Bug#42733: Type-punning warnings when compiling MySQL --

            strict aliasing violations.

One somewhat major source of strict-aliasing violations and
related warnings is the SQL_LIST structure. For example,
consider its member function `link_in_list` which takes
a pointer to pointer of type T (any type) as a pointer to
pointer to unsigned char. Dereferencing this pointer, which
is done to reset the next field, violates strict-aliasing
rules and might cause problems for surrounding code that
uses the next field of the object being added to the list.

The solution is to use templates to parametrize the SQL_LIST
structure in order to deference the pointers with compatible
types. As a side bonus, it becomes possible to remove quite
a few casts related to acessing data members of SQL_LIST.

1 files changed, 18 insertions, 20 deletions

diff --git a/sql/sql_union.cc b/sql/sql_union.cc
index 1760670f9c8..948ba1d9d9c 100644
--- a/sql/sql_union.cc
+++ b/sql/sql_union.cc
@@ -144,20 +144,19 @@ void
 st_select_lex_unit::init_prepare_fake_select_lex(THD *thd_arg) 
 {
   thd_arg->lex->current_select= fake_select_lex;
-  fake_select_lex->table_list.link_in_list((uchar *)&result_table_list,
-					   (uchar **)
-					   &result_table_list.next_local);
+  fake_select_lex->table_list.link_in_list(&result_table_list,
+                                           &result_table_list.next_local);
   fake_select_lex->context.table_list= 
     fake_select_lex->context.first_name_resolution_table= 
     fake_select_lex->get_table_list();
   if (!fake_select_lex->first_execution)
   {
-    for (ORDER *order= (ORDER *) global_parameters->order_list.first;
+    for (ORDER *order= global_parameters->order_list.first;
          order;
          order= order->next)
       order->item= &order->item_ptr;
   }
-  for (ORDER *order= (ORDER *)global_parameters->order_list.first;
+  for (ORDER *order= global_parameters->order_list.first;
        order;
        order=order->next)
   {
@@ -249,18 +248,18 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
     can_skip_order_by= is_union_select && !(sl->braces && sl->explicit_limit);
 
     saved_error= join->prepare(&sl->ref_pointer_array,
-                               (TABLE_LIST*) sl->table_list.first,
+                               sl->table_list.first,
                                sl->with_wild,
                                sl->where,
                                (can_skip_order_by ? 0 :
                                 sl->order_list.elements) +
                                sl->group_list.elements,
                                can_skip_order_by ?
-                               (ORDER*) 0 : (ORDER *)sl->order_list.first,
-                               (ORDER*) sl->group_list.first,
+                               NULL : sl->order_list.first,
+                               sl->group_list.first,
                                sl->having,
-                               (is_union_select ? (ORDER*) 0 :
-                                (ORDER*) thd_arg->lex->proc_list.first),
+                               (is_union_select ? NULL :
+                                thd_arg->lex->proc_list.first),
                                sl, this);
     /* There are no * in the statement anymore (for PS) */
     sl->with_wild= 0;
@@ -354,7 +353,7 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
     {
       ORDER *ord;
       Item_func::Functype ft=  Item_func::FT_FUNC;
-      for (ord= (ORDER*)global_parameters->order_list.first; ord; ord= ord->next)
+      for (ord= global_parameters->order_list.first; ord; ord= ord->next)
         if ((*ord->item)->walk (&Item::find_function_processor, FALSE, 
                                 (uchar *) &ft))
         {
@@ -416,12 +415,11 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
 	thd_arg->lex->current_select= fake_select_lex;
 	saved_error= fake_select_lex->join->
 	  prepare(&fake_select_lex->ref_pointer_array,
-		  (TABLE_LIST*) fake_select_lex->table_list.first,
+		  fake_select_lex->table_list.first,
 		  0, 0,
 		  fake_select_lex->order_list.elements,
-		  (ORDER*) fake_select_lex->order_list.first,
-		  (ORDER*) NULL, NULL,
-                  (ORDER*) NULL,
+		  fake_select_lex->order_list.first,
+		  NULL, NULL, NULL,
 		  fake_select_lex, this);
 	fake_select_lex->table_list.empty();
       }
@@ -597,8 +595,8 @@ bool st_select_lex_unit::exec()
                               &result_table_list,
                               0, item_list, NULL,
                               global_parameters->order_list.elements,
-                              (ORDER*)global_parameters->order_list.first,
-                              (ORDER*) NULL, NULL, (ORDER*) NULL,
+                              global_parameters->order_list.first,
+                              NULL, NULL, NULL,
                               fake_select_lex->options | SELECT_NO_UNLOCK,
                               result, this, fake_select_lex);
       }
@@ -620,8 +618,8 @@ bool st_select_lex_unit::exec()
                                 &result_table_list,
                                 0, item_list, NULL,
                                 global_parameters->order_list.elements,
-                                (ORDER*)global_parameters->order_list.first,
-                                (ORDER*) NULL, NULL, (ORDER*) NULL,
+                                global_parameters->order_list.first,
+                                NULL, NULL, NULL,
                                 fake_select_lex->options | SELECT_NO_UNLOCK,
                                 result, this, fake_select_lex);
         }
@@ -697,7 +695,7 @@ bool st_select_lex_unit::cleanup()
     if (global_parameters->order_list.elements)
     {
       ORDER *ord;
-      for (ord= (ORDER*)global_parameters->order_list.first; ord; ord= ord->next)
+      for (ord= global_parameters->order_list.first; ord; ord= ord->next)
         (*ord->item)->walk (&Item::cleanup_processor, 0, 0);
     }
   }