Extend stack to ss-5, and remove conditions

The `&& (ss-1)->killers[0] ` conditions are there seemingly to protect
accessing ss-5.

This is unneeded and not so intuitive (as the killer is checked for equality
with currentMove, and that one is non-zero once we're high enough in the stack,
this protects access to ss-5). We can just extend the stack from ss-4 to ss-5,
so we can call update_continuation_histories(ss-1, ..) always in search.

This goes a bit further than #1881 and addresses a comment in #1878.

passed STC:
http://tests.stockfishchess.org/tests/view/5c1aa8d50ebc5902ba127ad0
LLR: 3.12 (-2.94,2.94) [-3.00,1.00]
Total: 53515 W: 11734 L: 11666 D: 30115

passed LTC:
http://tests.stockfishchess.org/tests/view/5c1b272c0ebc5902ba12858d
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 140176 W: 23123 L: 23192 D: 93861

Bench: 3451321

diff --git a/src/search.cpp b/src/search.cpp
index 756b2d5769fb6f44b854cad829f9e1b1242746ef..4a541c5b90697137371c47cfb53662d066472e27 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -303,7 +303,11 @@ void MainThread::search() {
 
 void Thread::search() {
 
 
 void Thread::search() {
 

-  Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
+  // To allow access to (ss-5) up to (ss+2), the stack must be oversized.
+  // The former is needed to allow update_continuation_histories(ss-1, ...),
+  // which accesses its argument at ss-4, also near the root.
+  // The latter is needed for statScores and killer initialization.
+  Stack stack[MAX_PLY+8], *ss = stack+5;

   Move  pv[MAX_PLY+1];
   Value bestValue, alpha, beta, delta;
   Move  lastBestMove = MOVE_NONE;
   Move  pv[MAX_PLY+1];
   Value bestValue, alpha, beta, delta;
   Move  lastBestMove = MOVE_NONE;


@@ -313,8 +317,8 @@ void Thread::search() {
   Color us = rootPos.side_to_move();
   bool failedLow;
 
   Color us = rootPos.side_to_move();
   bool failedLow;
 

-  std::memset(ss-4, 0, 7 * sizeof(Stack));
-  for (int i = 4; i > 0; i--)
+  std::memset(ss-5, 0, 8 * sizeof(Stack));
+  for (int i = 5; i > 0; i--)

      (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
   ss->pv = pv;
 
      (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
   ss->pv = pv;
 


@@ -657,9 +661,8 @@ namespace {
                     update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
 
                 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
                     update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
 
                 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted

-                if (   (ss-1)->moveCount == 1
-                    || ((ss-1)->currentMove == (ss-1)->killers[0] && (ss-1)->killers[0]))
-                    if (!pos.captured_piece())
+                if (    ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
+                     && !pos.captured_piece())

                         update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
             }
             // Penalty for a quiet ttMove that fails low
                         update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
             }
             // Penalty for a quiet ttMove that fails low


@@ -1192,16 +1195,14 @@ moves_loop: // When in check, search starts from here
         update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
 
         // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
         update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
 
         // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted

-        if (   (ss-1)->moveCount == 1
-            || ((ss-1)->currentMove == (ss-1)->killers[0] && (ss-1)->killers[0]))
-            if (!pos.captured_piece())
+        if (   ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
+            && !pos.captured_piece())

                 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
 
     }
     // Bonus for prior countermove that caused the fail low
     else if (   (depth >= 3 * ONE_PLY || PvNode)
                 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
 
     }
     // Bonus for prior countermove that caused the fail low
     else if (   (depth >= 3 * ONE_PLY || PvNode)

-             && !pos.captured_piece()
-             && is_ok((ss-1)->currentMove))
+             && !pos.captured_piece())

         update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
 
     if (PvNode)
         update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
 
     if (PvNode)