Use less reduction for escaping moves

This patch reuses the threatenedPieces variable (which is calculated in movepicker)
to reduce less in the search tree the moves which escape a capture.

passed STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 314352 W: 84042 L: 83328 D: 146982
Ptnml(0-2): 1105, 35084, 84207, 35552, 1228
https://tests.stockfishchess.org/tests/view/63355f37a004bed9a2e4a17f

passed LTC:
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 90752 W: 24556 L: 24147 D: 42049
Ptnml(0-2): 59, 8855, 27123, 9296, 43
https://tests.stockfishchess.org/tests/view/63383a7735f43d649ff5fa8b

closes https://github.com/official-stockfish/Stockfish/pull/4181

bench: 4114228

src/movepick.cpp

@@ -69,6 +69,7 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHist
 
   stage = (pos.checkers() ? EVASION_TT : MAIN_TT) +
           !(ttm && pos.pseudo_legal(ttm));
+  threatenedPieces = 0;
 }
 
 /// MovePicker constructor for quiescence search
@@ -106,19 +107,17 @@ void MovePicker::score() {
 
   static_assert(Type == CAPTURES || Type == QUIETS || Type == EVASIONS, "Wrong type");
 
-  [[maybe_unused]] Bitboard threatened, threatenedByPawn, threatenedByMinor, threatenedByRook;
+  [[maybe_unused]] Bitboard threatenedByPawn, threatenedByMinor, threatenedByRook;
   if constexpr (Type == QUIETS)
   {
       Color us = pos.side_to_move();
-      // squares threatened by pawns
+
       threatenedByPawn  = pos.attacks_by<PAWN>(~us);
-      // squares threatened by minors or pawns
       threatenedByMinor = pos.attacks_by<KNIGHT>(~us) | pos.attacks_by<BISHOP>(~us) | threatenedByPawn;
-      // squares threatened by rooks, minors or pawns
       threatenedByRook  = pos.attacks_by<ROOK>(~us) | threatenedByMinor;
 
-      // pieces threatened by pieces of lesser material value
+      // Pieces threatened by pieces of lesser material value
-      threatened =  (pos.pieces(us, QUEEN) & threatenedByRook)
+      threatenedPieces = (pos.pieces(us, QUEEN) & threatenedByRook)
                        | (pos.pieces(us, ROOK)  & threatenedByMinor)
                        | (pos.pieces(us, KNIGHT, BISHOP) & threatenedByPawn);
   }
@@ -134,7 +133,7 @@ void MovePicker::score() {
                    +     (*continuationHistory[1])[pos.moved_piece(m)][to_sq(m)]
                    +     (*continuationHistory[3])[pos.moved_piece(m)][to_sq(m)]
                    +     (*continuationHistory[5])[pos.moved_piece(m)][to_sq(m)]
-                   +     (threatened & from_sq(m) ?
+                   +     (threatenedPieces & from_sq(m) ?
                            (type_of(pos.moved_piece(m)) == QUEEN && !(to_sq(m) & threatenedByRook)  ? 50000
                           : type_of(pos.moved_piece(m)) == ROOK  && !(to_sq(m) & threatenedByMinor) ? 25000
                           :                                         !(to_sq(m) & threatenedByPawn)  ? 15000

src/movepick.h

@@ -131,6 +131,8 @@ public:
   MovePicker(const Position&, Move, Value, Depth, const CapturePieceToHistory*);
   Move next_move(bool skipQuiets = false);
 
+  Bitboard threatenedPieces;
+
 private:
   template<PickType T, typename Pred> Move select(Pred);
   template<GenType> void score();

src/search.cpp

@@ -789,7 +789,7 @@ namespace {
         &&  depth < 8
         &&  eval - futility_margin(depth, improving) - (ss-1)->statScore / 303 >= beta
         &&  eval >= beta
-        &&  eval < 28031) // larger than VALUE_KNOWN_WIN, but smaller than TB wins.
+        &&  eval < 28031) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
         return eval;
 
     // Step 9. Null move search with verification search (~22 Elo)
@@ -1163,7 +1163,12 @@ moves_loop: // When in check, search starts here
           if (singularQuietLMR)
               r--;
 
-          // Increase reduction if next ply has a lot of fail high else reset count to 0
+          // Dicrease reduction if we move a threatened piece (~1 Elo)
+          if (   depth > 9
+              && (mp.threatenedPieces & from_sq(move)))
+              r--;
+
+          // Increase reduction if next ply has a lot of fail high
           if ((ss+1)->cutoffCnt > 3 && !PvNode)
               r++;