Remove ttPv tree shrinking.

Via the ttPv flag an implicit tree of current and former PV nodes is maintained. In addition this tree is grown or shrinked at the leafs dependant on the search results. But now the shrinking step has been removed.

As the frequency of ttPv nodes decreases with depth the shown scaling behavior (STC barely passed but LTC scales well) of the tests was expected.

STC:
LLR: 2.93 (-2.94,2.94) <-2.25,0.25>
Total: 270408 W: 71593 L: 71785 D: 127030
Ptnml(0-2): 1339, 31024, 70630, 30912, 1299
https://tests.stockfishchess.org/tests/view/622fbf9dc9e950cbfc2376d6

LTC:
LLR: 2.96 (-2.94,2.94) <-2.25,0.25>
Total: 34368 W: 9135 L: 8992 D: 16241
Ptnml(0-2): 28, 3423, 10135, 3574, 24
https://tests.stockfishchess.org/tests/view/62305257c9e950cbfc238964

closes https://github.com/official-stockfish/Stockfish/pull/3963

Bench: 7044203

diff --git a/src/search.cpp b/src/search.cpp
index 3008079e9c4ade413f2ee829ba5af2a7b8ad4f42..672abc05b27296165df1286890d1f658f56187bc 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1359,10 +1359,6 @@ moves_loop: // When in check, search starts here
     // opponent move is probably good and the new position is added to the search tree.
     if (bestValue <= alpha)
         ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
-    // Otherwise, a counter move has been found and if the position is the last leaf
-    // in the search tree, remove the position from the search tree.
-    else if (depth > 3)
-        ss->ttPv = ss->ttPv && (ss+1)->ttPv;
 
     // Write gathered information in transposition table
     if (!excludedMove && !(rootNode && thisThread->pvIdx))