From 7970236e9ea64796d5c7597cb1aedde737751f07 Mon Sep 17 00:00:00 2001 From: Stefan Geschwentner Date: Thu, 16 Nov 2023 08:40:25 +0100 Subject: [PATCH] Fix undefined behavior in search. We use following line to clamp the search depth in some range: Depth d = std::clamp(newDepth - r, 1, newDepth + 1); Through negative extension its possible that the maximum value becomes smaller than the minimum value but then the behavior is undefined (see https://en.cppreference.com/w/cpp/algorithm/clamp). So replace this line with a safe implementation. Remark: We have in recent master already one line where up to 3 negative extensions are possible which could trigger this undefined behavior but this can only be happen for completed depth > 24 so its not discovered by our default bench. Recent negative extension tests by @fauzi shows then this undefined behavior with wrong bench numbers. closes https://github.com/official-stockfish/Stockfish/pull/4877 No functional change --- src/search.cpp | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/src/search.cpp b/src/search.cpp index 5bea5945..fa479c4b 100644 --- a/src/search.cpp +++ b/src/search.cpp @@ -1178,7 +1178,9 @@ moves_loop: // When in check, search starts here // In general we want to cap the LMR depth search at newDepth, but when // reduction is negative, we allow this move a limited search extension // beyond the first move depth. This may lead to hidden double extensions. - Depth d = std::clamp(newDepth - r, 1, newDepth + 1); + // To prevent problems when the max value is less than the min value, + // std::clamp has been replaced by a more robust implementation. + Depth d = std::max(1, std::min(newDepth - r, newDepth + 1)); value = -search(pos, ss + 1, -(alpha + 1), -alpha, d, true); -- 2.39.2