Author: Volker Böhm
Date: 08:04:50 06/03/04
Go up one level in this thread
On June 01, 2004 at 19:17:09, Gerd Isenberg wrote:
><snip>
>>>Hi Volker,
>>>
>>>Not sure, i guess current compiler are not aware of the RCL trick.
>>>So you may still use (inline) assembler for best perfomance.
>>>
>>>Something like this in C may be result in a chain of cmp, setnz and lea
>>>instructions:
>>>
>>>bitfield = (q8 != 0);
>>>bitfield <<= 1;
>>>bitfield += (q7 != 0);
>>>bitfield <<= 1;
>>>...
>>>bitfield += (q0 != 0);
>>>
>>>or more compact:
>>>
>>>bitfield = ...((((((q8!=0)<<1)+(q7!=0))<<1)+(q6!=0))<<1)...
>>>
>>>Btw. have you considered using bitboards?
>>>
>>>Cheers,
>>>Gerd
>>
>>No I use 0x88. Seems best to me to build incremental attack tables. Thanks for
>>the answer, yes adding a boolean may work ... testing ...
>>
>>Volker
>
>If you introduce more of that stuff from packing square metrics to setwise
>metrics, you may consider bitboards one time ;-)
>
>Anyway, even if you avoid (difficult or easy to predict?) branches with
>cmp-setCC/rcl/adc instructions the problem is a kind of stall, because setCC
>needs to wait for the flag outcome of cmp.
>
>Therefore to break dependencies it may be smarter to build "boolean" {-1,0}
>values inside a register instead of {true,false} carry flag. That makes the code
>much more able to work simultaniuosly.
>
>assuming 32-bit int, signed shift and positive q values:
>
>bitfield = ((-q8>>31) & 128)
> | ((-q7>>31) & 64)
> | ((-q6>>31) & 32)
> | ((-q5>>31) & 16)
> | ((-q4>>31) & 8)
> | ((-q3>>31) & 4)
> | ((-q2>>31) & 2)
> | ((-q1>>31) & 1);
>
>OTOH if the conditional jumps are most often predicted correctly...
In this case it is if ((Field is not own piece or border) | (Field is not
attacked by opponent)) king may escape here ...
This is hardly predictable.
>
>Cheers,
>Gerd
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