Fix incorrect StateInfo

We use Position::set() to set root position across
threads. But there are some StateInfo fields (previous,
pliesFromNull, capturedPiece) that cannot be deduced
from a fen string, so set() clears them and to not lose
the info we need to backup and later restore setupStates->back().
Note that setupStates is shared by threads but is accessed
in read-only mode.

This fixes regression introduced by df6cb446eaf21

Tested with 3 threads at STC:
LLR: 2.95 (-2.94,2.94) [-4.00,0.00]
Total: 14436 W: 2304 L: 2196 D: 9936

Bench: 5608839

diff --git a/src/thread.cpp b/src/thread.cpp
index d358dec3f5b2da4f2644e4759b8269dd0855eeab..208cb0b1279d07f23f0b530c8b599b1444ab9bcb 100644 (file)
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -158,10 +158,12 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
   if (states.get())
       setupStates = std::move(states); // Ownership transfer, states is now empty
 
-  // We use Position::set() to set root position across threads. So we
-  // need to save and later to restore st->previous, cleared by set().
-  // Note that setupStates is shared by threads but is accessed in read-only mode.
-  StateInfo* previous = setupStates->back().previous;
+  // We use Position::set() to set root position across threads. But there are
+  // some StateInfo fields (previous, pliesFromNull, capturedPiece) that cannot
+  // be deduced from a fen string, so set() clears them and to not lose the info
+  // we need to backup and later restore setupStates->back(). Note that setupStates
+  // is shared by threads but is accessed in read-only mode.
+  StateInfo tmp = setupStates->back();
 
   for (Thread* th : Threads)
   {
@@ -171,7 +173,7 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
       th->rootPos.set(pos.fen(), pos.is_chess960(), &setupStates->back(), th);
   }
 
-  setupStates->back().previous = previous;
+  setupStates->back() = tmp;
 
   main()->start_searching();
 }