Do less LMR extensions

This patch restricts LMR extensions (of non-transposition table moves) from being
used when the transposition table move was extended by two plies via singular
extension. This may serve to limit search explosions in certain positions.

This makes a lot of sense because the precondition for the tt-move to have been
singular extended by two plies is that the result of the alternate search (with
excluded the tt-move) has been a hard fail low: it is natural to later search less
for non tt-moves in this situation.

The current state of depth/extensions/reductions management is getting quite tricky
in our search algo, see https://github.com/official-stockfish/Stockfish/pull/3546#issuecomment-860174549
for some discussion. Suggestions welcome!

Passed STC
https://tests.stockfishchess.org/tests/view/60c3f293457376eb8bcaac8d
LLR: 2.95 (-2.94,2.94) <-0.50,2.50>
Total: 117984 W: 9698 L: 9430 D: 98856
Ptnml(0-2): 315, 7708, 42703, 7926, 340

passed LTC
https://tests.stockfishchess.org/tests/view/60c46ea5457376eb8bcaacc7
LLR: 2.97 (-2.94,2.94) <0.50,3.50>
Total: 11280 W: 401 L: 302 D: 10577
Ptnml(0-2): 2, 271, 4998, 364, 5

closes https://github.com/official-stockfish/Stockfish/pull/3546

Bench: 4709974

diff --git a/src/search.cpp b/src/search.cpp
index d04898e8570a13486f0b11088760138d4d5c768a..b8756d381397bd9aeed94a899a1e361695ff3b8d 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -954,6 +954,7 @@ moves_loop: // When in check, search starts from here
 
     value = bestValue;
     singularQuietLMR = moveCountPruning = false;
+    bool doubleExtension = false;
 
     // Indicate PvNodes that will probably fail low if the node was searched
     // at a depth equal or greater than the current depth, and the result of this search was a fail low.
@@ -1080,7 +1081,10 @@ moves_loop: // When in check, search starts from here
               if (   !PvNode
                   && value < singularBeta - 93
                   && ss->doubleExtensions < 3)
+              {
                   extension = 2;
+                  doubleExtension = true;
+              }
           }
 
           // Multi-cut pruning
@@ -1188,8 +1192,8 @@ moves_loop: // When in check, search starts from here
 
           // In general we want to cap the LMR depth search at newDepth. But if
           // reductions are really negative and movecount is low, we allow this move
-          // to be searched deeper than the first move.
-          Depth d = std::clamp(newDepth - r, 1, newDepth + (r < -1 && moveCount <= 5));
+          // to be searched deeper than the first move, unless ttMove was extended by 2.
+          Depth d = std::clamp(newDepth - r, 1, newDepth + (r < -1 && moveCount <= 5 && !doubleExtension));
 
           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);