Don't overwrite exsisting TT with null search value

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index aaff2f547dfd5c90c2cf9539a80957076728a2ef..4c7e1e463dc553ae53c2efee850c215fb5f24223 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1306,7 +1306,7 @@ namespace {
     tte = TT.retrieve(posKey);
     ttMove = (tte ? tte->move() : MOVE_NONE);
 
-    if (tte && ok_to_use_TT(tte, depth, beta, ply, nullStatus == ALLOW_NULLMOVE))
+    if (tte && ok_to_use_TT(tte, depth, beta, ply, nullStatus != VERIFY_NULLMOVE))
     {
         ss[ply].currentMove = ttMove; // Can be MOVE_NONE
         return value_from_tt(tte->value(), ply);
@@ -1389,13 +1389,15 @@ namespace {
             // Do zugzwang verification search for high depths, don't store in TT
             // if search was stopped.
             if (   (   depth < 6 * OnePly
-                    || search(pos, ss, beta, depth-5*OnePly, ply, FORBID_NULLMOVE, threadID) >= beta)
+                    || search(pos, ss, beta, depth-5*OnePly, ply, VERIFY_NULLMOVE, threadID) >= beta)
                 && !AbortSearch
                 && !TM.thread_should_stop(threadID))
             {
                 assert(value_to_tt(nullValue, ply) == nullValue);
 
-                TT.store(posKey, nullValue, VALUE_TYPE_NS_LO, depth, MOVE_NONE);
+                if (!tte)
+                    TT.store(posKey, nullValue, VALUE_TYPE_NS_LO, depth, MOVE_NONE);
+
                 return nullValue;
             }
         } else {