Simplify Singular Extension condition

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 43193f1059540e0a92e4895e78b313044ced29f8..435b720695f52caf5bd8fd3154a9f8a266726d62 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -521,7 +521,7 @@ namespace {
     Move ttMove, move, excludedMove, bestMove;
     Depth extension, newDepth;
     Value bestValue, value, ttValue, eval, maxValue;
-    bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
+    bool ttHit, inCheck, givesCheck, improving;
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
     Piece movedPiece;
     int moveCount, captureCount, quietCount;
@@ -820,13 +820,6 @@ moves_loop: // When in check, search starts from here
     MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
     value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
 
-    singularExtensionNode =   !rootNode
-                           &&  depth >= 8 * ONE_PLY
-                           &&  ttMove != MOVE_NONE
-                           &&  ttValue != VALUE_NONE
-                           && !excludedMove // Recursive singular search is not allowed
-                           && (tte->bound() & BOUND_LOWER)
-                           &&  tte->depth() >= depth - 3 * ONE_PLY;
     skipQuiets = false;
     ttCapture = false;
     pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
@@ -866,13 +859,18 @@ moves_loop: // When in check, search starts from here
 
       // Step 13. Extensions (~70 Elo)
 
-      // Singular extension search (~60 Elo). If all moves but one fail low on a search
-      // of (alpha-s, beta-s), and just one fails high on (alpha, beta), then
-      // that move is singular and should be extended. To verify this we do a
-      // reduced search on on all the other moves but the ttMove and if the
+      // Singular extension search (~60 Elo). If all moves but one fail low on a
+      // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
+      // then that move is singular and should be extended. To verify this we do
+      // a reduced search on on all the other moves but the ttMove and if the
       // result is lower than ttValue minus a margin then we will extend the ttMove.
-      if (    singularExtensionNode
+      if (    depth >= 8 * ONE_PLY
           &&  move == ttMove
+          && !rootNode
+          && !excludedMove // Recursive singular search is not allowed
+          &&  ttValue != VALUE_NONE
+          && (tte->bound() & BOUND_LOWER)
+          &&  tte->depth() >= depth - 3 * ONE_PLY
           &&  pos.legal(move))
       {
           Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);