From 7b278aab9f61620b9dba31896b38aeea1eb911e2 Mon Sep 17 00:00:00 2001
From: Joost VandeVondele <Joost.VandeVondele@gmail.com>
Date: Sun, 7 Nov 2021 11:01:03 +0100
Subject: [PATCH] Reduce use of lazyEval

In case the evaluation at root is large, discourage the use of lazyEval.

This fixes https://github.com/official-stockfish/Stockfish/issues/3772
or at least improves it significantly. In this case, poor play with large
odds can be observed, in extreme cases leading to a loss despite large
advantage:

r1bq1b1r/ppp3p1/3p1nkp/n3p3/2B1P2N/2NPB3/PPP2PPP/R3K2R b KQ - 5 9

With this patch the poor move is only considered up to depth 13, in master
up to depth 28.

The patch did not pass at LTC with Elo gainer bounds, but with slightly
positive Elo nevertheless (95% LOS).

STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.50>
Total: 40368 W: 10318 L: 10041 D: 20009
Ptnml(0-2): 103, 4493, 10725, 4750, 113
https://tests.stockfishchess.org/tests/view/61800ad259e71df00dcc420d

LTC:
LLR: -2.94 (-2.94,2.94) <0.50,3.00>
Total: 212288 W: 52997 L: 52692 D: 106599
Ptnml(0-2): 112, 22038, 61549, 22323, 122
https://tests.stockfishchess.org/tests/view/618050d959e71df00dcc426d

closes https://github.com/official-stockfish/Stockfish/pull/3780

Bench: 7127040
---
 src/evaluate.cpp | 5 ++++-
 src/search.cpp   | 2 +-
 src/thread.h     | 1 +
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 2f1d5067..a503b0c8 100644
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -988,7 +988,9 @@ namespace {
 
     // Early exit if score is high
     auto lazy_skip = [&](Value lazyThreshold) {
-        return abs(mg_value(score) + eg_value(score)) > lazyThreshold + pos.non_pawn_material() / 32;
+        return abs(mg_value(score) + eg_value(score)) >   lazyThreshold
+                                                        + std::abs(pos.this_thread()->bestValue) * 5 / 4
+                                                        + pos.non_pawn_material() / 32;
     };
 
     if (lazy_skip(LazyThreshold1))
@@ -1126,6 +1128,7 @@ std::string Eval::trace(Position& pos) {
   std::memset(scores, 0, sizeof(scores));
 
   pos.this_thread()->trend = SCORE_ZERO; // Reset any dynamic contempt
+  pos.this_thread()->bestValue = VALUE_ZERO; // Reset bestValue for lazyEval
 
   v = Evaluation<TRACE>(pos).value();
 
diff --git a/src/search.cpp b/src/search.cpp
index 090a7931..11d1df32 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -286,7 +286,7 @@ void Thread::search() {
   // The latter is needed for statScore and killer initialization.
   Stack stack[MAX_PLY+10], *ss = stack+7;
   Move  pv[MAX_PLY+1];
-  Value bestValue, alpha, beta, delta;
+  Value alpha, beta, delta;
   Move  lastBestMove = MOVE_NONE;
   Depth lastBestMoveDepth = 0;
   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
diff --git a/src/thread.h b/src/thread.h
index fae866c4..38793739 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -64,6 +64,7 @@ public:
   uint64_t nodesLastExplosive;
   uint64_t nodesLastNormal;
   std::atomic<uint64_t> nodes, tbHits, bestMoveChanges;
+  Value bestValue;
   int selDepth, nmpMinPly;
   Color nmpColor;
   ExplosionState state;
-- 
2.39.2