Introduce post-lmr extensions

This idea is somewhat similar to extentions in LMR but has a different flavour.
If result of LMR was really good - thus exceeded alpha by some pretty
big given margin, we can extend move after LMR in full depth search with 0 window.
The idea is that this move is probably a fail high with somewhat of a big
probability so extending it makes a lot of sense

passed STC
https://tests.stockfishchess.org/tests/view/61ad45ea56fcf33bce7d74b7
LLR: 2.94 (-2.94,2.94) <0.00,2.50>
Total: 59680 W: 15531 L: 15215 D: 28934
Ptnml(0-2): 193, 6711, 15734, 6991, 211

passed LTC
https://tests.stockfishchess.org/tests/view/61ad9ff356fcf33bce7d8646
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 59104 W: 15321 L: 14992 D: 28791
Ptnml(0-2): 53, 6023, 17065, 6364, 47

closes https://github.com/official-stockfish/Stockfish/pull/3838

bench 4881329

diff --git a/src/search.cpp b/src/search.cpp
index edeed9c030446a9e015cf99218ce2a5ca402315b..f3a96ae29fe441bb13fc1899dcc94f804a63bd32 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1165,6 +1165,8 @@ moves_loop: // When in check, search starts here
       // Step 15. Make the move
       pos.do_move(move, st, givesCheck);
 
+      bool doDeeperSearch = false;
+
       // Step 16. Late moves reduction / extension (LMR, ~200 Elo)
       // We use various heuristics for the sons of a node after the first son has
       // been searched. In general we would like to reduce them, but there are many
@@ -1237,6 +1239,7 @@ moves_loop: // When in check, search starts here
 
           // If the son is reduced and fails high it will be re-searched at full depth
           doFullDepthSearch = value > alpha && d < newDepth;
+          doDeeperSearch = value > alpha + 88;
           didLMR = true;
       }
       else
@@ -1248,7 +1251,7 @@ moves_loop: // When in check, search starts here
       // Step 17. Full depth search when LMR is skipped or fails high
       if (doFullDepthSearch)
       {
-          value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
+          value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth + doDeeperSearch, !cutNode);
 
           // If the move passed LMR update its stats
           if (didLMR && !captureOrPromotion)