Author: Zappa
Date: 10:13:26 12/06/05
Go up one level in this thread
On December 06, 2005 at 03:31:29, Gerd Isenberg wrote:
>On December 05, 2005 at 23:24:52, Zappa wrote:
>
>>I am getting really, really tired of coding all my evaluation twice (once for
>>white and once for black). However, one of the things that is keeping me from
>>switching to a for(i < 2) loop is that I can't do a shift!
>>
>>For example, if I have some pattern based on (pawns << 8) for white, than that
>>is (pawns >> 8) for black, and you can't do a negative shift in IA32.
>>
>>My ideas:
>>
>>Eugene will happily point out that on the Itanium doing two shifts and selecting
>>the correct value is 1 (2?) bundles.
>>
>>Otherwise on AMD64 I could do
>>
>>a) two shifts & cmov. I think 5 instructions (as compared to 1, and I have a
>>LOT of shifts).
>>
>>b) << followed by >>. 1 extra instruction but I have twice as many loads for
>>constants.
>>
>>c) rotate (X | 64-x) (but then I have the possibility of things ending up
>>rotating around).
>>
>>d) your name here . . . :)
>>
>>I am not that concerned about latency because there would usually be alot of
>>stuff around that could be rescheduled, but if I have to do 5 instructions for
>>every shift my code size will triple.
>>
>>anthony
>
>
>
>a) mixture of a and b
>
>// assuming color ::= {0,1} := {white, black}
>shiftCountWhite = shiftCount & (color-1);
>shiftCountBlack = shiftCount & -color; // shiftCountWhite ^ shiftCount
>x <<= shiftCountWhite;
>x >>= shiftCountBlack;
>
>d) conditional generalized shift.
>
>if (color)
> x >>= shiftCount;
>else
> x <<= shiftCount;
>
>If the routine is inlined and color is a compile time constant (due to unrolling
>color loops) the compiler will optimize the none taken branch away - otherwise
>how likely is a misprediction?
>
>Gerd
I think the cmov solution is still better:
srl
sll
test
cmov
vs
test
jmp
srl
jmp
sll
Of course all of these will be really slow on 32-bits anyway :(
anthony
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