Decrease Depth more for positions not in TT.

If the position is not in TT, decrease depth by 2
or by 4 if the TT entry for the current position was hit
and the stored depth is greater than or equal to the current depth.

Many thanks to Vizvezdenec as the main idea was his.

Passed STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 70664 W: 18995 L: 18639 D: 33030
Ptnml(0-2): 228, 7712, 19090, 8080, 222
https://tests.stockfishchess.org/tests/view/64258a8bdb43ab2ba6f9b682

Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 85040 W: 23227 L: 22836 D: 38977
Ptnml(0-2): 26, 8115, 25867, 8466, 46
https://tests.stockfishchess.org/tests/view/64262057db43ab2ba6f9d0e7

closes https://github.com/official-stockfish/Stockfish/pull/4482

bench: 4380438

diff --git a/src/search.cpp b/src/search.cpp
index 3136046d2f52f6e82ec9cc70ced94eb70b73edb1..46cca50f4b23040c60ae9c25fce257ef98f33ef9 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -899,11 +899,11 @@ namespace {
         Eval::NNUE::hint_common_parent_position(pos);
     }
 
         Eval::NNUE::hint_common_parent_position(pos);
     }
 

-    // Step 11. If the position is not in TT, decrease depth by 3.
+    // Step 11. If the position is not in TT, decrease depth by 2 (or by 4 if the TT entry for the current position was hit and the stored depth is greater than or equal to the current depth).

     // Use qsearch if depth is equal or below zero (~9 Elo)
     if (    PvNode
         && !ttMove)
     // Use qsearch if depth is equal or below zero (~9 Elo)
     if (    PvNode
         && !ttMove)

-        depth -= 3;
+        depth -= 2 + 2 * (ss->ttHit &&  tte->depth() >= depth);

 
     if (depth <= 0)
         return qsearch<PV>(pos, ss, alpha, beta);
 
     if (depth <= 0)
         return qsearch<PV>(pos, ss, alpha, beta);