Reduce the space allocated for the runtime virtual machine stack. (CVS 2428)

FossilOrigin-Name: 7d6818da33a87076d1faf35ffc15a3aada0533b3

src/vdbeaux.c

@@ -166,6 +166,47 @@ void sqlite3VdbeResolveLabel(Vdbe *p, int x){
   }
 }
 
+/*
+** Return non-zero if opcode 'op' is guarenteed not to push more values
+** onto the VDBE stack than it pops off.
+*/
+static int opcodeUsesStack(u8 op){
+  /* The 10 STACK_MASK_n constants are defined in the automatically
+  ** generated header file opcodes.h. Each is a 16-bit bitmask, one
+  ** bit corresponding to each opcode implemented by the virtual
+  ** machine in vdbe.c. The bit is true if the word "stack" appears
+  ** in a comment on the same line as the "case OP_XXX:" in 
+  ** sqlite3VdbeExec() in vdbe.c.
+  **
+  ** If the bit is true, then the corresponding opcode is guarenteed not
+  ** to grow the stack when it is executed. Otherwise, it may grow the
+  ** stack by at most one entry.
+  **
+  ** STACK_MASK_0 corresponds to opcodes 0 to 15. STACK_MASK_1 contains
+  ** one bit for opcodes 16 to 31, and so on.
+  **
+  ** 16-bit bitmasks (rather than 32-bit) are specified in opcodes.h 
+  ** because the file is generated by an awk program. Awk manipulates
+  ** all numbers as floating-point and we don't want to risk a rounding
+  ** error if someone builds with an awk that uses (for example) 32-bit 
+  ** IEEE floats.
+  */ 
+  static u32 masks[5] = {
+    STACK_MASK_0 + (STACK_MASK_1<<16),
+    STACK_MASK_2 + (STACK_MASK_3<<16),
+    STACK_MASK_4 + (STACK_MASK_5<<16),
+    STACK_MASK_6 + (STACK_MASK_7<<16),
+    STACK_MASK_8 + (STACK_MASK_9<<16)
+  };
+  return (masks[op>>5] & (1<<(op&0x1F)));
+}
+
+#ifndef NDEBUG
+int sqlite3VdbeOpcodeUsesStack(u8 op){
+  return opcodeUsesStack(op);
+}
+#endif
+
 /*
 ** Loop through the program looking for P2 values that are negative.
 ** Each such value is a label.  Resolve the label by setting the P2
@@ -176,22 +217,30 @@ void sqlite3VdbeResolveLabel(Vdbe *p, int x){
 ** Variable *pMaxFuncArgs is set to the maximum value of any P1 argument 
 ** to an OP_Function or P2 to an OP_AggFunc opcode. This is used by 
 ** sqlite3VdbeMakeReady() to size the Vdbe.apArg[] array.
+**
+** The integer *pMaxStack is set to the maximum number of vdbe stack
+** entries that static analysis reveals this program might need.
 */
-static void resolveP2Values(Vdbe *p, int *pMaxFuncArgs){
+static void resolveP2Values(Vdbe *p, int *pMaxFuncArgs, int *pMaxStack){
   int i;
-  int nMax = 0;
+  int nMaxArgs = 0;
+  int nMaxStack = p->nOp;
   Op *pOp;
   int *aLabel = p->aLabel;
-  if( aLabel==0 ) return;
   for(pOp=p->aOp, i=p->nOp-1; i>=0; i--, pOp++){
     u8 opcode = pOp->opcode;
 
     /* Todo: Maybe OP_AggFunc should change to use P1 in the same
-     * way as OP_Function. */
+     * way as OP_Function. 
+     */
     if( opcode==OP_Function ){
-      if( pOp->p1>nMax ) nMax = pOp->p1;
+      if( pOp->p1>nMaxArgs ) nMaxArgs = pOp->p1;
     }else if( opcode==OP_AggFunc ){
-      if( pOp->p2>nMax ) nMax = pOp->p2;
+      if( pOp->p2>nMaxArgs ) nMaxArgs = pOp->p2;
+    }
+
+    if( opcodeUsesStack(opcode) ){
+      nMaxStack--;
     }
 
     if( pOp->p2>=0 ) continue;
@@ -200,7 +249,9 @@ static void resolveP2Values(Vdbe *p, int *pMaxFuncArgs){
   }
   sqliteFree(p->aLabel);
   p->aLabel = 0;
-  *pMaxFuncArgs = nMax;
+
+  *pMaxFuncArgs = nMaxArgs;
+  *pMaxStack = nMaxStack;
 }
 
 /*
@@ -641,12 +692,14 @@ void sqlite3VdbeMakeReady(
   */
   if( p->aStack==0 ){
     int nArg;       /* Maximum number of args passed to a user function. */
-    resolveP2Values(p, &nArg);
+    int nStack;     /* Maximum number of stack entries required */
+    resolveP2Values(p, &nArg, &nStack);
     resizeOpArray(p, p->nOp);
     assert( nVar>=0 );
-    n = isExplain ? 10 : p->nOp;
+    assert( nStack<p->nOp );
+    nStack = isExplain ? 10 : nStack;
     p->aStack = sqliteMalloc(
-        n*sizeof(p->aStack[0])         /* aStack */
+        nStack*sizeof(p->aStack[0])    /* aStack */
       + nArg*sizeof(Mem*)              /* apArg */
       + nVar*sizeof(Mem)               /* aVar */
       + nVar*sizeof(char*)             /* azVar */
@@ -655,7 +708,7 @@ void sqlite3VdbeMakeReady(
       + nAgg*sizeof(Agg)               /* Aggregate contexts */
     );
     if( !sqlite3_malloc_failed ){
-      p->aMem = &p->aStack[n];
+      p->aMem = &p->aStack[nStack];
       p->nMem = nMem;
       p->aVar = &p->aMem[nMem];
       p->nVar = nVar;