Change the flow in wich moves are generated and picked

In MovePicker we get the next move with pick_move_from_list(),
then check if the return value is equal to MOVE_NONE and
in this case we update the state to the new phase.

This patch reorders the flow so that now from pick_move_from_list()
renamed get_next_move() we directly call go_next_phase() to
generate and sort the next bunch of moves when there are no more
move to try. This avoids to always check for pick_move_from_list()
returned value and the flow is more linear and natural.

Also use a local variable instead of a pointer dereferencing in a
time critical switch statement in get_next_move()

With this patch alone we have an incredible speed up of 3.2% !!!

No functional change.

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

src/movepick.cpp

@@ -43,12 +43,12 @@ namespace {
   /// Variables
 
   CACHE_LINE_ALIGNMENT
-  const MovegenPhaseT MainSearchPhaseTable[] = { PH_STOP, PH_NULL_MOVE, PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
+  const MovegenPhaseT MainSearchPhaseTable[] = { PH_NULL_MOVE, PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
-  const MovegenPhaseT MainSearchNoNullPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
+  const MovegenPhaseT MainSearchNoNullPhaseTable[] = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
-  const MovegenPhaseT LowSearchPhaseTable[]  = { PH_STOP, PH_TT_MOVES, PH_GOOD_CAPTURES, PH_NULL_MOVE, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
+  const MovegenPhaseT LowSearchPhaseTable[]  = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_NULL_MOVE, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
-  const MovegenPhaseT EvasionsPhaseTable[] = { PH_STOP, PH_EVASIONS, PH_STOP};
+  const MovegenPhaseT EvasionsPhaseTable[] = { PH_EVASIONS, PH_STOP};
-  const MovegenPhaseT QsearchWithChecksPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
+  const MovegenPhaseT QsearchWithChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
-  const MovegenPhaseT QsearchWithoutChecksPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
+  const MovegenPhaseT QsearchWithoutChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
 }
 
 
@@ -75,9 +75,14 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
   } else
       ttMoves[1] = killers[0] = killers[1] = MOVE_NONE;
 
-  movesPicked = numOfMoves = numOfBadCaptures = 0;
+  numOfBadCaptures = 0;
   finished = false;
 
+  Color us = pos.side_to_move();
+
+  dc = p.discovered_check_candidates(us);
+  pinned = p.pinned_pieces(us);
+
   if (p.is_check())
       phasePtr = EvasionsPhaseTable;
   else if (d >= Depth(3 * OnePly))
@@ -89,118 +94,71 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
   else
       phasePtr = QsearchWithoutChecksPhaseTable;
 
-  Color us = pos.side_to_move();
+  phasePtr--;
-
+  go_next_phase();
-  dc = p.discovered_check_candidates(us);
-  pinned = p.pinned_pieces(us);
 }
 
 
-/// MovePicker::get_next_move() is the most important method of the MovePicker
+/// MovePicker::go_next_phase() generates, scores and sorts the next bunch
-/// class.  It returns a new legal move every time it is called, until there
+/// of moves when there are no more moves to try for the currrent phase.
-/// are no more moves left of the types we are interested in.
 
-Move MovePicker::get_next_move() {
+void MovePicker::go_next_phase() {
 
-  Move move;
+  movesPicked = 0;
-
+  phase = *(++phasePtr);
-  while (true)
+  switch (phase) {
-  {
-    // If we already have a list of generated moves, pick the best move from
-    // the list, and return it.
-    move = pick_move_from_list();
-    if (move != MOVE_NONE)
-    {
-        assert(move_is_ok(move));
-        return move;
-    }
-
-    // Next phase
-    phasePtr++;
-    switch (*phasePtr) {
 
   case PH_NULL_MOVE:
-        return MOVE_NULL;
-
   case PH_TT_MOVES:
-        movesPicked = 0; // This is used as index to ttMoves[]
+      return;
-        break;
 
   case PH_GOOD_CAPTURES:
       numOfMoves = generate_captures(pos, moves);
       score_captures();
       std::sort(moves, moves + numOfMoves);
-        movesPicked = 0;
+      return;
-        break;
 
   case PH_KILLERS:
-        movesPicked = 0; // This is used as index to killers[]
+      return;
-        break;
 
   case PH_NONCAPTURES:
       numOfMoves = generate_noncaptures(pos, moves);
       score_noncaptures();
       std::sort(moves, moves + numOfMoves);
-        movesPicked = 0;
+      return;
-        break;
 
   case PH_BAD_CAPTURES:
       // Bad captures SEE value is already calculated so just sort them
       // to get SEE move ordering.
       std::sort(badCaptures, badCaptures + numOfBadCaptures);
-        movesPicked = 0;
+      return;
-        break;
 
   case PH_EVASIONS:
       assert(pos.is_check());
       numOfMoves = generate_evasions(pos, moves, pinned);
       score_evasions();
       std::sort(moves, moves + numOfMoves);
-        movesPicked = 0;
+      return;
-        break;
 
   case PH_QCAPTURES:
       numOfMoves = generate_captures(pos, moves);
       score_captures();
       std::sort(moves, moves + numOfMoves);
-        movesPicked = 0;
+      return;
-        break;
 
   case PH_QCHECKS:
       // Perhaps we should order moves move here?  FIXME
       numOfMoves = generate_non_capture_checks(pos, moves, dc);
-        movesPicked = 0;
+      return;
-        break;
 
   case PH_STOP:
-        return MOVE_NONE;
+      return;
 
   default:
       assert(false);
-        return MOVE_NONE;
+      return;
   }
 }
-}
-
-
-/// A variant of get_next_move() which takes a lock as a parameter, used to
-/// prevent multiple threads from picking the same move at a split point.
-
-Move MovePicker::get_next_move(Lock &lock) {
-
-   lock_grab(&lock);
-   if (finished)
-   {
-       lock_release(&lock);
-       return MOVE_NONE;
-   }
-   Move m = get_next_move();
-   if (m == MOVE_NONE)
-       finished = true;
-
-   lock_release(&lock);
-   return m;
-}
 
 
 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
@@ -276,19 +234,25 @@ void MovePicker::score_evasions() {
   }
 }
 
+/// MovePicker::get_next_move() is the most important method of the MovePicker
+/// class. It returns a new legal move every time it is called, until there
+/// are no more moves left.
+/// It picks the move with the biggest score from a list of generated moves taking
+/// care not to return the tt move if that has already been serched previously.
 
-/// MovePicker::pick_move_from_list() picks the move with the biggest score
+Move MovePicker::get_next_move() {
-/// from a list of generated moves (moves[] or badCaptures[], depending on
-/// the current move generation phase).  It takes care not to return the
-/// transposition table move if that has already been serched previously.
-
-Move MovePicker::pick_move_from_list() {
 
   assert(movesPicked >= 0);
   assert(!pos.is_check() || *phasePtr == PH_EVASIONS || *phasePtr == PH_STOP);
   assert( pos.is_check() || *phasePtr != PH_EVASIONS);
 
-  switch (*phasePtr) {
+  while (true)
+  {
+      switch (phase) {
+
+      case PH_NULL_MOVE:
+          go_next_phase();
+          return MOVE_NULL;
 
       case PH_TT_MOVES:
             while (movesPicked < 2) {
@@ -368,8 +332,33 @@ Move MovePicker::pick_move_from_list() {
           }
           break;
 
+      case PH_STOP:
+          return MOVE_NONE;
+
       default:
+          assert(false);
           break;
       }
+      go_next_phase();
+  }
   return MOVE_NONE;
 }
+
+/// A variant of get_next_move() which takes a lock as a parameter, used to
+/// prevent multiple threads from picking the same move at a split point.
+
+Move MovePicker::get_next_move(Lock &lock) {
+
+   lock_grab(&lock);
+   if (finished)
+   {
+       lock_release(&lock);
+       return MOVE_NONE;
+   }
+   Move m = get_next_move();
+   if (m == MOVE_NONE)
+       finished = true;
+
+   lock_release(&lock);
+   return m;
+}

src/movepick.h

@@ -75,13 +75,13 @@ private:
   void score_captures();
   void score_noncaptures();
   void score_evasions();
-  Move pick_move_from_list();
+  void go_next_phase();
 
   const Position& pos;
   const History& H;
   Move ttMoves[2], killers[2];
   const MovegenPhaseT* phasePtr;
-  int movesPicked, numOfMoves, numOfBadCaptures;
+  int phase, movesPicked, numOfMoves, numOfBadCaptures;
   bool finished;
   Bitboard dc, pinned;
   MoveStack moves[256], badCaptures[64];