Use of ttvalue as static evaluation

[Fabio Gobbato]

When is correct to use the ttvalue as static evaluation without calling the evaluation function?

The ttvalue is not returned to the lower level if (ttdepth<depth) or (ttnode==CUTNODE && ttvalue<beta) or (ttnode==ALLNODE && ttvalue>alpha).

if ttdepth<depth && ttnode==PVNODE I have an exact score and I can use as static evaluation without problems but in the other cases It's better to call the evaluation function or it's good to use a lower or upper bound without calling the evaluation?

[elcabesa]

I could be wrong, but it looks to me that Stockfish always call eval function and after it try to refine eval with ttvalue.

Code: Select all

if (ttValue != VALUE_NONE)
    if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
      eval = ttValue;

[Fabio Gobbato]

Code: Select all

	// TT lookup
	uint64_t key = get_key(B);
	hash_t *tte = tt_find(key);
	if (tte) {
		if (tte->depth > 0)
			si->tt = tte->move;
		current_eval = tte->eval;
		if (ply > 0 && can_return_tt(is_pv, tte, depth, beta, ply)) {
			tt_refresh(tte);
			return adjust_tt_score(tte->score, ply);
		}
	} else
		current_eval = in_check ? -INF : calc_eval(B);

This is the same idea in DiscoCheck and it seems it always uses the tteval without calling the evaluation.
It seems correct to me to use only a PVNODE eval

[elcabesa]

Hi fabio, I don't understand your post.

in the code you posted from discocheck it looks like tte->eval if the result of calc_eval(B) saved in TT.
so it simply avoid a calc_eval call because it has previously saved the result in the TT.

The code i posted is from stckfish and it is eval refinement, it replace static-eval with result of search

[Fabio Gobbato]

It seems to me that stockfish and discocheck use a similar idea. In the code from discocheck curret_eval = tte->eval if there is a tt hit. Else current_eval is the result from calc_eval(). Where current_eval is the static evaluation of the position used for pruning decisions.

I don't understand at all why is a good thing to update always static eval with the ones in the tt entry.
If the tt entry is a lower or upper bound you could have a big difference between this value and a good one, maybe in this cases it's good to update the static evaluation making a call to the evaluation function .

[elcabesa]

you don't understand

in discocheck tte->score is the score of the search, which could be exact result, a lower bound or an higher bound

tte->eval is the result of calc_eval(B)

he saves both score and static eval in the Trasposition table!!