Author: Ed Schröder
Date: 11:32:37 01/30/03
Go up one level in this thread
On January 30, 2003 at 13:03:49, Robert Hyatt wrote:
>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.
>
>I would hope *not*.
>it should generate:
>
> mov eax, piece_type
> cmp eax,0
> jb skip
> cmp eax,12
> jna table[eax]
> skip:
Of course, but no need to go into ASM details. The point is its speed, replacing
a whole block of c-code into a few ASM instructions.
Also in ASM I don't have to do the 0-12 range check because I already know the
range is valid, so just 2 instructions :)
>Somebody had _better check the switch values.
>BTW the jna table[eax] is not particularly efficient from a prediction point of
>view.
???
Please explain.
Ed
>>
>>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
>>
>>
>>>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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