Author: Russell Reagan
Date: 05:36:55 01/30/03
Go up one level in this thread
On January 30, 2003 at 07:02:39, Ed Schröder wrote:
>On January 29, 2003 at 23:29:36, Russell Reagan wrote:
>
>>http://members.home.nl/matador/chess840.htm#INTRO
>>
>>From Ed's page...
>>
>>switch (piece_type) { case 0 : goto empty;
>> case 1 : goto white_pawn; // evaluate white pawn
>> case 2 : goto white_knight; // evaluate white knight
>> case 3 : goto white_bishop;
>> case 4 : goto white_rook;
>> case 5 : goto white_queen;
>> case 6 : goto white_king;
>> case 7 : goto black_pawn; // evaluate black pawn
>> case 8 : goto black_knight;
>> case 9 : goto black_bishop;
>> case 10 : goto black_rook;
>> case 11 : goto black_queen;
>> case 12 : goto black_king; }
>
>Any reasonable compiler will translate the above into 2 assembler statements,
>someling like:
>
> mov EAX, dword ptr piece_type
> JMP TABLE [EAX]
>
>Nothing can beat that. Just generate an ASM file to see it work.
>
>Explanation: the trick is that the compiler will generate an internal table (not
>visible for the programmer) where it calculates all the effective addresses of
>the labels mentioned in the switch/case statement.
>
>Then using the "piece_type" in register EAX it does an "indirect jump", only a
>few cycles.
>
>Of course, the sequence must be in reasonable order otherwise the compiler will
>not recognize the possibility.
>
>Ed
Hi Ed,
Let me see if I understand. It is an indirect jump, which will be at least as
slow as a mispredicted conditional. The reason this is faster is because since
you have 13 possible values for piece_type, you do ONE indirect jump as opposed
to (potentially) 12 mispredicted conditionals. Is this your reasoning?
Thanks,
Russell
>
>>On one portion of Ed's discussion of Rebel (see above), he talks about using
>>"indirect addressing". I get the impression from Ed's words that this method is
>>supposed to fast. I understand his discussion to mean that if you create a
>>switch statement like he does, you create a jump table and avoid a bunch of
>>conditionals.
>>
>>However, in past discussions, I recall hearing that using a function pointer is
>>going to be at least as slow as conditional, so I asked someone, and was told
>>that Ed's example should be no different than using a function pointer or
>>virtual functions.
>>
>>Ed talks about this method as if it is a good thing to use. So what is the
>>advantage of it? Either someone is mistaken, or Ed and the guy I talked to are
>>talking about different things.
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