Try to prefetch as soon as position key is ready

Move prefetching code inside do_move() so to allow a
very early prefetching and to put as many instructions
as possible between prefetching and following retrieve().

With this patch retrieve() times are cutted of another 25%

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/position.cpp

@@ -34,6 +34,7 @@
 #include "position.h"
 #include "psqtab.h"
 #include "san.h"
+#include "tt.h"
 #include "ucioption.h"
 
 using std::string;
@@ -71,6 +72,14 @@ Position::Position(const string& fen) {
 }
 
 
+/// Position::setTranspositionTable() is used by search functions to pass
+/// the pointer to the used TT so that do_move() will prefetch TT access.
+
+void Position::setTranspositionTable(TranspositionTable* tt) {
+  TT = tt;
+}
+
+
 /// Position::from_fen() initializes the position object with the given FEN
 /// string. This function is not very robust - make sure that input FENs are
 /// correct (this is assumed to be the responsibility of the GUI).
@@ -743,6 +752,32 @@ void Position::do_move(Move m, StateInfo& newSt, Bitboard dcCandidates) {
     if (st->capture)
       do_capture_move(st->capture, them, to);
 
+    // Update hash key
+    st->key ^= zobrist[us][pt][from] ^ zobrist[us][pt][to];
+    st->key ^= zobSideToMove;
+
+    // Reset en passant square
+    if (st->epSquare != SQ_NONE)
+    {
+        st->key ^= zobEp[st->epSquare];
+        st->epSquare = SQ_NONE;
+    }
+
+    // Update castle rights, try to shortcut a common case
+    if ((castleRightsMask[from] & castleRightsMask[to]) != ALL_CASTLES)
+    {
+        st->key ^= zobCastle[st->castleRights];
+        st->castleRights &= castleRightsMask[from];
+        st->castleRights &= castleRightsMask[to];
+        st->key ^= zobCastle[st->castleRights];
+    }
+
+    bool checkEpSquare = (pt == PAWN && abs(int(to) - int(from)) == 16);
+
+    // Prefetch TT access as soon as we know key is updated
+    if (!checkEpSquare && TT)
+        TT->prefetch(st->key);
+
     // Move the piece
     Bitboard move_bb = make_move_bb(from, to);
     do_move_bb(&(byColorBB[us]), move_bb);
@@ -752,24 +787,6 @@ void Position::do_move(Move m, StateInfo& newSt, Bitboard dcCandidates) {
     board[to] = board[from];
     board[from] = EMPTY;
 
-    // Update hash key
-    st->key ^= zobrist[us][pt][from] ^ zobrist[us][pt][to];
-
-    // Update incremental scores
-    st->mgValue += pst_delta<MidGame>(piece, from, to);
-    st->egValue += pst_delta<EndGame>(piece, from, to);
-
-    // If the moving piece was a king, update the king square
-    if (pt == KING)
-        kingSquare[us] = to;
-
-    // Reset en passant square
-    if (st->epSquare != SQ_NONE)
-    {
-        st->key ^= zobEp[st->epSquare];
-        st->epSquare = SQ_NONE;
-    }
-
     // If the moving piece was a pawn do some special extra work
     if (pt == PAWN)
     {
@@ -780,7 +797,7 @@ void Position::do_move(Move m, StateInfo& newSt, Bitboard dcCandidates) {
         st->pawnKey ^= zobrist[us][PAWN][from] ^ zobrist[us][PAWN][to];
 
         // Set en passant square, only if moved pawn can be captured
-        if (abs(int(to) - int(from)) == 16)
+        if (checkEpSquare)
         {
             if (   (us == WHITE && (pawn_attacks(WHITE, from + DELTA_N) & pawns(BLACK)))
                 || (us == BLACK && (pawn_attacks(BLACK, from + DELTA_S) & pawns(WHITE))))
@@ -791,19 +808,22 @@ void Position::do_move(Move m, StateInfo& newSt, Bitboard dcCandidates) {
         }
     }
 
+    // Prefetch only here in the few cases we needed zobEp[] to update the key
+    if (checkEpSquare && TT)
+        TT->prefetch(st->key);
+
+    // Update incremental scores
+    st->mgValue += pst_delta<MidGame>(piece, from, to);
+    st->egValue += pst_delta<EndGame>(piece, from, to);
+
+    // If the moving piece was a king, update the king square
+    if (pt == KING)
+        kingSquare[us] = to;
+
     // Update piece lists
     pieceList[us][pt][index[from]] = to;
     index[to] = index[from];
 
-    // Update castle rights, try to shortcut a common case
-    if ((castleRightsMask[from] & castleRightsMask[to]) != ALL_CASTLES)
-    {
-        st->key ^= zobCastle[st->castleRights];
-        st->castleRights &= castleRightsMask[from];
-        st->castleRights &= castleRightsMask[to];
-        st->key ^= zobCastle[st->castleRights];
-    }
-
     // Update checkers bitboard, piece must be already moved
     st->checkersBB = EmptyBoardBB;
     Square ksq = king_square(them);
@@ -820,7 +840,6 @@ void Position::do_move(Move m, StateInfo& newSt, Bitboard dcCandidates) {
   }
 
   // Finish
-  st->key ^= zobSideToMove;
   sideToMove = opposite_color(sideToMove);
   gamePly++;
 
@@ -960,6 +979,8 @@ void Position::do_castle_move(Move m) {
 
   // Update checkers BB
   st->checkersBB = attacks_to(king_square(them), us);
+
+  st->key ^= zobSideToMove;
 }
 
 
@@ -1050,6 +1071,8 @@ void Position::do_promotion_move(Move m) {
 
   // Update checkers BB
   st->checkersBB = attacks_to(king_square(them), us);
+
+  st->key ^= zobSideToMove;
 }
 
 
@@ -1127,6 +1150,8 @@ void Position::do_ep_move(Move m) {
 
   // Update checkers BB
   st->checkersBB = attacks_to(king_square(them), us);
+
+  st->key ^= zobSideToMove;
 }
 
 
@@ -1407,14 +1432,15 @@ void Position::do_null_move(StateInfo& backupSt) {
   history[gamePly] = st->key;
 
   // Update the necessary information
-  sideToMove = opposite_color(sideToMove);
   if (st->epSquare != SQ_NONE)
       st->key ^= zobEp[st->epSquare];
 
+  st->key ^= zobSideToMove;
+  TT->prefetch(st->key);
+  sideToMove = opposite_color(sideToMove);
   st->epSquare = SQ_NONE;
   st->rule50++;
   gamePly++;
-  st->key ^= zobSideToMove;
 
   st->mgValue += (sideToMove == WHITE)? TempoValueMidgame : -TempoValueMidgame;
   st->egValue += (sideToMove == WHITE)? TempoValueEndgame : -TempoValueEndgame;
@@ -1654,6 +1680,7 @@ void Position::clear() {
   initialKFile = FILE_E;
   initialKRFile = FILE_H;
   initialQRFile = FILE_A;
+  TT = NULL;
 }
 
 

src/position.h

@@ -98,6 +98,7 @@ struct StateInfo {
   StateInfo* previous;
 };
 
+class TranspositionTable;
 
 /// The position data structure. A position consists of the following data:
 ///
@@ -258,6 +259,7 @@ public:
   void undo_move(Move m);
   void do_null_move(StateInfo& st);
   void undo_null_move();
+  void setTranspositionTable(TranspositionTable* tt);
 
   // Static exchange evaluation
   int see(Square from, Square to) const;
@@ -356,6 +358,7 @@ private:
   File initialKFile, initialKRFile, initialQRFile;
   StateInfo startState;
   StateInfo* st;
+  TranspositionTable* TT;
 
   // Static variables
   static int castleRightsMask[64];

src/search.cpp

@@ -663,6 +663,7 @@ namespace {
 
     // Initialize
     TT.new_search();
+    p.setTranspositionTable(&TT);
     H.clear();
     for (int i = 0; i < 3; i++)
     {
@@ -1126,7 +1127,6 @@ namespace {
       // Make and search the move
       StateInfo st;
       pos.do_move(move, st, dcCandidates);
-      TT.prefetch(pos.get_key());
 
       if (moveCount == 1) // The first move in list is the PV
           value = -search_pv(pos, ss, -beta, -alpha, newDepth, ply+1, threadID);
@@ -1297,8 +1297,6 @@ namespace {
 
         StateInfo st;
         pos.do_null_move(st);
-        TT.prefetch(pos.get_key());
-
         int R = (depth >= 5 * OnePly ? 4 : 3); // Null move dynamic reduction
 
         Value nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
@@ -1413,7 +1411,6 @@ namespace {
       // Make and search the move
       StateInfo st;
       pos.do_move(move, st, dcCandidates);
-      TT.prefetch(pos.get_key());
 
       // Try to reduce non-pv search depth by one ply if move seems not problematic,
       // if the move fails high will be re-searched at full depth.
@@ -1623,7 +1620,6 @@ namespace {
       // Make and search the move.
       StateInfo st;
       pos.do_move(move, st, dcCandidates);
-      TT.prefetch(pos.get_key());
       Value value = -qsearch(pos, ss, -beta, -alpha, depth-OnePly, ply+1, threadID);
       pos.undo_move(move);
 

src/tt.cpp

@@ -93,6 +93,16 @@ void TranspositionTable::clear() {
 }
 
 
+/// TranspositionTable::first_entry returns a pointer to the first
+/// entry of a cluster given a position. The low 32 bits of the key
+/// are used to get the index in the table.
+
+inline TTEntry* TranspositionTable::first_entry(const Key posKey) const {
+
+  return entries + ((uint32_t(posKey) & (size - 1)) * ClusterSize);
+}
+
+
 /// TranspositionTable::store writes a new entry containing a position,
 /// a value, a value type, a search depth, and a best move to the
 /// transposition table. Transposition table is organized in clusters of
@@ -145,7 +155,7 @@ void TranspositionTable::store(const Key posKey, Value v, ValueType t, Depth d,
 TTEntry* TranspositionTable::retrieve(const Key posKey) const {
 
   uint32_t posKey32 = posKey >> 32;
-  TTEntry *tte = first_entry(posKey);
+  TTEntry* tte = first_entry(posKey);
 
   for (int i = 0; i < ClusterSize; i++, tte++)
       if (tte->key() == posKey32)
@@ -154,6 +164,7 @@ TTEntry* TranspositionTable::retrieve(const Key posKey) const {
   return NULL;
 }
 
+
 /// TranspositionTable::prefetch looks up the current position in the
 /// transposition table and load it in L1/L2 cache. This is a non
 /// blocking function and do not stalls the CPU waiting for data
@@ -166,14 +177,6 @@ void TranspositionTable::prefetch(const Key posKey) const {
   _mm_prefetch((char*)first_entry(posKey), _MM_HINT_T0);
 }
 
-/// TranspositionTable::first_entry returns a pointer to the first
-/// entry of a cluster given a position. The low 32 bits of the key
-/// are used to get the index in the table.
-
-inline TTEntry* TranspositionTable::first_entry(const Key posKey) const {
-
-  return entries + ((uint32_t(posKey) & (size - 1)) * ClusterSize);
-}
 
 /// TranspositionTable::new_search() is called at the beginning of every new
 /// search. It increments the "generation" variable, which is used to