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

for prepared statements.

FossilOrigin-Name: 0e5e18ea12c70559d4c63981c0bb5f9430c5bcbb

manifest

@@ -1,5 +1,8 @@
-C Add\stests\sto\simprove\scoverage\sof\sfts3.\sAssociated\sbugfixes.
-D 2009-12-08T19:05:54
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA1
+
+C Add\scomments\sto\sbetter\sexplain\sthe\stwo-pass\smemory\sallocation\sapproach\nfor\sprepared\sstatements.
+D 2009-12-08T19:58:20
 F Makefile.arm-wince-mingw32ce-gcc fcd5e9cd67fe88836360bb4f9ef4cb7f8e2fb5a0
 F Makefile.in c5827ead754ab32b9585487177c93bb00b9497b3
 F Makefile.linux-gcc d53183f4aa6a9192d249731c90dbdffbd2c68654
@@ -213,7 +216,7 @@ F src/vdbe.c 5ed06318aac5d57849170a8bf39e807c22c5fedd
 F src/vdbe.h bea1f0cd530775bdb58a340265f3cf3ee920e9b2
 F src/vdbeInt.h d7ea821ac7813c9bea0fe87558c35e07b2c7c44d
 F src/vdbeapi.c bb128b819b9ef1a2ce211a36a6cb70a1643fa239
-F src/vdbeaux.c 86d43cdf9615235e1def993a945dfaabc20ab079
+F src/vdbeaux.c 42ed644fea54c3fbfa70af7c65456ec4ab089c77
 F src/vdbeblob.c 84f924700a7a889152aeebef77ca5f4e3875ffb4
 F src/vdbemem.c 1e16e3a16e55f4c3452834f0e041726021aa66e0
 F src/vdbetrace.c 864cef96919323482ebd9986f2132435115e9cc2
@@ -777,7 +780,14 @@ F tool/speedtest2.tcl ee2149167303ba8e95af97873c575c3e0fab58ff
 F tool/speedtest8.c 2902c46588c40b55661e471d7a86e4dd71a18224
 F tool/speedtest8inst1.c 293327bc76823f473684d589a8160bde1f52c14e
 F tool/vdbe-compress.tcl d70ea6d8a19e3571d7ab8c9b75cba86d1173ff0f
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-U dan
-Z d714a86f89f40b4ba38c62abeeffc91f
+P f0eac4175aee6c50ee68acc253f76fbe44574250
+R 74e90ca8b896b5a20c44aa66d10e0006
+U drh
+Z 0320ff4e0cbee5e0cb2eb5909024d39a
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manifest.uuid

@@ -1 +1 @@
-f0eac4175aee6c50ee68acc253f76fbe44574250
+0e5e18ea12c70559d4c63981c0bb5f9430c5bcbb

src/vdbeaux.c

@@ -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);