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Commit ID 16b31bb249ccb9f4f625001f9772799d286e2f04
Author DU-jdto
Date 2018-02-25 00:15:38 UTC
More robust interaction of singular search and iid When iid (Internal iterative deepening) is invoked, the prior value of ttValue is not guaranteed to be VALUE_NONE. As such, it is currently possible to enter a state in which ttValue has a specific value which is inconsistent with tte->bound() and tte->depth(). Currently, ttValue is only used within the search in a context that prevents this situation from making a difference (and so this change is non-functional, but this is not guaranteed to remain the case in the future. For instance, just changing the tt depth condition in singular extension node to be tte->depth() >= depth - 4 * ONE_PLY instead of tte->depth() >= depth - 3 * ONE_PLY interacts badly with the absence of ttMove in iid. For the ttMove to become a singular extension candidate, singularExtensionNode needs to be true. With the current master, this requires that tte->depth() >= depth - 3 * ONE_PLY. This is not currently possible if tte comes from IID, since the depth 'd' used for the IID search is always less than depth - 4 * ONE_PLY for depth >= 8 * ONE_PLY (below depth 8 singularExtensionNode can never be true anyway). However, with DU-jdto/Stockfish@251281a , this condition can be met, and it is possible for singularExtensionNode to become true after IID. There are then two mechanisms by which this patch can affect the search: • If ttValue was VALUE_NONE prior to IID, the fact that this patch sets ttValue allows the 'ttValue != VALUE_NONE' condition of singularExtensionNode to be met. • If ttValue wasn't VALUE_NONE prior to IID, the fact that this patch modifies ttValue's value causes a different 'rBeta' to be calculated if the singular extension search is performed. Tested at STC for non-regression: LLR: 2.95 (-2.94,2.94) [-3.00,1.00] Total: 76981 W: 17060 L: 17048 D: 42873 No functional change
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