Author: Tony Werten
Date: 08:06:54 07/01/04
Go up one level in this thread
On July 01, 2004 at 10:52:09, Gerd Isenberg wrote:
>On July 01, 2004 at 08:42:38, Vincent Diepeveen wrote:
>
>>On July 01, 2004 at 04:39:24, Gerd Isenberg wrote:
>>
>>>On July 01, 2004 at 02:50:35, Tony Werten wrote:
>>>
>>>>Hi all,
>>>>
>>>>although I like the principle of bitboards, it really bothers me that I can't
>>>>seem to find a decent/fast way to evaluate weighted safe squares.
>>>>
>>>>Suppose I want to (simple) evaluate a rook, I generate a bitboard with all
>>>>reachable squares and mask off the squares attacked by lower pieces (that's no
>>>>problem).
>>>>
>>>>(This doesn't exacly generate safe squares, only the ones that aren't attacked
>>>>at all by opponents pieces are, for the remaining squares one would need a SEE,
>>>>but that's not the point )
>>>>
>>>>Now I can use this bitboard ( say rook on e4 ), mask the rank state, and look in
>>>>a precomputed table how this rankstate scores on an e rank. No problem.
>>>>
>>>>But how to do the files ? If I use the rotated board, I need to have the
>>>>opponents attackboard in this rotated board as well, wich would be very costly
>>>>to compute (ie also for the bishops,queens ) and very complicated.
>>>>
>>>>Any ideas ? Am I missing something ?
>>>>
>>>>BTW, doing a popcount isn't a solution, since it violates the elegance of
>>>>bitboards ( and is slow ?)
>>>
>>>Ok, bitscan traversing and psq-lookup is a even more violation IMHO ;-)
>>>
>>>For safe rook attacks one may look for boolean pattern/properties (depending on
>>>the game state). Intersect the attack set with several small areas of the board,
>>>like center, remaining extended center, squares near opposite or own king,
>>>passers, squares behind passers or other movable pawns e.g. to support a
>>>minority attack, squares on open/halfopen files, squares on some rook
>>>trajetories to opposite king and what ever else.
>>>
>>>With those sets, look whether they are empty and if not, probably look whether
>>>the population count is greater one.
>>>
>>>That may be implemented loopless and with some preconditions:
>>>
>>>if ( safeRookAttacks )
>>>{
>>> set = safeRookAttacks & someAreaOfInterest;
>>> bunus += f1*(set!=0) + f2*((set&(set-1))!=0);
>>
>>2 times a 64 bits multiply AUCH. That's slow.
>
>oups bunus, hehe, of course "and" with -1|0 masks.
>Or is there any compiler generating boolean muls here?
>But agreed, i should make it more readable:
>
> bonus += (f1 & atleastOne(set)) + (f2 & atLeastTwo(set));
>
>with inlined
>
> int atleastOne(BitBoard set) {return -(set!=0);}
> int atleastTwo(BitBoard set) {return -((set&(set-1))!=0);}
>
>>
>>> ...
>>>}
>>>
>>
>>I assume Tony isn't doing such simple things as you propose here.
>>
>>Basically Tony might want to use some chessknowledge evaluating each square.
>>
>>And the reality is that scanning in bitboards is just too slow.
>
>With bitboards you have several choices, to count weighted attacks.
>I agree that serialising the whole bitboard using some kind of piece square
>tables is too slow. Disjoint population counting with some area bitboards is
>possible. For small area sets, it is most often enough to check whether there is
>one or more attack - that was my point, a kind of cheap popcount.
>
>Of course ranks attacks may be easily computed by extracting appropriate bytes
>from several bitboards and to do some lookup into precalculated tables.
>
>
>>
>>>Of course such statements "cry" for MMX,SSE2,Itanium or AltiVec SIMD
>>>instructions like pcmpeq to build -1|0 masks to "and" with. Itanium has popcnt
>>>instruction, and there are rumors that AMD64 has undocumented popcnt too,
>>>somewhere in the bt opcode range.
>>
>>So in a 32/64 bits programs you can execute with 3 instructions a cycle without
>>any problems using 16 registers and skip complex patterns with a single 'if',
>>and with your 'advanced SSE code' for the opteron you can do 1 multiply+add per
>>cycle (note a multiply eats 2 cycles) in the meantime you must convert back and
>>forth from SSE to normal registers.
>>
>>In short you cripple your program to something doing 1 instruction a cycle at
>>maximum and you extensivly use 64 bits multiplications to achieve your goal and
>>all kind of fancy tricks.
>>
>>But even the simplest pattern in C like asked by Tony you write unreadable code
>>like :
>>
>> set = safeRookAttacks & someAreaOfInterest;
>> bonus += f1*(set!=0) + f2*((set&(set-1))!=0);
>>
>>How do you plan to debug thousands of patterns?
>
>I don't see the point, where is the difference in debugging?
Suppose you want to give a bonus if you attack a square, 1 away from the king
and a smaller bonus for 2 away and an even smaller 1 for 3 away.
If you loop over the squares (with fe 0x88) you will get some thing like
bonus+=king_attack_bonus[king_dist[square-kingsquare+0x77]]
I only need a k_dist table (used very often) and a bonus table wich I can keep
at a different place
With your method it would be
if (attackbb & one_from_king[kingsquare]) bonus += F
if (attackbb & two_from_king[kingsquare]) bonus += G
if (attackbb & three_from_king[kingsquare]) bonus += H
where a lot more gets stuffed inside the code and in addition some rarely used
big tables.
Tony
>
>if ( condition ) bonus += F;
>
>versus
>
>bonus += F & ConditionMinusOneorZero;
>
>The latter is more SIMD-wise, so with SSE2 one may process 16-byte masks at
>once, so pcmpeqb, pand, psadbw.
>
>>
>>Another question, when do you get parallel with Isichess?
>
>I'm working on it. The problem is the 40h++ job.
>Ready to take off, wake up time is half past four tomorrow ;-(
>
>Gerd
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