Move internal iterative reduction before probcut

Vizvezdenec · vondele · commit 8dea070538dc · 2023-06-04T23:12:23.000+02:00
This patch moves IIR before probcut which allows probcut to be produced at lower depths. Comments in IIR are also slightly updated. Passed STC: https://tests.stockfishchess.org/tests/view/6472d604d29264e4cfa749fd LLR: 2.94 (-2.94,2.94) <0.00,2.00> Total: 387616 W: 103295 L: 102498 D: 181823 Ptnml(0-2): 976, 42322, 106381, 43187, 942 Passed LTC: https://tests.stockfishchess.org/tests/view/6475eb8c4a36543c4c9f42e8 LLR: 2.94 (-2.94,2.94) <0.50,2.50> Total: 202836 W: 54901 L: 54281 D: 93654 Ptnml(0-2): 85, 19609, 61422, 20205, 97 closes #4597 bench 2551691
diff --git a/src/search.cpp b/src/search.cpp
@@ -822,9 +822,24 @@ namespace {
         }
     }
 
+    // Step 10. If the position doesn't a have ttMove, decrease depth by 2
+    // (or by 4 if the TT entry for the current position was hit and the stored depth is greater than or equal to the current depth).
+    // Use qsearch if depth is equal or below zero (~9 Elo)
+    if (    PvNode
+        && !ttMove)
+        depth -= 2 + 2 * (ss->ttHit && tte->depth() >= depth);
+
+    if (depth <= 0)
+        return qsearch<PV>(pos, ss, alpha, beta);
+
+    if (    cutNode
+        &&  depth >= 8
+        && !ttMove)
+        depth -= 2;
+
     probCutBeta = beta + 168 - 61 * improving;
 
-    // Step 10. ProbCut (~10 Elo)
+    // 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.
     if (   !PvNode
@@ -875,20 +890,6 @@ namespace {
         Eval::NNUE::hint_common_parent_position(pos);
     }
 
-    // Step 11. If the position is not in TT, decrease depth by 2 (or by 4 if the TT entry for the current position was hit and the stored depth is greater than or equal to the current depth).
-    // Use qsearch if depth is equal or below zero (~9 Elo)
-    if (    PvNode
-        && !ttMove)
-        depth -= 2 + 2 * (ss->ttHit && tte->depth() >= depth);
-
-    if (depth <= 0)
-        return qsearch<PV>(pos, ss, alpha, beta);
-
-    if (    cutNode
-        &&  depth >= 8
-        && !ttMove)
-        depth -= 2;
-
 moves_loop: // When in check, search starts here
 
     // Step 12. A small Probcut idea, when we are in check (~4 Elo)
