Performance improvement by unwinding a loop in jsonAppendString().

FossilOrigin-Name: 190ab3c08431a0ba24d76392eab251f5c1792add05e4ec780998b299208eca95

src/json.c

@@ -627,8 +627,33 @@ static void jsonAppendString(JsonString *p, const char *zIn, u32 N){
   p->zBuf[p->nUsed++] = '"';
   while( 1 /*exit-by-break*/ ){
     k = 0;
-    while( k+1<N && jsonIsOk[z[k]] && jsonIsOk[z[k+1]] ){ k += 2; } /* <--, */
-    while( k<N && jsonIsOk[z[k]] ){ k++; }    /* <-- loop unwound for speed  */
+    /* The following while() is the 4-way unwound equivalent of
+    **
+    **     while( k<N && jsonIsOk[z[k]] ){ k++; }
+    */
+    while( 1 /* Exit by break */ ){
+      if( k+3>=N ){
+        while( k<N && jsonIsOk[z[k]] ){ k++; }
+        break;
+      }
+      if( !jsonIsOk[z[k]] ){
+        break;
+      }
+      if( !jsonIsOk[z[k+1]] ){
+        k += 1;
+        break;
+      }
+      if( !jsonIsOk[z[k+2]] ){
+        k += 2;
+        break;
+      }
+      if( !jsonIsOk[z[k+3]] ){
+        k += 3;
+        break;
+      }else{
+        k += 4;
+      }
+    }
     if( k>=N ){
       if( k>0 ){
         memcpy(&p->zBuf[p->nUsed], z, k);