Improve multiPV mode

Skip all moves during the Non-PV (zero-window) search which will be
searched as PV moves later anyways. We also wake sure the moves will
be reported to the GUI despite they're not being searched — some GUIs
may get confused otherwise, and it would unnecessarily complicate the
code.

Tested with MultiPV=4
STC
http://tests.stockfishchess.org/tests/view/5ce7137c0ebc5925cf070d69
LLR: 2.95 (-2.94,2.94) [0.50,4.50]
Total: 8233 W: 3708 L: 3424 D: 1101

LTC
http://tests.stockfishchess.org/tests/view/5ce798d60ebc5925cf071d17
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 7369 W: 3197 L: 2911 D: 1261

Closes official-stockfish#2163

No functional change. (in single PV mode)

src/search.cpp

@@ -292,7 +292,7 @@ void Thread::search() {
   bestValue = delta = alpha = -VALUE_INFINITE;
   beta = VALUE_INFINITE;
 
-  size_t multiPV = Options["MultiPV"];
+  multiPV = Options["MultiPV"];
   Skill skill(Options["Skill Level"]);
 
   // When playing with strength handicap enable MultiPV search that we will
@@ -870,6 +870,12 @@ namespace {
           sync_cout << "info depth " << depth / ONE_PLY
                     << " currmove " << UCI::move(move, pos.is_chess960())
                     << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
+
+      // In MultiPV mode also skip moves which will be searched later as PV moves
+      if (rootNode && std::count(thisThread->rootMoves.begin() + thisThread->pvIdx + 1,
+                                 thisThread->rootMoves.begin() + thisThread->multiPV, move))
+          continue;
+
       if (PvNode)
           (ss+1)->pv = nullptr;
 

src/thread.h

@@ -59,7 +59,7 @@ class Thread {
 
   Pawns::Table pawnsTable;
   Material::Table materialTable;
-  size_t pvIdx, pvLast, shuffleExts;
+  size_t pvIdx, multiPV, pvLast, shuffleExts;
   int selDepth, nmpMinPly;
   Color nmpColor;
   std::atomic<uint64_t> nodes, tbHits, bestMoveChanges;