Author: Gerd Isenberg
Date: 10:05:18 06/25/04
Go up one level in this thread
On June 25, 2004 at 11:25:10, Volker Böhm wrote:
>On June 24, 2004 at 12:25:25, Gerd Isenberg wrote:
>
>>On June 23, 2004 at 18:30:57, Russell Reagan wrote:
>>
>>>I am curious about a few design choices in Crafty. Some of the questions are
>>>general questions that anyone who is knowledgable could answer, so I ask this
>>>publicly instead of via email to Dr. Hyatt.
>>>
>>>First issue. Crafty is written with a lot of color-specific code, like:
>>>
>>>if (wtm) {
>>> // ...
>>>}
>>>else {
>>> // ...
>>>}
>>>
>>>This is what I think. The downside to this approach is that you double the size
>>>of the code, which is not cache friendly. This should be a bigger problem in a
>>>rotated bitboard engine where there are already cache issues.
>>
>>
>>I did that color-specific code too but maintainability is an issue here.
>>Meanwhile, for a lot of functions i use inlines with actual color parameter and
>>access (precalculated) data via color index.
>>
>> doColorSpecificStuff(color2move);
>>
>>One may easily duplicate code, e.g. by some conditional compiled macro this way
>>
>> if ( color2move == white )
>> doColorSpecificStuff(white);
>> else
>> doColorSpecificStuff(black);
>>
>>and to look from time to time what is actually faster.
>>
>>If an actual parameter of an inlined function is a compile time constant,
>>compiler may be able to optimize the additional register usage away in that
>>special inlined incarnation of that function.
>>
>>Another way to introduce compile time constants is to use integer templates with
>>C++ compiler supporting this correctly:
>>
>> doColorSpecificStuff<int color2move> () {...}
>>and to call
>>
>> if ( color2move == white )
>> doColorSpecificStuff<white>();
>> else
>> doColorSpecificStuff<black>();
>>
>>
>>>
>>>Another potential downside is that you introduce an extra branch. However, I
>>>think that this branch would be easily predictable, since the side to move will
>>>alternate back and forth, and so the branch decision will alternate.
>I may be wrong but I think the branch does not need to be predicted. The flag
>does not change right before the jump. Thus it could be loaded in a register
>verry early and the jump target can be calculated very early before the jump is
>reached. Thus no prediction needed, no failure thus very fast.
I have only a vague half knowledege about that issue.
The (AMD64) processor fetches "next" 16 bytes each cycle. Form where?
Even an unconditional return has some potential to get misspredicted.
Read chapter 4.5 and following of Hans de Vries paper.
Gerd
http://chip-architect.com/news/2003_09_21_Detailed_Architecture_of_AMDs_64bit_Core.html
Chapter 4, Opteron's Instruction Cache and Decoding
...
4.5 Large Workload Branch Prediction
Branch Prediction is the technique that makes it possible to design pipelined
processors. The outcome of a conditional branch is generally only known at the
very end of the pipeline while we need to have this information at the very
beginning of the pipeline. We need the branch outcome to know which line of
instructions to load next.
The loading of a line of instructions already takes two cycles. If we don't want
to loose anymore cycles then we must have decided on a new instruction pointer
at the end of the cycle when 16 instruction byte line arrives from the
instruction cache.
This means that there is no time at all to even look at the instruction bytes,
to try to identify conditional branches, and then to look up what the behavior
was of these branches in recent history in order to make a prediction. Doing
this alone would cost us several cycles....
>>
>>I guess for leaf-nodes e.g. as childs of all nodes, those functions are often
>>called with same color in a row for N previous moves. Miss-predictions are
>>relative more expensive, if the bodies are really small.
>>
>>Gerd
>>
>><snip>
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