This one is simple and based on some testing. If in a PVS search and in a PV node, always update the history values if a move is found above alpha. In non-PV nodes, only do this when a score>=beta is found. I have not decided what to do if in check and in a non-PV node, but I think maybe not poluting the history values then is a good idea, and not updating them.
Does anyone have a specific "in check killer"? I have not tested that but it might be good in positions with multiple say king moves to get out of check. This could help prevent overriding killers from positions where you are not in check.
Mark
history move ordering tweak
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sje
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Re: history move ordering tweak
This would be helpful only in those cases where there were two or more check response moves and none of them was an obvious capture that could be easily determined and generated. I don't think this happens often enough to make a separate killer worthwhile, but I could be wrong and so an experiment might be in order.