Tweak doubly singular condition (Topo's patch)

This patch relax a little bit the condition for doubly singular moves
(ie moves that are so forced that we think that they deserve a local
double extension of the search). We lower the margin and allow up to
six such double extensions in the path between the root and the critical
node.

Original idea by Siad Daboul (@TopoIogist) in PR #3709

Tested with the previous commit:

passed STC:
LLR: 2.94 (-2.94,2.94) <-0.50,2.50>
Total: 33048 W: 8458 L: 8236 D: 16354
Ptnml(0-2): 120, 3701, 8660, 3923, 120
https://tests.stockfishchess.org/tests/view/614b24347bdc23e77ceb88fe

passed LTC:
LLR: 2.95 (-2.94,2.94) <0.50,3.50>
Total: 54176 W: 13712 L: 13406 D: 27058
Ptnml(0-2): 36, 5653, 15399, 5969, 31
https://tests.stockfishchess.org/tests/view/614b3b727bdc23e77ceb8911

closes https://github.com/official-stockfish/Stockfish/pull/3714

Bench: 5792377

diff --git a/src/search.cpp b/src/search.cpp
index 10a9027b94922cd1982a35b2e42957be22f42564..9b56bd2b08325cf3d871a8a49ec57b352c0c67b6 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1107,8 +1107,8 @@ moves_loop: // When in check, search starts here
 
               // Avoid search explosion by limiting the number of double extensions
               if (   !PvNode
-                  && value < singularBeta - 93
-                  && ss->doubleExtensions < 3)
+                  && value < singularBeta - 75
+                  && ss->doubleExtensions <= 6)
               {
                   extension = 2;
                   noLMRExtension = true;