Performance improvements to the comparison operators in the bytecode engine.

FossilOrigin-Name: 37803b19d219f4107f29b240d5d314600e4bd236

src/vdbeaux.c

@@ -3716,15 +3716,47 @@ static int vdbeCompareMemString(
   }
 }
 
+/*
+** The input pBlob is guaranteed to be a Blob that is not marked
+** with MEM_Zero.  Return true if it could be a zero-blob.
+*/
+static int isZeroBlob(const Mem *pBlob){
+  int i;
+  for(i=0; i<pBlob->n && pBlob->z[i]==0; i++){}
+  return i==pBlob->n;
+}
+
 /*
 ** Compare two blobs.  Return negative, zero, or positive if the first
 ** is less than, equal to, or greater than the second, respectively.
 ** If one blob is a prefix of the other, then the shorter is the lessor.
 */
 static SQLITE_NOINLINE int sqlite3BlobCompare(const Mem *pB1, const Mem *pB2){
-  int c = memcmp(pB1->z, pB2->z, pB1->n>pB2->n ? pB2->n : pB1->n);
+  int c;
+  int n1 = pB1->n;
+  int n2 = pB2->n;
+
+  /* It is possible to have a Blob value that has some non-zero content
+  ** followed by zero content.  But that only comes up for Blobs formed
+  ** by the OP_MakeRecord opcode, and such Blobs never get passed into
+  ** sqlite3MemCompare(). */
+  assert( (pB1->flags & MEM_Zero)==0 || n1==0 );
+  assert( (pB2->flags & MEM_Zero)==0 || n2==0 );
+
+  if( (pB1->flags|pB2->flags) & MEM_Zero ){
+    if( pB1->flags & pB2->flags & MEM_Zero ){
+      return pB1->u.nZero - pB2->u.nZero;
+    }else if( pB1->flags & MEM_Zero ){
+      if( !isZeroBlob(pB2) ) return -1;
+      return pB1->u.nZero - n2;
+    }else{
+      if( !isZeroBlob(pB1) ) return +1;
+      return n1 - pB2->u.nZero;
+    }
+  }
+  c = memcmp(pB1->z, pB2->z, n1>n2 ? n2 : n1);
   if( c ) return c;
-  return pB1->n - pB2->n;
+  return n1 - n2;
 }
 
 /*