bench 1324656

src/search.cpp

@@ -836,19 +836,17 @@ Value Search::Worker::search(
     }
 
     // Step 10. Internal iterative reductions (~9 Elo)
-    // For PV nodes without a ttMove, we decrease depth.
-    if (PvNode && !ttData.move)
-        depth -= 3;
+    // For PV nodes without a ttMove as well as for deep enough cutNodes, we decrease depth.
+    // This heuristic is known to scale non-linearly, current version was tested at VVLTC.
+    // Further improvements need to be tested at similar time control if they make IIR
+    // more aggressive.
+    if ((PvNode || (cutNode && depth >= 7)) && !ttData.move)
+        depth -= 2;
 
     // Use qsearch if depth <= 0
     if (depth <= 0)
         return qsearch<PV>(pos, ss, alpha, beta);
 
-    // For cutNodes, if depth is high enough, decrease depth by 2 if there is no ttMove,
-    // or by 1 if there is a ttMove with an upper bound.
-    if (cutNode && depth >= 7 && (!ttData.move || ttData.bound == BOUND_UPPER))
-        depth -= 1 + !ttData.move;
-
     // Step 11. ProbCut (~10 Elo)
     // If we have a good enough capture (or queen promotion) and a reduced search
     // returns a value much above beta, we can (almost) safely prune the previous move.