Reduction instead of cutoff

In master, during singular move analysis, when both the transposition value
and a reduced search for the other moves seem to indicate a fail high, we
heuristically prune the whole subtree and return an fail high score.

This patch is a little bit more cautious in this case, and instead of the
risky cutoff, we now search the ttMove with a reduced depth (by two plies).

STC:
https://tests.stockfishchess.org/tests/view/614dafe07bdc23e77ceb8a89
LLR: 2.94 (-2.94,2.94) <-0.50,2.50>
Total: 46728 W: 11909 L: 11666 D: 23153
Ptnml(0-2): 181, 5288, 12168, 5561, 166

LTC:
https://tests.stockfishchess.org/tests/view/614dc84abe4c07e0ecac3c95
LLR: 2.94 (-2.94,2.94) <0.50,3.50>
Total: 74520 W: 18809 L: 18450 D: 37261
Ptnml(0-2): 45, 7735, 21346, 8084, 50

closes https://github.com/official-stockfish/Stockfish/pull/3718

Bench: 5499262

diff --git a/src/search.cpp b/src/search.cpp
index ffeb511095da93cd4e0d125402c018fdf434ced4..8db295f1fd8ee47043fbce706c7b8c632bd357c6 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1125,7 +1125,8 @@ moves_loop: // When in check, search starts here
               return singularBeta;
 
           // If the eval of ttMove is greater than beta we try also if there is another
-          // move that pushes it over beta, if so also produce a cutoff.
+          // move that pushes it over beta, if so the position also has probably multiple
+          // moves giving fail highs. We will then reduce the ttMove (negative extension).
           else if (ttValue >= beta)
           {
               ss->excludedMove = move;
@@ -1133,7 +1134,7 @@ moves_loop: // When in check, search starts here
               ss->excludedMove = MOVE_NONE;
 
               if (value >= beta)
-                  return beta;
+                  extension = -2;
           }
       }