Revert futility pruning patches

vondele · vondele · commit dabaf2220fe0 · 2021-08-05T16:41:07.000+02:00
reverts 09b6d28 and dbd7f60 that significantly impact mate finding capabilities. For example on ChestUCI_23102018.epd, at 1M nodes, the number of mates found is nearly reduced 2x without these depth conditions: sf6 2091 sf7 2093 sf8 2107 sf9 2062 sf10 2208 sf11 2552 sf12 2563 sf13 2509 sf14 2427 master 1246 patched 2467 (script for testing at https://github.com/official-stockfish/Stockfish/files/6936412/matecheck.zip) closes #3641 fixes #3627 Bench: 5467570
diff --git a/src/search.cpp b/src/search.cpp
@@ -779,8 +779,10 @@ namespace {
                ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
                : ss->staticEval > (ss-2)->staticEval;
 
-    // Step 7. Futility pruning: child node (~50 Elo)
+    // Step 7. Futility pruning: child node (~50 Elo).
+    // The depth condition is important for mate finding.
     if (   !PvNode
+        &&  depth < 9
         &&  eval - futility_margin(depth, improving) >= beta
         &&  eval < VALUE_KNOWN_WIN) // Do not return unproven wins
         return eval;
@@ -989,7 +991,7 @@ namespace {
       // Calculate new depth for this move
       newDepth = depth - 1;
 
-      // Step 13. Pruning at shallow depth (~200 Elo)
+      // Step 13. Pruning at shallow depth (~200 Elo). Depth conditions are important for mate finding.
       if (  !rootNode
           && pos.non_pawn_material(us)
           && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
@@ -1023,6 +1025,7 @@ namespace {
 
               // Futility pruning: parent node (~5 Elo)
               if (   !ss->inCheck
+                  && lmrDepth < 7
                   && ss->staticEval + 174 + 157 * lmrDepth <= alpha)
                   continue;
 
