Avoid unnecessary zeroing of fields in the MemPage object that are going

to be reinitialized before use anyhow.  A smaller and faster binary results.

FossilOrigin-Name: 01ada3d1068476f90dcae02cb089001ea4bcc23d

4 files changed, 32 insertions, 26 deletions

diff --git a/src/btree.c b/src/btree.c
index 7a68331c3..999ce159c 100644
--- a/src/btree.c
+++ b/src/btree.c
@@ -2282,7 +2282,7 @@ int sqlite3BtreeOpen(
       goto btree_open_out;
     }
     rc = sqlite3PagerOpen(pVfs, &pBt->pPager, zFilename,
-                          EXTRA_SIZE, flags, vfsFlags, pageReinit);
+                          sizeof(MemPage), flags, vfsFlags, pageReinit);
     if( rc==SQLITE_OK ){
       sqlite3PagerSetMmapLimit(pBt->pPager, db->szMmap);
       rc = sqlite3PagerReadFileheader(pBt->pPager,sizeof(zDbHeader),zDbHeader);
@@ -6259,7 +6259,6 @@ static void dropCell(MemPage *pPage, int idx, int sz, int *pRC){
   int hdr;        /* Beginning of the header.  0 most pages.  100 page 1 */
 
   if( *pRC ) return;
-
   assert( idx>=0 && idx<pPage->nCell );
   assert( CORRUPT_DB || sz==cellSize(pPage, idx) );
   assert( sqlite3PagerIswriteable(pPage->pDbPage) );
@@ -6343,7 +6342,10 @@ static void insertCell(
       put4byte(pCell, iChild);
     }
     j = pPage->nOverflow++;
-    assert( j<(int)(sizeof(pPage->apOvfl)/sizeof(pPage->apOvfl[0])) );
+    /* Comparison against ArraySize-1 since we hold back one extra slot
+    ** as a contingency.  In other words, never need more than 3 overflow
+    ** slots but 4 are allocated, just to be safe. */
+    assert( j < ArraySize(pPage->apOvfl)-1 );
     pPage->apOvfl[j] = pCell;
     pPage->aiOvfl[j] = (u16)i;
 
diff --git a/src/btreeInt.h b/src/btreeInt.h
index 6cd090257..fc2182b63 100644
--- a/src/btreeInt.h
+++ b/src/btreeInt.h
@@ -259,37 +259,39 @@ typedef struct CellInfo CellInfo;
 #define PTF_LEAF      0x08
 
 /*
-** As each page of the file is loaded into memory, an instance of the following
-** structure is appended and initialized to zero.  This structure stores
-** information about the page that is decoded from the raw file page.
+** An instance of this object stores information about each a single database
+** page that has been loaded into memory.  The information in this object
+** is derived from the raw on-disk page content.
 **
-** The pParent field points back to the parent page.  This allows us to
-** walk up the BTree from any leaf to the root.  Care must be taken to
-** unref() the parent page pointer when this page is no longer referenced.
-** The pageDestructor() routine handles that chore.
+** As each database page is loaded into memory, the pager allocats an
+** instance of this object and zeros the first 8 bytes.  (This is the
+** "extra" information associated with each page of the pager.)
 **
 ** Access to all fields of this structure is controlled by the mutex
 ** stored in MemPage.pBt->mutex.
 */
 struct MemPage {
   u8 isInit;           /* True if previously initialized. MUST BE FIRST! */
-  u8 nOverflow;        /* Number of overflow cell bodies in aCell[] */
+  u8 bBusy;            /* Prevent endless loops on corrupt database files */
   u8 intKey;           /* True if table b-trees.  False for index b-trees */
   u8 intKeyLeaf;       /* True if the leaf of an intKey table */
+  Pgno pgno;           /* Page number for this page */
+  /* Only the first 8 bytes (above) are zeroed by pager.c when a new page
+  ** is allocated. All fields that follow must be initialized before use */
   u8 leaf;             /* True if a leaf page */
   u8 hdrOffset;        /* 100 for page 1.  0 otherwise */
   u8 childPtrSize;     /* 0 if leaf==1.  4 if leaf==0 */
   u8 max1bytePayload;  /* min(maxLocal,127) */
-  u8 bBusy;            /* Prevent endless loops on corrupt database files */
+  u8 nOverflow;        /* Number of overflow cell bodies in aCell[] */
   u16 maxLocal;        /* Copy of BtShared.maxLocal or BtShared.maxLeaf */
   u16 minLocal;        /* Copy of BtShared.minLocal or BtShared.minLeaf */
   u16 cellOffset;      /* Index in aData of first cell pointer */
   u16 nFree;           /* Number of free bytes on the page */
   u16 nCell;           /* Number of cells on this page, local and ovfl */
   u16 maskPage;        /* Mask for page offset */
-  u16 aiOvfl[5];       /* Insert the i-th overflow cell before the aiOvfl-th
+  u16 aiOvfl[4];       /* Insert the i-th overflow cell before the aiOvfl-th
                        ** non-overflow cell */
-  u8 *apOvfl[5];       /* Pointers to the body of overflow cells */
+  u8 *apOvfl[4];       /* Pointers to the body of overflow cells */
   BtShared *pBt;       /* Pointer to BtShared that this page is part of */
   u8 *aData;           /* Pointer to disk image of the page data */
   u8 *aDataEnd;        /* One byte past the end of usable data */
@@ -298,17 +300,9 @@ struct MemPage {
   DbPage *pDbPage;     /* Pager page handle */
   u16 (*xCellSize)(MemPage*,u8*);             /* cellSizePtr method */
   void (*xParseCell)(MemPage*,u8*,CellInfo*); /* btreeParseCell method */
-  Pgno pgno;           /* Page number for this page */
 };
 
 /*
-** The in-memory image of a disk page has the auxiliary information appended
-** to the end.  EXTRA_SIZE is the number of bytes of space needed to hold
-** that extra information.
-*/
-#define EXTRA_SIZE sizeof(MemPage)
-
-/*
 ** A linked list of the following structures is stored at BtShared.pLock.
 ** Locks are added (or upgraded from READ_LOCK to WRITE_LOCK) when a cursor 
 ** is opened on the table with root page BtShared.iTable. Locks are removed
diff --git a/src/pager.c b/src/pager.c
index 9975f3fcb..aa989f4ef 100644
--- a/src/pager.c
+++ b/src/pager.c
@@ -3955,7 +3955,8 @@ static int pagerAcquireMapPage(
     *ppPage = p = pPager->pMmapFreelist;
     pPager->pMmapFreelist = p->pDirty;
     p->pDirty = 0;
-    memset(p->pExtra, 0, pPager->nExtra);
+    assert( pPager->nExtra>=8 );
+    memset(p->pExtra, 0, 8);
   }else{
     *ppPage = p = (PgHdr *)sqlite3MallocZero(sizeof(PgHdr) + pPager->nExtra);
     if( p==0 ){
@@ -4555,7 +4556,9 @@ int sqlite3PagerFlush(Pager *pPager){
 **
 ** The nExtra parameter specifies the number of bytes of space allocated
 ** along with each page reference. This space is available to the user
-** via the sqlite3PagerGetExtra() API.
+** via the sqlite3PagerGetExtra() API.  When a new page is allocated, the
+** first 8 bytes of this space are zeroed but the remainder is uninitialized.
+** (The extra space is used by btree as the MemPage object.)
 **
 ** The flags argument is used to specify properties that affect the
 ** operation of the pager. It should be passed some bitwise combination
@@ -4785,8 +4788,8 @@ act_like_temp_file:
 
   /* Initialize the PCache object. */
   if( rc==SQLITE_OK ){
-    assert( nExtra<1000 );
     nExtra = ROUND8(nExtra);
+    assert( nExtra>=8 && nExtra<1000 );
     rc = sqlite3PcacheOpen(szPageDflt, nExtra, !memDb,
                        !memDb?pagerStress:0, (void *)pPager, pPager->pPCache);
   }
diff --git a/src/pcache.c b/src/pcache.c
index 68f4b51dc..70cf0f5a0 100644
--- a/src/pcache.c
+++ b/src/pcache.c
@@ -284,6 +284,12 @@ int sqlite3PcacheSize(void){ return sizeof(PCache); }
 ** has already been allocated and is passed in as the p pointer. 
 ** The caller discovers how much space needs to be allocated by 
 ** calling sqlite3PcacheSize().
+**
+** szExtra is some extra space allocated for each page.  The first
+** 8 bytes of the extra space will be zeroed as the page is allocated,
+** but remaining content will be uninitialized.  Though it is opaque
+** to this module, the extra space really ends up being the MemPage
+** structure in the pager.
 */
 int sqlite3PcacheOpen(
   int szPage,                  /* Size of every page */
@@ -296,6 +302,7 @@ int sqlite3PcacheOpen(
   memset(p, 0, sizeof(PCache));
   p->szPage = 1;
   p->szExtra = szExtra;
+  assert( szExtra>=8 );  /* First 8 bytes will be zeroed */
   p->bPurgeable = bPurgeable;
   p->eCreate = 2;
   p->xStress = xStress;
@@ -465,7 +472,7 @@ static SQLITE_NOINLINE PgHdr *pcacheFetchFinishWithInit(
   pPgHdr->pPage = pPage;
   pPgHdr->pData = pPage->pBuf;
   pPgHdr->pExtra = (void *)&pPgHdr[1];
-  memset(pPgHdr->pExtra, 0, pCache->szExtra);
+  memset(pPgHdr->pExtra, 0, 8);
   pPgHdr->pCache = pCache;
   pPgHdr->pgno = pgno;
   pPgHdr->flags = PGHDR_CLEAN;