Limit double extensions

Double extensions can lead to search explosions, for specific positions.
Currently, however, these double extensions are worth about 10Elo and cannot
be removed. This patch instead limits the number of double extensions given
to a maximum of 3.

This fixes https://github.com/official-stockfish/Stockfish/issues/3532
where the following testcase was shown to be problematic:

```
uci
setoption name Hash value 4
setoption name Contempt value 0
ucinewgame
position fen 8/Pk6/8/1p6/8/P1K5/8/6B1 w - - 37 130
go depth 20
```

passed STC:
https://tests.stockfishchess.org/tests/view/60c13161457376eb8bcaaa0f
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 73256 W: 6114 L: 6062 D: 61080
Ptnml(0-2): 222, 4912, 26306, 4968, 220

passed LTC:
https://tests.stockfishchess.org/tests/view/60c196fb457376eb8bcaaa6b
LLR: 2.94 (-2.94,2.94) <-2.50,0.50>
Total: 166440 W: 5559 L: 5594 D: 155287
Ptnml(0-2): 106, 4921, 73197, 4894, 102

closes https://github.com/official-stockfish/Stockfish/pull/3544

Bench: 5067605

diff --git a/src/search.cpp b/src/search.cpp
index 5306bb9a0084b2ee50ae7b4fc83cbd7324ba2bd0..d04898e8570a13486f0b11088760138d4d5c768a 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -610,10 +610,11 @@ namespace {
 
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
-    (ss+1)->ttPv = false;
+    (ss+1)->ttPv         = false;
     (ss+1)->excludedMove = bestMove = MOVE_NONE;
-    (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
-    Square prevSq = to_sq((ss-1)->currentMove);
+    (ss+2)->killers[0]   = (ss+2)->killers[1] = MOVE_NONE;
+    ss->doubleExtensions = (ss-1)->doubleExtensions;
+    Square prevSq        = to_sq((ss-1)->currentMove);
 
     // Initialize statScore to zero for the grandchildren of the current position.
     // So statScore is shared between all grandchildren and only the first grandchild
@@ -1074,7 +1075,11 @@ moves_loop: // When in check, search starts from here
           {
               extension = 1;
               singularQuietLMR = !ttCapture;
-              if (!PvNode && value < singularBeta - 93)
+
+              // Avoid search explosion by limiting the number of double extensions to at most 3
+              if (   !PvNode
+                  && value < singularBeta - 93
+                  && ss->doubleExtensions < 3)
                   extension = 2;
           }
 
@@ -1105,6 +1110,7 @@ moves_loop: // When in check, search starts from here
 
       // Add extension to new depth
       newDepth += extension;
+      ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
 
       // Speculative prefetch as early as possible
       prefetch(TT.first_entry(pos.key_after(move)));

diff --git a/src/search.h b/src/search.h
index 811b2e2a9460ef0437322fde8d76ce8731303eb5..801baacc7f1cccfb8ed0852cab3d250ea6ca4e83 100644 (file)
--- a/src/search.h
+++ b/src/search.h
@@ -52,6 +52,7 @@ struct Stack {
   bool inCheck;
   bool ttPv;
   bool ttHit;
+  int doubleExtensions;
 };