A small simplification: perform two actions at once, register a

change, and perform it (the new Item changes registry).

sql/item.cc

@@ -1332,13 +1332,11 @@ bool Item_field::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
 	  return -1;
 	}
 
-	Item_ref *rf;
-	*ref= rf= new Item_ref(last->ref_pointer_array + counter,
-			       (char *)table_name,
-			       (char *)field_name);
-        thd->register_item_tree_change(ref, this, &thd->mem_root);
+	Item_ref *rf= new Item_ref(last->ref_pointer_array + counter,
+                                   (char *)table_name, (char *)field_name);
 	if (!rf)
 	  return 1;
+        thd->change_item_tree(ref, rf);
 	/*
 	  rf is Item_ref => never substitute other items (in this case)
 	  during fix_fields() => we can use rf after fix_fields()
@@ -1355,11 +1353,11 @@ bool Item_field::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
 	if (last->having_fix_field)
 	{
 	  Item_ref *rf;
-          thd->register_item_tree_change(ref, *ref, &thd->mem_root);
-          *ref= rf= new Item_ref((where->db[0] ? where->db : 0),
+          rf= new Item_ref((where->db[0] ? where->db : 0),
                            (char*) where->alias, (char*) field_name);
 	  if (!rf)
 	    return 1;
+          thd->change_item_tree(ref, rf);
 	  /*
 	    rf is Item_ref => never substitute other items (in this case)
 	    during fix_fields() => we can use rf after fix_fields()
@@ -1992,10 +1990,10 @@ bool Item_ref::fix_fields(THD *thd,TABLE_LIST *tables, Item **reference)
       else if (tmp != not_found_field)
       {
 	ref= 0; // To prevent "delete *ref;" on ~Item_ref() of this item
-	Item_field* fld;
-	if (!((*reference)= fld= new Item_field(tmp)))
+	Item_field* fld= new Item_field(tmp);
+	if (!fld)
 	  return 1;
-	thd->register_item_tree_change(reference, this, &thd->mem_root);
+	thd->change_item_tree(reference, fld);
 	mark_as_dependent(thd, last, thd->lex->current_select, fld);
 	return 0;
       }
@@ -2250,10 +2248,7 @@ void resolve_const_item(THD *thd, Item **ref, Item *comp_item)
                new Item_real(name, result, decimals, length));
   }
   if (new_item)
-  {
-    thd->register_item_tree_change(ref, item, &thd->mem_root);
-    *ref= new_item;
-  }
+    thd->change_item_tree(ref, new_item);
 }
 
 /*

sql/item_cmpfunc.cc

@@ -153,10 +153,7 @@ static bool convert_constant_item(THD *thd, Field *field, Item **item)
     {
       Item *tmp=new Item_int_with_ref(field->val_int(), *item);
       if (tmp)
-      {
-        thd->register_item_tree_change(item, *item, &thd->mem_root);
-	*item=tmp;
-      }
+        thd->change_item_tree(item, tmp);
       return 1;					// Item was replaced
     }
   }
@@ -2033,10 +2030,10 @@ void Item_cond::split_sum_func(THD *thd, Item **ref_pointer_array,
     {
       Item **ref= li.ref();
       uint el= fields.elements;
+      Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name);
       fields.push_front(item);
       ref_pointer_array[el]= item;
-      thd->register_item_tree_change(ref, *ref, &thd->mem_root);
-      li.replace(new Item_ref(ref_pointer_array + el, 0, item->name));
+      thd->change_item_tree(ref, new_item);
     }
     item->update_used_tables();
     used_tables_cache|=item->used_tables();

sql/item_func.cc

@@ -269,10 +269,10 @@ void Item_func::split_sum_func(THD *thd, Item **ref_pointer_array,
     else if (item->used_tables() || item->type() == SUM_FUNC_ITEM)
     {
       uint el= fields.elements;
+      Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name);
       fields.push_front(item);
       ref_pointer_array[el]= item;
-      thd->register_item_tree_change(arg, *arg, &thd->mem_root);
-      *arg= new Item_ref(ref_pointer_array + el, 0, item->name);
+      thd->change_item_tree(arg, new_item);
     }
   }
 }

sql/item_row.cc

@@ -95,10 +95,10 @@ void Item_row::split_sum_func(THD *thd, Item **ref_pointer_array,
     else if ((*arg)->used_tables() || (*arg)->type() == SUM_FUNC_ITEM)
     {
       uint el= fields.elements;
+      Item *new_item= new Item_ref(ref_pointer_array + el, 0, (*arg)->name);
       fields.push_front(*arg);
       ref_pointer_array[el]= *arg;
-      thd->register_item_tree_change(arg, *arg, &thd->mem_root);
-      *arg= new Item_ref(ref_pointer_array + el, 0, (*arg)->name);
+      thd->change_item_tree(arg, new_item);
     }
   }
 }

sql/item_strfunc.cc

@@ -643,10 +643,10 @@ void Item_func_concat_ws::split_sum_func(THD *thd, Item **ref_pointer_array,
   else if (separator->used_tables() || separator->type() == SUM_FUNC_ITEM)
   {
     uint el= fields.elements;
+    Item *new_item= new Item_ref(ref_pointer_array + el, 0, separator->name);
     fields.push_front(separator);
     ref_pointer_array[el]= separator;
-    thd->register_item_tree_change(&separator, separator, &thd->mem_root);
-    separator= new Item_ref(ref_pointer_array + el, 0, separator->name);
+    thd->change_item_tree(&separator, new_item);
   }
   Item_str_func::split_sum_func(thd, ref_pointer_array, fields);
 }
@@ -1779,10 +1779,10 @@ void Item_func_make_set::split_sum_func(THD *thd, Item **ref_pointer_array,
   else if (item->used_tables() || item->type() == SUM_FUNC_ITEM)
   {
     uint el= fields.elements;
+    Item *new_item= new Item_ref(ref_pointer_array + el, 0, item->name);
     fields.push_front(item);
     ref_pointer_array[el]= item;
-    thd->register_item_tree_change(&item, item, &thd->mem_root);
-    item= new Item_ref(ref_pointer_array + el, 0, item->name);
+    thd->change_item_tree(&item, new_item);
   }
   Item_str_func::split_sum_func(thd, ref_pointer_array, fields);
 }

sql/sql_class.h

@@ -1054,11 +1054,12 @@ public:
   inline CHARSET_INFO *charset() { return variables.character_set_client; }
   void update_charset();
 
-  void register_item_tree_change(Item **place, Item *old_value,
-                                 MEM_ROOT *runtime_memroot)
+  void change_item_tree(Item **place, Item *new_value)
   {
+    /* TODO: check for OOM condition here */
     if (!current_arena->is_conventional_execution())
-      nocheck_register_item_tree_change(place, old_value, runtime_memroot);
+      nocheck_register_item_tree_change(place, *place, &mem_root);
+    *place= new_value;
   }
   void nocheck_register_item_tree_change(Item **place, Item *old_value,
                                          MEM_ROOT *runtime_memroot);

sql/sql_select.cc

@@ -4203,8 +4203,7 @@ change_cond_ref_to_const(THD *thd, I_List<COND_CMP> *save_list,
     Item *tmp=value->new_item();
     if (tmp)
     {
-      thd->register_item_tree_change(args + 1, args[1], &thd->mem_root);
-      args[1]= tmp;
+      thd->change_item_tree(args + 1, tmp);
       func->update_used_tables();
       if ((functype == Item_func::EQ_FUNC || functype == Item_func::EQUAL_FUNC)
 	  && and_father != cond && !left_item->const_item())
@@ -4225,15 +4224,14 @@ change_cond_ref_to_const(THD *thd, I_List<COND_CMP> *save_list,
     Item *tmp=value->new_item();
     if (tmp)
     {
-      thd->register_item_tree_change(args, args[0], &thd->mem_root);
-      args[0]= value= tmp;
+      thd->change_item_tree(args, tmp);
+      value= tmp;
       func->update_used_tables();
       if ((functype == Item_func::EQ_FUNC || functype == Item_func::EQUAL_FUNC)
 	  && and_father != cond && !right_item->const_item())
       {
         args[0]= args[1];                       // For easy check
-        thd->register_item_tree_change(args + 1, args[1], &thd->mem_root);
-        args[1]= value;
+        thd->change_item_tree(args + 1, value);
 	cond->marker=1;
 	COND_CMP *tmp2;
 	if ((tmp2=new COND_CMP(and_father,func)))
@@ -4959,8 +4957,7 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
 	    *blob_field++= new_field;
 	    blob_count++;
 	  }
-          thd->register_item_tree_change(argp, arg, &thd->mem_root);
-	  *argp= new Item_field(new_field);
+          thd->change_item_tree(argp, new Item_field(new_field));
 	  if (!(new_field->flags & NOT_NULL_FLAG))
           {
 	    null_count++;