Author: Eugene Nalimov
Date: 13:50:46 06/20/02
Go up one level in this thread
Ok, here is why I don't expect the speedup will be more than ~10%. I already
gave that example here a year or two ago, but I am posting it again.
Let's assume that we have a superscalar CPU with (moderate) pipeline length of
12. Each cycle it can issue 2 instructions. Each memory reference takes 2 cycles
(data is mainly in the L1 cache), register operation takes 1 cycle,
hard-to-predict branch takes 6 cycles (in half of the cases we get it right, so
there is no penalty, in other half there is 12 cycles penalty for the
mispredicten branch). Model is oversimplifcated, but it will give you an idea.
In the source we have something like
if (p->pawns & some_bit_mask)
...
On the 64-bit CPU we'll have
ld R1=[R2+offsetof(pawns)]
and R3=R1,some_bit_mask
cmp R3,0
bne ...
Total time is 10 cycles.
On the 32-bit CPU we'll have
ld R1=[R2+offsetof(pawns)]
ld R3=[R2+offsetof(pawns)+4]
and R4=R1,low_4_bytes(some_bit_mask)
and R5=R3,high_4_bytes(some_bit_mask)
or R6=R4,R5
cmp R6,0
bne ...
Total time is 11 cycles.
And there is a surprisingly lot of places in the code where even
bitboard-oriented program works with "native length" data -- all operations with
alpha, beta, score, loop counters, etc.
Eugene
On June 20, 2002 at 16:02:04, Robert Hyatt wrote:
>On June 20, 2002 at 15:58:29, Tom Kerrigan wrote:
>
>>On June 20, 2002 at 15:25:36, Sune Fischer wrote:
>>
>>>So what you are saying is that you can't just count the number of operation and
>>>use that to pridict the speed?
>>
>>Counting the operations is difficult. You can't just go through the source code
>>and count them because that doesn't tell you how often the code is run.
>>
>>>Now if Crafty is 50% 64 bit operations then we can expect a factor of two in
>>>speedup on that 50%, right?
>>
>>I think Crafty must be << 50% 64 bit operations. Think about all the data that
>>Crafty must operate on that isn't bitboards...
>>
>>-Tom
>
>
>Which data is that? The top 80% in profiling depends on bitboards heavily.
>generating moves, evaluating positions, updating the bitmaps in make/unmake,
>detecting checks, evaluating Swap().
>
>I doubt it is << 50% (where << typically means "much less than").
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