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Overview
Comment:Small performance improvement and size reduction for pageFindSlot() - the routine in btree.c that locates a free slot for a cell on a btree page.
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SHA3-256:1969372ac72d25cc642a0268f4bb0ae4b59f2dca568c119ef61b67183b3a8bd9
User & Date: drh 2019-02-08 22:34:59
Context
2019-02-09
19:23
Change a few assert() statements in fts3 that might fail if the database is corrupt. check-in: db74a56a user: dan tags: trunk
2019-02-08
22:34
Small performance improvement and size reduction for pageFindSlot() - the routine in btree.c that locates a free slot for a cell on a btree page. check-in: 1969372a user: drh tags: trunk
17:28
Further simplifications to sqlite3VdbeMemSetStr(). check-in: 1d212957 user: drh tags: trunk
Changes
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Changes to src/btree.c.

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** detected then *pRc is set to SQLITE_CORRUPT and NULL is returned.
**
** Slots on the free list that are between 1 and 3 bytes larger than nByte
** will be ignored if adding the extra space to the fragmentation count
** causes the fragmentation count to exceed 60.
*/
static u8 *pageFindSlot(MemPage *pPg, int nByte, int *pRc){
  const int hdr = pPg->hdrOffset;
  u8 * const aData = pPg->aData;
  int iAddr = hdr + 1;
  int pc = get2byte(&aData[iAddr]);
  int x;
  int usableSize = pPg->pBt->usableSize;
  int size;            /* Size of the free slot */

  assert( pc>0 );
  while( pc<=usableSize-4 ){
    /* EVIDENCE-OF: R-22710-53328 The third and fourth bytes of each
    ** freeblock form a big-endian integer which is the size of the freeblock
    ** in bytes, including the 4-byte header. */
    size = get2byte(&aData[pc+2]);
    if( (x = size - nByte)>=0 ){
      testcase( x==4 );
      testcase( x==3 );
      if( size+pc > usableSize ){
        *pRc = SQLITE_CORRUPT_PAGE(pPg);
        return 0;
      }else if( x<4 ){
        /* EVIDENCE-OF: R-11498-58022 In a well-formed b-tree page, the total
        ** number of bytes in fragments may not exceed 60. */
        if( aData[hdr+7]>57 ) return 0;

        /* Remove the slot from the free-list. Update the number of
        ** fragmented bytes within the page. */
        memcpy(&aData[iAddr], &aData[pc], 2);
        aData[hdr+7] += (u8)x;




      }else{
        /* The slot remains on the free-list. Reduce its size to account
         ** for the portion used by the new allocation. */
        put2byte(&aData[pc+2], x);
      }
      return &aData[pc + x];
    }
    iAddr = pc;
    pc = get2byte(&aData[pc]);
    if( pc<iAddr+size ) break;
  }
  if( pc ){

    *pRc = SQLITE_CORRUPT_PAGE(pPg);
  }







  return 0;
}

/*
** Allocate nByte bytes of space from within the B-Tree page passed
** as the first argument. Write into *pIdx the index into pPage->aData[]
** of the first byte of allocated space. Return either SQLITE_OK or







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** detected then *pRc is set to SQLITE_CORRUPT and NULL is returned.
**
** Slots on the free list that are between 1 and 3 bytes larger than nByte
** will be ignored if adding the extra space to the fragmentation count
** causes the fragmentation count to exceed 60.
*/
static u8 *pageFindSlot(MemPage *pPg, int nByte, int *pRc){
  const int hdr = pPg->hdrOffset;            /* Offset to page header */
  u8 * const aData = pPg->aData;             /* Page data */
  int iAddr = hdr + 1;                       /* Address of ptr to pc */
  int pc = get2byte(&aData[iAddr]);          /* Address of a free slot */
  int x;                                     /* Excess size of the slot */
  int maxPC = pPg->pBt->usableSize - nByte;  /* Max address for a usable slot */
  int size;                                  /* Size of the free slot */

  assert( pc>0 );
  while( pc<=maxPC ){
    /* EVIDENCE-OF: R-22710-53328 The third and fourth bytes of each
    ** freeblock form a big-endian integer which is the size of the freeblock
    ** in bytes, including the 4-byte header. */
    size = get2byte(&aData[pc+2]);
    if( (x = size - nByte)>=0 ){
      testcase( x==4 );
      testcase( x==3 );



      if( x<4 ){
        /* EVIDENCE-OF: R-11498-58022 In a well-formed b-tree page, the total
        ** number of bytes in fragments may not exceed 60. */
        if( aData[hdr+7]>57 ) return 0;

        /* Remove the slot from the free-list. Update the number of
        ** fragmented bytes within the page. */
        memcpy(&aData[iAddr], &aData[pc], 2);
        aData[hdr+7] += (u8)x;
      }else if( x+pc > maxPC ){
        /* This slot extends off the end of the usable part of the page */
        *pRc = SQLITE_CORRUPT_PAGE(pPg);
        return 0;
      }else{
        /* The slot remains on the free-list. Reduce its size to account
        ** for the portion used by the new allocation. */
        put2byte(&aData[pc+2], x);
      }
      return &aData[pc + x];
    }
    iAddr = pc;
    pc = get2byte(&aData[pc]);
    if( pc<iAddr+size ){

      if( pc ){
        /* The next slot in the chain is not past the end of the current slot */
        *pRc = SQLITE_CORRUPT_PAGE(pPg);
      }
      return 0;
    }
  }
  if( pc>maxPC+nByte-4 ){
    /* The free slot chain extends off the end of the page */
    *pRc = SQLITE_CORRUPT_PAGE(pPg);
  }
  return 0;
}

/*
** Allocate nByte bytes of space from within the B-Tree page passed
** as the first argument. Write into *pIdx the index into pPage->aData[]
** of the first byte of allocated space. Return either SQLITE_OK or