Author: Eugene Nalimov
Date: 22:53:30 06/21/00
Go up one level in this thread
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
Here is relevant part of Crafty (FirstOne(), the LastOne() is virtually the
same):
(1) Plain C
int FirstOne(BITBOARD arg1) {
union doub {
unsigned short i[4];
BITBOARD d;
};
register union doub x;
x.d=arg1;
#if defined(LITTLE_ENDIAN_ARCH)
if (x.i[3])
return (first_ones[x.i[3]]);
if (x.i[2])
return (first_ones[x.i[2]]+16);
if (x.i[1])
return (first_ones[x.i[1]]+32);
if (x.i[0])
return (first_ones[x.i[0]]+48);
#else
if (x.i[0])
return (first_ones[x.i[0]]);
if (x.i[1])
return (first_ones[x.i[1]]+16);
if (x.i[2])
return (first_ones[x.i[2]]+32);
if (x.i[3])
return (first_ones[x.i[3]]+48);
#endif
return(64);
}
(2) x86 assembly (MSVC)
__forceinline int FirstOne(BITBOARD a) {
__asm {
bsr edx, dword ptr a+4
mov eax, 31
jnz l1
bsr edx, dword ptr a
mov eax, 63
jnz l1
mov edx, -1
l1: sub eax, edx
}
}
Portable C version is not terribly efficient (it's trashing the cache), but good
enough. Assembly version is very fast on P6/PII/PIII, and you can speed it up if
you'll never pass zero as an argument (worst case would be ~15 CPU ticks,
average ~10). It's very easy to write portable C version that would be very
efficient on IA-64, something like this:
int FirstOne (__int64 arg) {
__int64 result = 0;
if (arg > 0xFFFFFFFF) {
arg >>= 32;
result = 32;
}
if (arg > 0xFFFF) {
arg >>= 16;
result += 16;
}
if (arg > 0xFF) {
arg >>= 8;
result += 8;
}
return first_one_8bit[arg];
}
There would be no branches after compiler would optimize the function, and
execution time would always be 9 CPU ticks (assuming that 256-bytes
first_one_8bit[] is in the L1 cache).
It's easy to write LastOne() in the same manner.
Eugene
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