var.cc
+++ b/sql/set_var.cc
@@ -2559,7 +2559,7 @@ bool update_sys_var_str_path(THD *thd, sys_var_str *var_str,
     file_log= logger.get_log_file_handler();
     break;
   default:
-    assert(0);                                  // Impossible
+    MY_ASSERT_UNREACHABLE();
   }
 
   if (!old_value)
diff --git a/sql/sql_acl.cc b/sql/sql_acl.cc
index ea002f59fe3..1733a0f74b4 100644
--- a/sql/sql_acl.cc
+++ b/sql/sql_acl.cc
@@ -5479,7 +5479,7 @@ static int handle_grant_struct(uint struct_no, bool drop,
       host= grant_name->host.hostname;
       break;
     default:
-      assert(0);
+      MY_ASSERT_UNREACHABLE();
     }
     if (! user)
       user= "";
diff --git a/sql/sql_help.cc b/sql/sql_help.cc
index 2818aa5082c..c21968cb86e 100644
--- a/sql/sql_help.cc
+++ b/sql/sql_help.cc
@@ -686,7 +686,7 @@ bool mysqld_help(THD *thd, const char *mask)
 
   if (count_topics == 0)
   {
-    int key_id;
+    int UNINIT_VAR(key_id);
     if (!(select=
           prepare_select_for_name(thd,mask,mlen,tables,tables[3].table,
                                   used_fields[help_keyword_name].field,
diff --git a/sql/sql_partition.cc b/sql/sql_partition.cc
index 76caa2b0c8d..702c3d99fda 100644
--- a/sql/sql_partition.cc
+++ b/sql/sql_partition.cc
@@ -6786,8 +6786,8 @@ int get_part_iter_for_interval_via_mapping(partition_info *part_info,
     }
   }
   else
-    assert(0);
-  
+    MY_ASSERT_UNREACHABLE();
+
   can_match_multiple_values= (flags || !min_value || !max_value ||
                               memcmp(min_value, max_value, field_len));
   if (can_match_multiple_values &&
diff --git a/sql/sql_union.cc b/sql/sql_union.cc
index 948ba1d9d9c..70598fbfcd4 100644
--- a/sql/sql_union.cc
+++ b/sql/sql_union.cc
@@ -173,7 +173,6 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
   SELECT_LEX *sl, *first_sl= first_select();
   select_result *tmp_result;
   bool is_union_select;
-  TABLE *empty_table= 0;
   DBUG_ENTER("st_select_lex_unit::prepare");
 
   describe= test(additional_options & SELECT_DESCRIBE);
@@ -275,14 +274,6 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
       types= first_sl->item_list;
     else if (sl == first_sl)
     {
-      /*
-        We need to create an empty table object. It is used
-        to create tmp_table fields in Item_type_holder.
-        The main reason of this is that we can't create
-        field object without table.
-      */
-      DBUG_ASSERT(!empty_table);
-      empty_table= (TABLE*) thd->calloc(sizeof(TABLE));
       types.empty();
       List_iterator_fast<Item> it(sl->item_list);
       Item *item_tmp;