Spend less time if we have found a TB win

[SFisGOD]

There is no need to spend lots of time if we have found a TB win so we reduce the allowable total time to 20% of optimum time.

Bench: 3834252

[syzygy1]

Does this cover both the case that the root position is a TB win and the case that a TB win is found in the search? (Probably it does... I did not try to verify.)

How about doing the same or similar in case of a TB loss? (Of course only if the root position is a TB loss or if a full ply was searched so there is no move that can avoid the TB loss.)

Perhaps same if the root position is a TB draw?

[vondele]

CI is failing on this one.

However, some comments for consideration:
This seem to trigger in the case of having a TB position in the root. These are not the cases relevant for TCEC, I think, since games are adjudicated at that point?

Doing the more general case, i.e. once search has found a TB win would cover these cases, one would wonder if finding a mate score would be useful as well.

I think we prove winning mate scores (and probably TB wins), but I'm not so sure about losses.

If search finds a mate score or a TB win, and the position is not searched well enough, it will miss shortest wins, and in a few rare cases, will actually not be able to find it back in the next move.

So, not so sure that this patch is a good idea.

What does seem to happen at TCEC is that search takes something a very long time once a TB score has been found, as if the iteration takes forever to complete. This would need to be understood IMO. Maybe these things are related to #3170 or #2717

[SFisGOD]

@vondele Updated. I increased it to 20% for concerns about the rare cases of losing mate score or TB win.

[vondele]

one side effect will be that once we have found a TB win, it will become much more rare to see a mate score. Personally, I like to see a #81 or similar to appear in a tournament, it is always mind-boggling to see how an engine can find lines that deep.

[vondele]

I'll not merge this one, I think searching for a better mate etc is worthwhile.