Author: Dann Corbit
Date: 11:52:40 01/19/06
Go up one level in this thread
On January 19, 2006 at 14:41:07, Gerd Isenberg wrote:
>On January 19, 2006 at 13:44:01, Dann Corbit wrote:
>
>>On January 19, 2006 at 12:45:10, Chan Rasjid wrote:
>>
>>>
>>>When using bitboards, often we have arrays that require only indices 0 / 1.
>>>eg if we have no underpromotions, then there is either 1/2 knights per color.
>>>Sometimes ,in such situations, codes may be re-written without using the if ()
>>>statement. But whether such simple tricks can avoid the cost of branch
>>>misprediction ( I only have a vague idea about this ).
>>>
>>>X[i] where i = 0 / 1;
>>>
>>>1) if ( a & b ){
>>> j = X[1];
>>> }else{
>>> j = X[0];
>>> }
>>> //etc.. codes dependent on j.
>>>
>>>2) Now without if() :-
>>>
>>> j = X[(a & b)!= 0];
>>> //etc.. codes dependent on j.
>>>
>>>How (if ? ) can such a simple re-coding cause an improvement. Can there
>>>be some situations related to the above with significanr misprediction
>>>overheads ?
>>
>>If you have only 0 and 1 outcomes:
>>
>>Use an array of 2 function pointers and do this:
>>
>>assert(expression == 0 || expression == 1);
>>foo[expression]();
>
>No - avoid that for simple expressions!
You might be surprised:
http://chessprogramming.org/cccsearch/ccc.php?art_id=178068
Depends on the compiler, of course.
>>Not any great shakes, but it might be worthwhile.
>
>Branch prediction is very poor with indirect branches.
>If expression toggles each time 100% misspredictions for amd64 (and P4), while
>it is perfectly predictable with conditional jumps. Also there is additional
>call/ret overhead - the ret needs branch prediction ressources as well.
>
>>
>>Most of the time, this sort of stuff is a complete waste of time.
>>
>>The best way to speed up your program is to improve the algorithms.
>>The best goal would be to reduce the branching factor while maintaining the good
>>decisions.
>
>Yes - of course assumed perfectly done already ;-)
>
>Gerd
>
>>
>>IMO-YMMV.
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