Author: Christophe Theron
Date: 17:00:12 08/20/03
Go up one level in this thread
On August 20, 2003 at 14:30:47, Robert Hyatt wrote:
>On August 20, 2003 at 13:05:17, Christophe Theron wrote:
>
>>On August 20, 2003 at 03:59:38, Johan de Koning wrote:
>>
>>>On August 19, 2003 at 22:11:14, Robert Hyatt wrote:
>>>
>>>>On August 19, 2003 at 20:06:58, Mathieu Pagé wrote:
>>>>
>>>>>Hi,
>>>>>
>>>>>The fact:
>>>>>
>>>>>I have this question i read at some place that it is faster to unmake a move
>>>>>than to save the state of the game before moving then restoring it when we want
>>>>>to unmake the move.
>>>>>
>>>>>For the moment my engines did not implement unmake() (it is still buggy).
>>>>>
>>>>>My thougth:
>>>>>
>>>>>Since bitboard computation are slow (on 32 hardware) i think that it can be
>>>>>slower to unmake the move than to save the state. I friend of me that is lot
>>>>>better than me at optimizing code also think that.
>>>>>
>>>>>My questions:
>>>>>
>>>>>Are you all using unmake() function or there is some of you that found that
>>>>>saving the state is better ?
>>>>
>>>>
>>>>
>>>>read the comments from Crafty in main.c. I started out using what is
>>>>commonly called "copy/make" as that worked well in Cray Blitz. But it
>>>>didn't work well in the PC. The PC has very limited memory bandwidth,
>>>>when you compare the speed of memory to the speed/demands of current
>>>>processors. If you keep the board in cache, and update it there, it is
>>>>more efficient than to copy it from real memory to real memory...
>>>
>>>I hate to play Vincent here, but real memory is not an issue.
>>>
>>>If you manage to keep the deepest few plies worth of position structs in L1
>>>cache, then bandwith is pretty decent on the PC. And it has been ever since them
>>>PCs were endowed with cache.
>>>
>>>Copying a struct does take time, and it can easily be pinpointed. Saving and
>>>restoring and unupdating also takes time, but is harder to identify. Especially
>>>since the stress on code cache and branch prediction don't show up in a run time
>>>profile.
>>>
>>>... Johan
>>
>>
>>
>>I'm not surprised by Bob results on this issue, as Crafty has a *lot* of things
>>to save/restore, and all of them are rather big structures.
>>
>>In a non-bitboard program like Chess Tiger, saving/restoring is probably faster.
>>At least it is in Chess Tiger.
>>
>>I do not know if you are using bitboards in The King. Do you?
>>
>>Actually I'm using a mix of undo and restore: I do not save the chessboard
>>itself because undoing a move involves very few read/write operations. So I undo
>>the move "manually" on the chessboard but restore with a memcpy a single
>>structure that holds the rest of the chessboard and a part of the search state.
>>I seem to remember that this structure is less than 40 bytes big, so restoring
>>it is really no problem, and as you pointed out most of the time the data to be
>>restored still lies in the L1 cache.
>>
>>In any case I cannot imagine that restoring the hash key for the current
>>position from memory could be slower than computing it again by undoing a
>>sequence of XORs (at least 2) on a 64 bits integer...
>>
>>
>>
>> Christophe
>
>
>I just checked. I think my previous 168 was wrong. I think a complete
>"structure" would now contain about 256 bytes which would have to be copied
>for each copy/make cycle.
>
>In Cray Blitz, without the bitmap stuff, we did a combination. We didn't copy
>the chess board, we did make/unmake there. But we did copy things like hash
>signatures and incrementally updated stuff (number of pawns on a file, open
>files, etc) so that we didn't have to unmake those.
Yes that's exactly what I do. The structure I'm talking about contains exactly
that: hash key, incremental score, the last computed part of the dynamical
score, king safety information, pawn structure information, last move, last
captured piece, ...
But I understand that if you have a bigger structure that saves various
bitboards at some point saving/restoring might be too costly because of memory
wait states.
That's a part I do not like much: when the hidden (internal) architecture of the
machine starts to have such a variable impact on the program's design. I mean
from one computer to another, and depending on the processor/memory/mainboard
manufacturer, one should completely redesign the program to get the best
performances.
I don't like the idea that someone just counting clock cycles could improve more
a chess program than someone thinking about the chess algorithms themselves.
As a matter of facts I had to do cycle-counting myself in Chess Tiger, and I
hate that.
Christophe
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