Implementation of Monty's idea about clear_alloc_root() optimization and cleanup of work

with memory roots in THD/Statement/Item_arena.
Added assertions preventing memory allocation on bzero'ed MEM_ROOT since it is worked by 
pure luck and was very ineffective.


include/my_sys.h:
  Reimplementation of Monty's optimization of clear_alloc_root().
  Now clear_alloc_root() can be used only for detaching memory associated with MEM_ROOT
  (e.g. to avoid its freeing). It can not be used for MEM_ROOT initialization any longer 
  (it was bad idea anyway since memory allocation on such MEM_ROOT was very ineffective 
  and worked by pure luck).
  Introduced ALLOC_ROOT_MIN_BLOCK_SIZE constant.
mysys/my_alloc.c:
  Added description of init_alloc_root().
  Added assertions to alloc_root() and reset_root_defaults() so now they can only be used
  on previosly initialized MEM_ROOT. (It worked for bzeroed MEM_ROOT before but 
  by pure luck and very inefficiently). Calling free_root() on bzero'ed MEM_ROOT
  is still ok (we can't remove this easily because of static MEM_ROOTs).
  Also now using ALLOC_ROOT_MIN_BLOCK_SIZE constant inside these functions.
sql/opt_range.cc:
  Fixed get_quick_select_for_ref() function to not use bzero'ed MEM_ROOT for allocation.
  Also QUICK_RANGEs created in this function should be created in memory root of QUICK_SELECT.
sql/sql_class.cc:
  Implementation of Monty's idea about clear_alloc_root() optimization and cleanup of work 
  with memory roots in THD/Statement/Item_arena.
  Now we are always initing THD::transaction.mem_root and THD::mem_root in THD constructor 
  (without memory allocation and with minimal block size) and then later change their
  parameters in THD::init_for_queries() (this is partially because we can't allocate anything
  on bzero'ed memory roots anymore).
  Item_arena() constructor is now trivial and is used only then Item_arena is created as backup
  storage for other Item_arena (we use Item_arena(bool) now if it is part of Statement).
  Both trivial Item_arena constructor and destructor are now inline.
  Removed unneeded clear_alloc_root from Item_arena::restore_backup_item_arena().
sql/sql_class.h:
  Both trivial Item_arena constructor and destructor are now inline.
  Commented various Item_arena constructors.

1 files changed, 2 insertions, 1 deletions

diff --git a/include/my_sys.h b/include/my_sys.h
index ad1966ba67f..271e0ea0bcb 100644
--- a/include/my_sys.h
+++ b/include/my_sys.h
@@ -725,7 +725,8 @@ extern void my_free_lock(byte *ptr,myf flags);
 #define my_free_lock(A,B) my_free((A),(B))
 #endif
 #define alloc_root_inited(A) ((A)->min_malloc != 0)
-#define clear_alloc_root(A) bzero((void *) (A), sizeof(MEM_ROOT))
+#define ALLOC_ROOT_MIN_BLOCK_SIZE (MALLOC_OVERHEAD + sizeof(USED_MEM) + 8)
+#define clear_alloc_root(A) do { (A)->free= (A)->used= (A)->pre_alloc= 0; } while(0)
 extern void init_alloc_root(MEM_ROOT *mem_root, uint block_size,
 			    uint pre_alloc_size);
 extern gptr alloc_root(MEM_ROOT *mem_root,unsigned int Size);