removed second killer move

STC with movepicker rewrite:

LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 46656 W: 12208 L: 11995 D: 22453
Ptnml(0-2): 203, 5461, 11777, 5694, 193
https://tests.stockfishchess.org/tests/view/668d98a15034141ae5999e68

Earlier version passed STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 468896 W: 120999 L: 120054 D: 227843
Ptnml(0-2): 1207, 55209, 120639, 56218, 1175
https://tests.stockfishchess.org/tests/view/668b17d2cf91c430fca58630

Earlier version passed LTC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 550524 W: 139553 L: 139877 D: 271094
Ptnml(0-2): 333, 61646, 151616, 61346, 321
https://tests.stockfishchess.org/tests/view/668b2e04cf91c430fca586b1

closes https://github.com/official-stockfish/Stockfish/pull/5472

bench 1234309

Co-authored-by: rn5f107s2 <clemens.lerchl@gmail.com>

src/movepick.cpp

@@ -20,7 +20,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <iterator>
 #include <utility>
 
 #include "bitboard.h"
@@ -35,7 +34,7 @@ enum Stages {
     MAIN_TT,
     CAPTURE_INIT,
     GOOD_CAPTURE,
-    REFUTATION,
+    KILLER,
     QUIET_INIT,
     GOOD_QUIET,
     BAD_CAPTURE,
@@ -91,14 +90,14 @@ MovePicker::MovePicker(const Position&              p,
                        const CapturePieceToHistory* cph,
                        const PieceToHistory**       ch,
                        const PawnHistory*           ph,
-                       const Move*                  killers) :
+                       Move                         km) :
     pos(p),
     mainHistory(mh),
     captureHistory(cph),
     continuationHistory(ch),
     pawnHistory(ph),
     ttMove(ttm),
-    refutations{{killers[0], 0}, {killers[1], 0}},
+    killer{km, 0},
     depth(d) {
     assert(d > 0);
 
@@ -268,19 +267,17 @@ top:
             }))
             return *(cur - 1);
 
-        // Prepare the pointers to loop over the refutations array
-        cur      = std::begin(refutations);
-        endMoves = std::end(refutations);
-
         ++stage;
         [[fallthrough]];
 
-    case REFUTATION :
+    case KILLER :
-        if (select<Next>([&]() {
+        // increment it before so if we aren't stuck here indefinitely
-                return *cur != Move::none() && !pos.capture_stage(*cur) && pos.pseudo_legal(*cur);
-            }))
-            return *(cur - 1);
         ++stage;
+
+        if (killer != ttMove && killer != Move::none() && !pos.capture_stage(killer)
+            && pos.pseudo_legal(killer))
+            return killer;
+
         [[fallthrough]];
 
     case QUIET_INIT :
@@ -297,8 +294,7 @@ top:
         [[fallthrough]];
 
     case GOOD_QUIET :
-        if (!skipQuiets
+        if (!skipQuiets && select<Next>([&]() { return *cur != killer; }))
-            && select<Next>([&]() { return *cur != refutations[0] && *cur != refutations[1]; }))
         {
             if ((cur - 1)->value > -7998 || (cur - 1)->value <= quiet_threshold(depth))
                 return *(cur - 1);
@@ -327,7 +323,7 @@ top:
 
     case BAD_QUIET :
         if (!skipQuiets)
-            return select<Next>([&]() { return *cur != refutations[0] && *cur != refutations[1]; });
+            return select<Next>([&]() { return *cur != killer; });
 
         return Move::none();
 

src/movepick.h

@@ -160,7 +160,7 @@ class MovePicker {
                const CapturePieceToHistory*,
                const PieceToHistory**,
                const PawnHistory*,
-               const Move*);
+               Move);
     MovePicker(const Position&,
                Move,
                Depth,
@@ -185,7 +185,7 @@ class MovePicker {
     const PieceToHistory**       continuationHistory;
     const PawnHistory*           pawnHistory;
     Move                         ttMove;
-    ExtMove refutations[2], *cur, *endMoves, *endBadCaptures, *beginBadQuiets, *endBadQuiets;
+    ExtMove killer, *cur, *endMoves, *endBadCaptures, *beginBadQuiets, *endBadQuiets;
     int     stage;
     int     threshold;
     Depth   depth;

src/search.cpp

@@ -123,7 +123,7 @@ 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_refutations(Stack* ss, Move move);
+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(
@@ -609,7 +609,7 @@ Value Search::Worker::search(
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
     bestMove            = Move::none();
-    (ss + 2)->killers[0] = (ss + 2)->killers[1] = 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;
@@ -934,7 +934,7 @@ moves_loop:  // When in check, search starts here
 
 
     MovePicker mp(pos, ttData.move, depth, &thisThread->mainHistory, &thisThread->captureHistory,
-                  contHist, &thisThread->pawnHistory, ss->killers);
+                  contHist, &thisThread->pawnHistory, ss->killer);
 
     value            = bestValue;
     moveCountPruning = false;
@@ -1157,7 +1157,7 @@ moves_loop:  // When in check, search starts here
         // Increase reduction for cut nodes (~4 Elo)
         if (cutNode)
             r += 2 - (ttData.depth >= depth && ss->ttPv)
-               + (!ss->ttPv && move != ttData.move && move != ss->killers[0]);
+               + (!ss->ttPv && move != ttData.move && move != ss->killer);
 
         // Increase reduction if ttMove is a capture (~3 Elo)
         if (ttCapture)
@@ -1801,7 +1801,7 @@ void update_all_stats(const Position& pos,
     // 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)->killers[0]))
+            || ((ss - 1)->currentMove == (ss - 1)->killer))
         && !pos.captured_piece())
         update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq, -quietMoveMalus);
 
@@ -1832,14 +1832,10 @@ void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
 }
 
 // Updates move sorting heuristics
-void update_refutations(Stack* ss, Move move) {
+void update_killer(Stack* ss, Move move) {
 
     // Update killers
-    if (ss->killers[0] != move)
+    ss->killer = move;
-    {
-        ss->killers[1] = ss->killers[0];
-        ss->killers[0] = move;
-    }
 }
 
 void update_quiet_histories(
@@ -1858,7 +1854,7 @@ void update_quiet_histories(
 void update_quiet_stats(
   const Position& pos, Stack* ss, Search::Worker& workerThread, Move move, int bonus) {
 
-    update_refutations(ss, move);
+    update_killer(ss, move);
     update_quiet_histories(pos, ss, workerThread, move, bonus);
 }
 

src/search.h

@@ -65,7 +65,7 @@ struct Stack {
     int             ply;
     Move            currentMove;
     Move            excludedMove;
-    Move            killers[2];
+    Move            killer;
     Value           staticEval;
     int             statScore;
     int             moveCount;