Author: Uri Blass
Date: 12:46:39 02/19/04
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On February 19, 2004 at 14:48:08, Robert Hyatt wrote: >On February 19, 2004 at 14:39:16, Uri Blass wrote: > >>Today I have an array direction[64][64] that gives me different numbers for >>different directions(possible directions are queen direction that get 0-7 knight >>directions that get -2,identical squares that get -9 or no direction that gets >>-1 ). >> >>I thought about the idea to change it to the following definition: >> >>#define direction((i)(j)) directionnumber[translate[i]-translate[j]+128] >> >>The result is that I can get instead of one array of 4096 entries >>2 arrays when translate is an array of 64 entries and directionnumber is an >>array of 256 entries. >> >>My question is if it is a good idea from speed point of view. >>It will probably be a simple change when I only need to construct the 2 arrays >>and the main problem is to construct the translate array. >> >>It is probably only few hours of work but I do not like to spend time on >>constructing these arrays only to discover later that it is not productive so I >>ask for your opinion about it. >> >>Uri > > >This is never easy to answer. And unfortunately, I would suggest that you >simply try it to see. Trading additional instructions for fewer memory >references is a good idea, but you are not exactly doing that. WIth your first >approach, you do one memory reference and you are finished, even though it is >into a fairly big array. With the second approach, you do three memory >references, although the two translate array references might be to the same >cache line if the thing is 64 chars rather than 64 ints. > >I wouldn't be surprised if the change turns out to be worse however, but it >might be a "close thing". And, of course, the answer can change as your program >changes as well, so that suddenly you start accessing something that dislodges >either of the two smaller arrays, or the one bigger array from cache, making it >more expensive to access one over the other. Thanks Without clear answer there are other things with bigger priority to try. Uri
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