how can IID with null allowed cause wrong mate move from tt

[Ras]

It's a little disingenious to say that this is what you suggested though, since what you said was "positions that arise from a null-move don't. Belong in the main transposition table", which is wrong.

Well I still don't see much use in storing them because they aren't "real" game positions. Though of course if the side to move is handled correctly, they won't cause harm. Maybe it's also a question of how much memory you have to spare even for unreal game positions.

[Ras]

How big is this effect? I would expect killers and hash moves to dominate very quickly.

I've measured that for the 3 ply depth pre-search, which is plain alpha-beta Negamax with move sorting. During normal openings, the number of QS entry nodes went down by 40%.

Doesn't sound much, given depth==3, but I have seen positions where the pre-sorting alone took more than 2 seconds. Which I noticed because I hadn't expected this, hadn't been triggering the 2 seconds watchdog during pre-search and sometimes got unexpected watchdog system resets. In blitz games, 2 seconds for the sorting alone is a lot.

[Evert]

Well I still don't see much use in storing them because they aren't "real" game positions.

What does that have to do with anything? Null move isn't legal, but that doesn't stop anyone from trying it.
The point is that the positions following null-move are still searched normally. As such, anything that helps the search normally will help there.

[Ras]

Null move isn't legal, but that doesn't stop anyone from trying it.

The issue is that IMO, it doesn't make sense to do a deep search with positions that are illegal anyway. But if the search isn't deep, then there isn't much to transpose, so hash tables won't be that useful (and entries will be blocked).

As such, anything that helps the search normally will help there.

Well if the null search is deep enough, yes. But I'd invest the limited time rather in deeper search for positions that actually could arise on the board.

[Karlo Bala]

Null move isn't legal, but that doesn't stop anyone from trying it.

The issue is that IMO, it doesn't make sense to do a deep search with positions that are illegal anyway. But if the search isn't deep, then there isn't much to transpose, so hash tables won't be that useful (and entries will be blocked).

As such, anything that helps the search normally will help there.

Well if the null search is deep enough, yes. But I'd invest the limited time rather in deeper search for positions that actually could arise on the board.

It seems that you are trying to disprove the well proven concept
The problem is that no one knows in advance which positions are relevant.