Author: Tom Kerrigan
Date: 10:24:46 06/22/00
Go up one level in this thread
On June 22, 2000 at 11:28:39, Eugene Nalimov wrote:
>On June 22, 2000 at 11:10:25, Ed Schröder wrote:
>
>>On June 22, 2000 at 08:41:18, Robert Hyatt wrote:
>>
>>>On June 22, 2000 at 01:06:43, Ed Schröder wrote:
>>>
>>>>On June 21, 2000 at 21:51:01, Robert Hyatt wrote:
>>>>
>>>>>On June 21, 2000 at 19:14:24, Tom Kerrigan wrote:
>>>>>
>>>>>>On June 21, 2000 at 17:16:05, Robert Hyatt wrote:
>>>>>>
>>>>>>>On June 21, 2000 at 14:52:34, Tom Kerrigan wrote:
>>>>>>>
>>>>>>>>On June 21, 2000 at 14:38:23, Christophe Theron wrote:
>>>>>>>>
>>>>>>>>>On June 21, 2000 at 13:33:14, Tom Kerrigan wrote:
>>>>>>>>>
>>>>>>>>>>On June 20, 2000 at 21:39:01, Robert Hyatt wrote:
>>>>>>>>>>
>>>>>>>>>>>On June 20, 2000 at 15:52:53, Tom Kerrigan wrote:
>>>>>>>>>>>
>>>>>>>>>>>>On June 20, 2000 at 15:03:48, Robert Hyatt wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>>On June 20, 2000 at 14:07:39, Tom Kerrigan wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>>On June 19, 2000 at 21:32:56, Robert Hyatt wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>On June 19, 2000 at 20:50:11, John Coffey wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>On June 19, 2000 at 19:48:36, Larry Griffiths wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>I have found bitboards to be an even trade-off on my Pentium system. I have to
>>>>>>>>>>>>>>>>>update about 6 bitboards when a piece moves and this generates a lot of
>>>>>>>>>>>>>>>>>instructions. I get it back in my IsKingInCheck code so it evens out. I like
>>>>>>>>>>>>>>>>>to have fast move generation code, but most of my gains have been through
>>>>>>>>>>>>>>>>>alpha-beta, hash-table, killer-move and movelist ordering etc.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>Larry.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>Maybe I am too much of a novice, but I don't see yet why I should convert over
>>>>>>>>>>>>>>>>to bitboards. Is move generation faster? If so, why? My program scans the
>>>>>>>>>>>>>>>>board and uses simple loops to generate moves. Do you not have to do loops
>>>>>>>>>>>>>>>>with bitboards?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>Not to generate moves, No. You generate all the sliding piece moves with two
>>>>>>>>>>>>>>>table lookups...
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>Hmmm. I do table lookups all over my program, and none of them seem to be
>>>>>>>>>>>>>>generating any moves...
>>>>>>>>>>>>>>The fact is that you DO need to loop to generate moves in a bitboard program.
>>>>>>>>>>>>>>Maybe it's not the same loop, but it's still a loop.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>-Tom
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>Who says so? Ask the Dark Thought guys.
>>>>>>>>>>>>>Or Slate/Atkin. You only need to loop if you want to take the attack bitmap
>>>>>>>>>>>>>and turn it into a list of moves. This is not the way _all_ programs operate
>>>>>>>>>>>>>(chess 4.x, Dark Thought, others, any of which generate a few moves at a time,
>>>>>>>>>>>>>then take one and search it, without enumerating the other moves.)
>>>>>>>>>>>>>
>>>>>>>>>>>>>So loops are something you do (with bitmaps) if you want to, not because you
>>>>>>>>>>>>>have to.
>>>>>>>>>>>>>
>>>>>>>>>>>>>As far as your table lookups not generating any moves, that is a programming
>>>>>>>>>>>>>issue. Mine do. :)
>>>>>>>>>>>>
>>>>>>>>>>>>Maybe your makemove() function can take bitboards as input (i.e., here is a set
>>>>>>>>>>>>of squares that my pieces can move to) but mine sure can't.
>>>>>>>>>>>>-Tom
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>You are missing the point. A move generator _can_ emit a single move, which
>>>>>>>>>>>can be fed into MakeMove(). Read "Chess Skill in Man and Machine", the chess
>>>>>>>>>>>4.x section. They explain this pretty well. It takes zero loops to emit a
>>>>>>>>>>>single chess move. You pick the source square. You do two table lookups for
>>>>>>>>>>>bishops (say) and you have all the target squares it can move to. A single
>>>>>>>>>>>FirstOne() and you have a <to> square, which is all you need to make the move,
>>>>>>>>>>>and recursively call Search().
>>>>>>>>>>
>>>>>>>>>>So you end up having to call gen() a mess of times. I don't see how that isn't a
>>>>>>>>>>loop.
>>>>>>>>>>-Tom
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>As I understand he says that in order to generate one move he doesn't have to
>>>>>>>>>loop. That's what James explains in another post.
>>>>>>>>>
>>>>>>>>>With 0x88 or 16x you have to loop thru empty squares, he says with bitboards you
>>>>>>>>>don't have to. For each rank, file or diagonal in any configuration (by
>>>>>>>>>configuration I mean set of empty squares in this rank/file/diagonal), you can
>>>>>>>>>have precomputed arrays that instantly give you the set of squares (a bitboard)
>>>>>>>>>a sliding piece can move to.
>>>>>>>>>
>>>>>>>>>Not that I support his point of view about bitboards. I prefer to "loop thru
>>>>>>>>>empty squares" in my L1 cache rather than clobbering the same cache with
>>>>>>>>>bitboards. And anyway, the time required to extract the rank/file/diagonal from
>>>>>>>>>the "occupied" bitboard and the time required to process the resulting set of
>>>>>>>>>"can move to" squares is not required in 0x88 or 16x. And for non-sliding pieces
>>>>>>>>>(which represent in average half of the pieces present on the board), the method
>>>>>>>>>does not apply.
>>>>>>>>
>>>>>>>>Exactly, it's necessary to process the resulting bitboards. Maybe you can do
>>>>>>>>some simple operations to get an interesting set of bits, but at some point, you
>>>>>>>>have to turn those bits into something useful. There has to be a loop somewhere
>>>>>>>>which extracts the bits and does appropriate things to them. If you're lucky,
>>>>>>>>your processor has BSF/BSR (or an equivalent) and this loop is relatively fast.
>>>>>>>>But if you don't have these instructions, I bet the pretty bit patterns aren't
>>>>>>>>helping you much. Personally, I'm a little sick of people saying, "oh, one AND
>>>>>>>>operation and I'm done!" and totally ignoring everything else that has to be
>>>>>>>>done.
>>>>>>>>
>>>>>>>>-Tom
>>>>>>>
>>>>>>>You are not nearly so sick of hearing that as I am sick of hearing people talk
>>>>>>>about what you can and can't do with bitboards _without_ ever having tried them.
>>>>>>>
>>>>>>>Again, there is _no_ need for a loop. I can generate a single move (capture)
>>>>>>>with no loop of any kind. Anybody can generate a non-capture (single move)
>>>>>>>without a loop, of course. But captures are way more common to want, since
>>>>>>>they are usually tried first.
>>>>>>
>>>>>>Fine, let's review something you said earlier:
>>>>>>>Not to generate moves, No. You generate all the sliding piece moves with two
>>>>>>>table lookups...
>>>>>>
>>>>>>So how about you tell me how you're going to generate multiple moves ("all the
>>>>>>sliding piece moves") in some sort of machine-usable form without doing a loop?
>>>>>>Remember, a loop around the move generator is still a loop. Nobody's asking
>>>>>>whether or not you can generate a single stupid capture without a loop, and
>>>>>>there's no practical value in that anyway, unless you can be sure that the
>>>>>>capture is generated in the correct order.
>>>>>
>>>>>
>>>>>Easy, again from Chess Skill in man and machine. I produce a 64 bit value
>>>>>for all the bishop moves, by doing two table lookups. I already know the
>>>>><from> square to produce these moves. I use a FirstOne() call to find one
>>>>>of the destination squares (<to> square). I clear this bit, save the 64 bit
>>>>>vector, and make this move. I recursively call search. When it returns, I
>>>>>regrab the 64 bit vector, FirstOne() to find the next destination, make this
>>>>>move and again call Search() recursively. The only loop I have is the same
>>>>>loop everyone has to select the next move. I have _zero_ loops to _generate_
>>>>>the moves.
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>>I've done 0x88, 8X8, 16x16, 10x12, and probably others. I don't think that
>>>>>>>move generation is the separating point for bitboards vs the others, except
>>>>>>>for the fact that I can generate captures far easier. Bitboards help in other
>>>>>>>places as well. And on 64 bit architectures, they make a lot of sense.
>>>>>>>
>>>>>>>You ought to do what I did 5 years ago. Say "I am going to try this for a
>>>>>>>couple of years, to see if this is worthwhile." It takes a lot of time and
>>>>>>
>>>>>>Remember, I did use bitboards for a while, I know many of the issues involved.
>>>>>
>>>>>
>>>>>I have used them for 5 years. I have learned far more. And I am still finding
>>>>>new ways to do things every few months. The learning doesn't stop after using
>>>>>them "for a while".
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>>experience and false starts to do bitboards. But they _can_ work quite well.
>>>>>>>I can point to several programs that prove this, from Kaissa and chess 4.x in
>>>>>>>the 1970's, thru Crafty and several others in 2000.
>>>>>>>
>>>>>>>But they _do_ take time to learn, just like a programming language does.
>>>>>>
>>>>>>You have your way, I have my way. In case you didn't notice, I'm not saying one
>>>>>>way is better or worse. (Not in this thread, anyway.) So I don't see why you're
>>>>>>being so violently pro-bitboard. All I'm saying is that you should not jump in
>>>>>>and say that you can solve the world's problems with a single table lookup,
>>>>>>because that's simply not accurate.
>>>>>>
>>>>>>-Tom
>>>>>
>>>>>
>>>>>I'm not violently pro-bitboard at all. I simply corrected some _wrong_
>>>>>information that was being posted here. I've said hundreds of times that
>>>>>until we talk about 64 bit cpus, bitboards probably do no better than break
>>>>>even with other good approaches. I have also said, hundreds of times, that
>>>>>move generation is _not_ the most important thing done in a chess board. At
>>>>>least for my code, it is not in the top 5 when you profile things.
>>>>>
>>>>>As far as a lookup goes, I can generate _all_ sliding piece moves for a bishop,
>>>>>with two 64 bit memory loads. I can generate all captures for a bishop just as
>>>>>easily, without having to traverse the empty squares. I didn't say any more
>>>>>or any less than that. Since in a chess engine, generating captures is a very
>>>>>common thing to do, bitboards are good there. They are good in other places.
>>>>>They also have their problems. But memory bandwidth is not particularly one of
>>>>>them. And on machines like the EV6-based 64 bit architectures, I think bitmaps
>>>>>might have a real advantage due to the inherent data density they have.
>>>>
>>>>Having never had the pleasure of dealing with BB's I can understand the
>>>>2 x AND to detect the capture. But how do you get the square from the
>>>>64 bit integer since you can't address a 64 bit table to get the square
>>>>immediately so you have to write a piece of clever code for it, right?
>>>>Looks (very) time consuming to me, or?
>>>>
>>>>Ed
>>>
>>>
>>>Intel has BSF/BSR which finds the number of the first one bit, from either end
>>>of the 32 bit value you want. For 64 bits you load the left-end, and if non-
>>>zero, BSF it. If zero, you load the right-end and if not zero, you BSF it.
>>>
>>>only a few instructions. Here is the code for X86:
>>
>>I was already afraid of such an answer as the principle to get a square
>>takes many cycles and if there are more bits set you have to loop anyway
>>to get the next square(s). Why this should be superior the simple approach
>>of adding the direction add checking the board passes me.
>
>You have to use 4 loops for 4 directions (or one loop with extra if inside, that
>doesn't change anything). Typically you have (say) half of mispredicted branch
>per such simple loop, so on average you'll have 2 of them. Mispredicted branches
>are evil -- 9 clock cycles on P6/PII/PIII, 20 on Willamette. And that penalty
>you have to pay regardless of the possibility of captures.
>
>With bitboards there would be *no* mispredicted branches as long as there are 0
>or 1 possible captures and no more than 2 identical pieces. Branch predictor in
>modern CPU is capable of recognizing such regular patterns.
>
>Eugene
It seems that Bob's code for using BSF on a 64-bit value has a branch that would
be mispredicted *frequently*.
-Tom
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