Fix greater than TB scores in null move pruning.

This patch is a simplification and a fix to dealing with null moves scores that returns proven mates or TB scores by preventing 'null move pruning' if the nullvalue is in that range.

Current solution downgrades nullValues on the non-PV node but the value can be used in a transposed PV-node to the same position afterwards (Triangulation), the later is prone to propagate a wrong score (96.05) to root that will not be refuted unless we search further.

Score of (96.05) can be obtained be two methods,

maxim static-eval returned on Pv update (mostly qSearch)
this downgrade (clamp) in NMP
and theoretically can happen with or without TBs but the second scenario is more dangerous than the first.
This fixes the reproducible case in very common scenarios with TBs as shown in the debugging at discord.

Fixes: #4699
Passed STC:
https://tests.stockfishchess.org/tests/view/64c1eca8dc56e1650abba6f9
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 670048 W: 171132 L: 171600 D: 327316
Ptnml(0-2): 2134, 75687, 179820, 75279, 2104

Passed LTC:
https://tests.stockfishchess.org/tests/view/64c5e130dc56e1650abc0438
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 92868 W: 23642 L: 23499 D: 45727
Ptnml(0-2): 52, 9509, 27171, 9648, 54

closes https://github.com/official-stockfish/Stockfish/pull/4715

Bench: 1327410

diff --git a/src/search.cpp b/src/search.cpp
index a1834ab99b303387f505cd4011ee2f1cc4a25026..1b019a08e829f466038536aa5fd35adb60e7bb7f 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -812,11 +812,9 @@ namespace {
 
         pos.undo_null_move();
 
-        if (nullValue >= beta)
+        // Do not return unproven mate or TB scores
+        if (nullValue >= beta && nullValue < VALUE_TB_WIN_IN_MAX_PLY)
         {
-            // Do not return unproven mate or TB scores
-            nullValue = std::min(nullValue, VALUE_TB_WIN_IN_MAX_PLY-1);
-
             if (thisThread->nmpMinPly || depth < 14)
                 return nullValue;