From 4c7a71a44bbe37a5e5dd971650c9b22790cae302 Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Fri, 26 Oct 2012 12:33:58 +0200
Subject: [PATCH] Fix asserts due to TT access races

In multi-threads runs with debug on we experience some
asserts due to the fact that TT access is intrinsecally
racy and its contents cannot be always trusted so must
be validated before to be used and this is what the
patch does.

No functional case.
---
 src/search.cpp | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/src/search.cpp b/src/search.cpp
index 2edaff8c..f82197f8 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -553,13 +553,13 @@ namespace {
     // smooth experience in analysis mode. We don't probe at Root nodes otherwise
     // we should also update RootMoveList to avoid bogus output.
     if (   !RootNode
-        && tte && tte->depth() >= depth
+        && tte
+        && tte->depth() >= depth
+        && ttValue != VALUE_NONE // Only in case of TT access race
         && (           PvNode ?  tte->type() == BOUND_EXACT
             : ttValue >= beta ? (tte->type() & BOUND_LOWER)
                               : (tte->type() & BOUND_UPPER)))
     {
-        assert(ttValue != VALUE_NONE); // Due to depth > DEPTH_NONE
-
         TT.refresh(tte);
         ss->currentMove = ttMove; // Can be MOVE_NONE
 
@@ -580,16 +580,22 @@ namespace {
 
     else if (tte)
     {
-        assert(tte->static_value() != VALUE_NONE);
-        assert(ttValue != VALUE_NONE || tte->type() == BOUND_NONE);
+        // Following asserts are valid only in single thread condition because
+        // TT access is always racy and its contents cannot be trusted.
+        assert(tte->static_value() != VALUE_NONE || Threads.size() > 1);
+        assert(ttValue != VALUE_NONE || tte->type() == BOUND_NONE || Threads.size() > 1);
 
         ss->staticEval = eval = tte->static_value();
         ss->evalMargin = tte->static_value_margin();
 
+        if (eval == VALUE_NONE || ss->evalMargin == VALUE_NONE) // Due to a race
+            eval = ss->staticEval = evaluate(pos, ss->evalMargin);
+
         // Can ttValue be used as a better position evaluation?
-        if (   ((tte->type() & BOUND_LOWER) && ttValue > eval)
-            || ((tte->type() & BOUND_UPPER) && ttValue < eval))
-            eval = ttValue;
+        if (ttValue != VALUE_NONE)
+            if (   ((tte->type() & BOUND_LOWER) && ttValue > eval)
+                || ((tte->type() & BOUND_UPPER) && ttValue < eval))
+                eval = ttValue;
     }
     else
     {
@@ -1118,13 +1124,13 @@ split_point_start: // At split points actual search starts from here
     // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
     ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
                                                   : DEPTH_QS_NO_CHECKS;
-    if (   tte && tte->depth() >= ttDepth
+    if (   tte
+        && tte->depth() >= ttDepth
+        && ttValue != VALUE_NONE // Only in case of TT access race
         && (           PvNode ?  tte->type() == BOUND_EXACT
             : ttValue >= beta ? (tte->type() & BOUND_LOWER)
                               : (tte->type() & BOUND_UPPER)))
     {
-        assert(ttValue != VALUE_NONE); // Due to ttDepth > DEPTH_NONE
-
         ss->currentMove = ttMove; // Can be MOVE_NONE
         return ttValue;
     }
@@ -1140,10 +1146,13 @@ split_point_start: // At split points actual search starts from here
     {
         if (tte)
         {
-            assert(tte->static_value() != VALUE_NONE);
+            assert(tte->static_value() != VALUE_NONE || Threads.size() > 1);
 
             ss->staticEval = bestValue = tte->static_value();
             ss->evalMargin = tte->static_value_margin();
+
+            if (ss->staticEval == VALUE_NONE || ss->evalMargin == VALUE_NONE) // Due to a race
+                ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
         }
         else
             ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
-- 
2.39.2