Remove Killers

The removal of killers on line 1774 resulted in a substantial decrease
in pre-LMR history average, so a negative history fill is applied to
counter it.

Passed Non-regression STC (vs official-stockfish#5513):
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 21984 W: 5886 L: 5645 D: 10453
Ptnml(0-2): 80, 2492, 5628, 2691, 101
https://tests.stockfishchess.org/tests/view/66a095894ff211be9d4ecb9d

Passed Non-regression LTC (vs official-stockfish#5513):
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 95430 W: 24141 L: 23995 D: 47294
Ptnml(0-2): 97, 10537, 26298, 10689, 94
https://tests.stockfishchess.org/tests/view/66a11c8d4ff211be9d4ecbf8

closes official-stockfish#5517

Bench: 1660869

src/search.cpp

@@ -98,11 +98,8 @@ Value value_to_tt(Value v, int ply);
 Value value_from_tt(Value v, int ply, int r50c);
 void  update_pv(Move* pv, Move move, const Move* childPv);
 void  update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
-void  update_killer(Stack* ss, Move move);
 void  update_quiet_histories(
    const Position& pos, Stack* ss, Search::Worker& workerThread, Move move, int bonus);
-void update_quiet_stats(
-  const Position& pos, Stack* ss, Search::Worker& workerThread, Move move, int bonus);
 void update_all_stats(const Position&      pos,
                       Stack*               ss,
                       Search::Worker&      workerThread,
@@ -222,7 +219,7 @@ void Search::Worker::iterative_deepening() {
 
     // Allocate stack with extra size to allow access from (ss - 7) to (ss + 2):
     // (ss - 7) is needed for update_continuation_histories(ss - 1) which accesses (ss - 6),
-    // (ss + 2) is needed for initialization of cutOffCnt and killers.
+    // (ss + 2) is needed for initialization of cutOffCnt.
     Stack  stack[MAX_PLY + 10] = {};
     Stack* ss                  = stack + 7;
 
@@ -490,7 +487,7 @@ void Search::Worker::clear() {
         for (StatsType c : {NoCaptures, Captures})
             for (auto& to : continuationHistory[inCheck][c])
                 for (auto& h : to)
-                    h->fill(-58);
+                    h->fill(-658);
 
     for (size_t i = 1; i < reductions.size(); ++i)
         reductions[i] = int((18.62 + std::log(size_t(options["Threads"])) / 2) * std::log(i));
@@ -584,7 +581,6 @@ Value Search::Worker::search(
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
     bestMove            = Move::none();
-    (ss + 1)->killer    = Move::none();
     (ss + 2)->cutoffCnt = 0;
     Square prevSq = ((ss - 1)->currentMove).is_ok() ? ((ss - 1)->currentMove).to_sq() : SQ_NONE;
     ss->statScore = 0;
@@ -615,7 +611,7 @@ Value Search::Worker::search(
         {
             // Bonus for a quiet ttMove that fails high (~2 Elo)
             if (!ttCapture)
-                update_quiet_stats(pos, ss, *this, ttData.move, stat_bonus(depth));
+                update_quiet_histories(pos, ss, *this, ttData.move, stat_bonus(depth));
 
             // Extra penalty for early quiet moves of
             // the previous ply (~1 Elo on STC, ~2 Elo on LTC)
@@ -1754,7 +1750,7 @@ void update_all_stats(const Position&      pos,
 
     if (!pos.capture_stage(bestMove))
     {
-        update_quiet_stats(pos, ss, workerThread, bestMove, quietMoveBonus);
+        update_quiet_histories(pos, ss, workerThread, bestMove, quietMoveBonus);
 
         // Decrease stats for all non-best quiet moves
         for (Move move : quietsSearched)
@@ -1767,12 +1763,9 @@ void update_all_stats(const Position&      pos,
         captureHistory[moved_piece][bestMove.to_sq()][captured] << quietMoveBonus;
     }
 
-    // Extra penalty for a quiet early move that was not a TT move or
-    // main killer move in previous ply when it gets refuted.
-    if (prevSq != SQ_NONE
-        && ((ss - 1)->moveCount == 1 + (ss - 1)->ttHit
-            || ((ss - 1)->currentMove == (ss - 1)->killer))
-        && !pos.captured_piece())
+    // Extra penalty for a quiet early move that was not a TT move in
+    // previous ply when it gets refuted.
+    if (prevSq != SQ_NONE && ((ss - 1)->moveCount == 1 + (ss - 1)->ttHit) && !pos.captured_piece())
         update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq, -quietMoveMalus);
 
     // Decrease stats for all non-best capture moves
@@ -1802,11 +1795,6 @@ void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
 }
 
 // Updates move sorting heuristics
-void update_killer(Stack* ss, Move move) {
-
-    // Update killers
-    ss->killer = move;
-}
 
 void update_quiet_histories(
   const Position& pos, Stack* ss, Search::Worker& workerThread, Move move, int bonus) {
@@ -1820,14 +1808,6 @@ void update_quiet_histories(
     workerThread.pawnHistory[pIndex][pos.moved_piece(move)][move.to_sq()] << bonus / 2;
 }
 
-// Updates move sorting heuristics
-void update_quiet_stats(
-  const Position& pos, Stack* ss, Search::Worker& workerThread, Move move, int bonus) {
-
-    update_killer(ss, move);
-    update_quiet_histories(pos, ss, workerThread, move, bonus);
-}
-
 }
 
 // When playing with strength handicap, choose the best move among a set of

src/search.h

@@ -66,7 +66,6 @@ struct Stack {
     int             ply;
     Move            currentMove;
     Move            excludedMove;
-    Move            killer;
     Value           staticEval;
     int             statScore;
     int             moveCount;