magic bitboards and nps

[benstoker]

The engine-that-dare-not-speak-its-name just implemented 'magic bitboards'. This appears to have decreased the nps. I would think the purpose of magic bitboards is to increase nps. Am I mistaken? Is there some other benefit to this beyond increase in search speed?

[bob]

The engine-that-dare-not-speak-its-name just implemented 'magic bitboards'. This appears to have decreased the nps. I would think the purpose of magic bitboards is to increase nps. Am I mistaken? Is there some other benefit to this beyond increase in search speed?

No reason it should slow down. There is essentially no speed difference between rotated bitboards and magic. At least when I converted to Crafty I saw no speed difference at all. There are some flexibility issues that favor magic bitboards, not the least of which is not needing to maintain the rotated stuff, which takes time and makes it easier to generate moves without having to do a lot of work updating 4 boards first.

[hgm]

Magic bitboards require far larger tables than rotated bitboards, and thus produce a much havier cache load. For an engine with a quite small cache footprint, where all the accesses neded for rotated bitboards would be L1 hits, the rotated bitboards could be much faster than magic.