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, 32 insertions, 5 deletions

diff --git a/mysys/my_alloc.c b/mysys/my_alloc.c
index c9784ddc9a0..f0bc62b10a0 100644
--- a/mysys/my_alloc.c
+++ b/mysys/my_alloc.c
@@ -22,6 +22,27 @@
 #undef EXTRA_DEBUG
 #define EXTRA_DEBUG
 
+
+/*
+  Initialize memory root
+
+  SYNOPSIS
+    init_alloc_root()
+      mem_root       - memory root to initialize
+      block_size     - size of chunks (blocks) used for memory allocation
+                       (It is external size of chunk i.e. it should include
+                        memory required for internal structures, thus it
+                        should be no less than ALLOC_ROOT_MIN_BLOCK_SIZE)
+      pre_alloc_size - if non-0, then size of block that should be
+                       pre-allocated during memory root initialization.
+
+  DESCRIPTION
+    This function prepares memory root for further use, sets initial size of
+    chunk for memory allocation and pre-allocates first block if specified.
+    Altough error can happen during execution of this function if pre_alloc_size
+    is non-0 it won't be reported. Instead it will be reported as error in first
+    alloc_root() on this memory root.
+*/
 void init_alloc_root(MEM_ROOT *mem_root, uint block_size,
 		     uint pre_alloc_size __attribute__((unused)))
 {
@@ -29,7 +50,7 @@ void init_alloc_root(MEM_ROOT *mem_root, uint block_size,
   DBUG_PRINT("enter",("root: 0x%lx", mem_root));
   mem_root->free= mem_root->used= mem_root->pre_alloc= 0;
   mem_root->min_malloc= 32;
-  mem_root->block_size= block_size-MALLOC_OVERHEAD-sizeof(USED_MEM)-8;
+  mem_root->block_size= block_size - ALLOC_ROOT_MIN_BLOCK_SIZE;
   mem_root->error_handler= 0;
   mem_root->block_num= 4;			/* We shift this with >>2 */
   mem_root->first_block_usage= 0;
@@ -54,9 +75,9 @@ void init_alloc_root(MEM_ROOT *mem_root, uint block_size,
   SYNOPSIS
     reset_root_defaults()
     mem_root        memory root to change defaults of
-    block_size      new value of block size. Must be 
-                    greater than ~68 bytes (the exact value depends on
-                    platform and compilation flags)
+    block_size      new value of block size. Must be greater or equal
+                    than ALLOC_ROOT_MIN_BLOCK_SIZE (this value is about
+                    68 bytes and depends on platform and compilation flags)
     pre_alloc_size  new size of preallocated block. If not zero,
                     must be equal to or greater than block size,
                     otherwise means 'no prealloc'.
@@ -70,7 +91,9 @@ void init_alloc_root(MEM_ROOT *mem_root, uint block_size,
 void reset_root_defaults(MEM_ROOT *mem_root, uint block_size,
                          uint pre_alloc_size __attribute__((unused)))
 {
-  mem_root->block_size= block_size-MALLOC_OVERHEAD-sizeof(USED_MEM)-8;
+  DBUG_ASSERT(alloc_root_inited(mem_root));
+
+  mem_root->block_size= block_size - ALLOC_ROOT_MIN_BLOCK_SIZE;
 #if !(defined(HAVE_purify) && defined(EXTRA_DEBUG))
   if (pre_alloc_size)
   {
@@ -123,6 +146,8 @@ gptr alloc_root(MEM_ROOT *mem_root,unsigned int Size)
   DBUG_ENTER("alloc_root");
   DBUG_PRINT("enter",("root: 0x%lx", mem_root));
 
+  DBUG_ASSERT(alloc_root_inited(mem_root));
+
   Size+=ALIGN_SIZE(sizeof(USED_MEM));
   if (!(next = (USED_MEM*) my_malloc(Size,MYF(MY_WME))))
   {
@@ -140,6 +165,8 @@ gptr alloc_root(MEM_ROOT *mem_root,unsigned int Size)
   reg1 USED_MEM *next= 0;
   reg2 USED_MEM **prev;
 
+  DBUG_ASSERT(alloc_root_inited(mem_root));
+
   Size= ALIGN_SIZE(Size);
   if ((*(prev= &mem_root->free)) != NULL)
   {