Prune illegal moves in qsearch earlier

The main idea is that illegal moves influencing search or
qsearch obviously can't be any sort of good. The only reason
why initially legality checks for search and qsearch were done
after they actually can influence some heuristics is because
legality check is expensive computationally. Eventually in
search it was moved to the place where it makes sure that
illegal moves can't influence search.

This patch shows that the same can be done for qsearch + it
passed STC with elo-gaining bounds + it removes 3 lines of code
because one no longer needs to increment/decrement movecount
on illegal moves.

passed STC with elo-gaining bounds
https://tests.stockfishchess.org/tests/view/60f20aefd1189bed71812da0
LLR: 2.94 (-2.94,2.94) <-0.50,2.50>
Total: 61512 W: 4688 L: 4492 D: 52332
Ptnml(0-2): 139, 3730, 22848, 3874, 165

The same version functionally but with moving condition ever earlier
passed LTC with simplification bounds.
https://tests.stockfishchess.org/tests/view/60f292cad1189bed71812de9
LLR: 2.98 (-2.94,2.94) <-2.50,0.50>
Total: 60944 W: 1724 L: 1685 D: 57535
Ptnml(0-2): 11, 1556, 27298, 1597, 10

closes https://github.com/official-stockfish/Stockfish/pull/3618

bench 4709569

diff --git a/src/search.cpp b/src/search.cpp
index fef1b518160abf00e7e6e4ac14a7e656d2ed4046..66402f0429a390d491aeaae86b26021e5f5de185 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1472,6 +1472,10 @@ moves_loop: // When in check, search starts from here
     {
       assert(is_ok(move));
 
+      // Check for legality
+      if (!pos.legal(move))
+          continue;
+
       givesCheck = pos.gives_check(move);
       captureOrPromotion = pos.capture_or_promotion(move);
 
@@ -1510,13 +1514,6 @@ moves_loop: // When in check, search starts from here
       // Speculative prefetch as early as possible
       prefetch(TT.first_entry(pos.key_after(move)));
 
-      // Check for legality just before making the move
-      if (!pos.legal(move))
-      {
-          moveCount--;
-          continue;
-      }
-
       ss->currentMove = move;
       ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
                                                                 [captureOrPromotion]