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Add comments to better explain the two-pass memory allocation approach

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for prepared statements.

FossilOrigin-Name: 0e5e18ea12c70559d4c63981c0bb5f9430c5bcbb
This commit is contained in:
drh
2009-12-08 19:58:19 +00:00
parent 84db21ec6a
commit 19875c8240
3 changed files with 35 additions and 13 deletions
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22
src/vdbeaux.c
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@@ -1266,8 +1266,9 @@ void sqlite3VdbeIOTraceSql(Vdbe *p){
**
** nByte is the number of bytes of space needed.
**
** *ppFrom point to available space and pEnd points to the end of the
** available space.
** *ppFrom points to available space and pEnd points to the end of the
** available space. When space is allocated, *ppFrom is advanced past
** the end of the allocated space.
**
** *pnByte is a counter of the number of bytes of space that have failed
** to allocate. If there is insufficient space in *ppFrom to satisfy the
@@ -1348,9 +1349,10 @@ void sqlite3VdbeMakeReady(
** being called from sqlite3_reset() to reset the virtual machine.
*/
if( nVar>=0 && ALWAYS(db->mallocFailed==0) ){
u8 *zCsr = (u8 *)&p->aOp[p->nOp];
u8 *zEnd = (u8 *)&p->aOp[p->nOpAlloc];
int nByte;
u8 *zCsr = (u8 *)&p->aOp[p->nOp]; /* Memory avaliable for alloation */
u8 *zEnd = (u8 *)&p->aOp[p->nOpAlloc]; /* First byte past available mem */
int nByte; /* How much extra memory needed */
resolveP2Values(p, &nArg);
p->usesStmtJournal = (u8)usesStmtJournal;
if( isExplain && nMem<10 ){
@@ -1360,6 +1362,16 @@ void sqlite3VdbeMakeReady(
zCsr += (zCsr - (u8*)0)&7;
assert( EIGHT_BYTE_ALIGNMENT(zCsr) );
/* Memory for registers, parameters, cursor, etc, is allocated in two
** passes. On the first pass, we try to reuse unused space at the
** end of the opcode array. If we are unable to satisfy all memory
** requirements by reusing the opcode array tail, then the second
** pass will fill in the rest using a fresh allocation.
**
** This two-pass approach that reuses as much memory as possible from
** the leftover space at the end of the opcode array can significantly
** reduce the amount of memory held by a prepared statement.
*/
do {
nByte = 0;
p->aMem = allocSpace(p->aMem, nMem*sizeof(Mem), &zCsr, zEnd, &nByte);