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
// 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.
// 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+ // std::clamp has been replaced by a more robust implementation.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<NonPV>(pos, ss + 1, -(alpha + 1), -alpha, d, true);
value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha, d, true);