From c67c1ec603d22202cc35e1ddfa685a705d9b4b3c Mon Sep 17 00:00:00 2001
From: "Robert Nurnberg @ elitebook"
 <28635489+robertnurnberg@users.noreply.github.com>
Date: Tue, 19 May 2026 18:35:23 +0200
Subject: [PATCH] [bugfix] Guard is_decisive() to avoid assert triggers

This PR fixes an oversight in #6818. At least in multithreaded searches the TT scores can be corrupted, and accessing them is racy.

Passed non-reg STC:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 61792 W: 15609 L: 15428 D: 30755
Ptnml(0-2): 134, 6534, 17373, 6727, 128
https://tests.stockfishchess.org/tests/view/6a0b15396524d21ee79b868b

We also increase the number of the threads in our instrumented CI tests to make these assert failures for buggy patches more likely in future.

closes https://github.com/official-stockfish/Stockfish/pull/6830

No functional change
---
 src/tt.cpp            | 8 ++++++--
 tests/instrumented.py | 2 +-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/tt.cpp b/src/tt.cpp
index fcf70f628..359399347 100644
--- a/src/tt.cpp
+++ b/src/tt.cpp
@@ -110,9 +110,13 @@ void TTEntry::save(
     // Secondary aging. Important for elementary mate finding.
     // (*Scaler) Secondary aging on entries relevant to singular extensions
     // generally scales poorly and requires VVLTC verification.
-    else if (depth8 + DEPTH_NONE >= 5 && is_decisive(value16)
+    else if (depth8 + DEPTH_NONE >= 5
              && Bound((genBound8 & BOUND_MASK) >> BOUND_SHIFT) != BOUND_EXACT)
-        depth8--;
+    {
+        auto v16 = value16;
+        if (std::abs(v16) < VALUE_INFINITE && is_decisive(v16))
+            depth8--;
+    }
 }
 
 
diff --git a/tests/instrumented.py b/tests/instrumented.py
index dd84845ba..ed8b40c82 100644
--- a/tests/instrumented.py
+++ b/tests/instrumented.py
@@ -31,7 +31,7 @@ def get_prefix():
 
 def get_threads():
     if args.valgrind_thread or args.sanitizer_thread:
-        return 2
+        return 4
     return 1