ChessUSA.com TalkChess.com
Hosted by Your Move Chess & Games
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

question about symmertic evaluation
Post new topic    TalkChess.com Forum Index -> Computer Chess Club: Programming and Technical Discussions Flat
View previous topic :: View next topic  
Author Message
Gerd Isenberg



Joined: 08 Mar 2006
Posts: 1785
Location: Hattingen, Germany
PostPost subject: Re: question about symmertic evaluation    Posted: Fri May 25, 2007 6:28 pm Reply to topic Reply with quote
Uri Blass wrote:
My mistake is that I simply did not know that using / has different meaning than using >>

I thought that it is simply the same and this is the reason that I even did not think about using / for getting symmetric evaluation.

In case of knowing that / make things symmetric I could simply use it without asking questions because I had no reason to believe that / can be significantly slower than >>(I trust the compiler to make good work for every function).

I have some /24 in my code and I never cared about speed of these cases
I used >>3 instead of /8 simply because I thought it is the same and did not care if I write >>3 and not /8

Uri


Even worse - in C arithmetical shift with signed int is not specified (even twos-complement might not be used for signed int types!). It might be compiler and/or target architecture depending.

With todays processors and compilers I guess this expression is almost ever true:

assert (-1 >> 1 == ~1 + 1)

If some rare systems have only logical shift right, so that always zeros are shifted in from left to right, one has to use something like this (adapted from http://swox.com/~tege/divcnst-pldi94.pdf):

Code:
int shiftArithmeticalRight(int x, int s)
{
   unsigned int u = x ^ 0x80000000;
   return (u >> s) - (1 << (s^31));
}

with K > 0
Code:
-K >> i == -K / 2**i
is only true, if the division goes without remainder, that is the i least significant bits are zero.
Otherwise shift rounds to -oo, while idiv truncates toward zero.

Many programmers are aware of some tricks to avoid very expensive instructions like division or modulo, eg. for making hash indices by "and" for power of two sized tables instead of modulo table size. Same for shift versus division by power of two divisors with unsigned values.

In the meantime recent compiler aka code generators will likely use similar algorithms like I posted from the amd-manual - to replace division by invariant integers with reciprocal imul or sar.

Therefor it is recommend to use "/" by constants rather than own shift- or whatever tricks. But i think it does not hurt to be aware of what the compiler does - and to be aware of div/idiv is really expensive - to better avoid variable divisors in critical code.

Btw. K10 idiv-latency is 23 + number of significant bits in absolute value of the dividend. 32*32=64bit imul still takes 3 cycles.

Gerd
Back to top
View user's profile Send private message
Display posts from previous:   
Subject Author Date/Time
question about symmertic evaluation Uri Blass Wed May 23, 2007 7:53 pm
      Re: question about symmertic evaluation Uri Blass Wed May 23, 2007 8:45 pm
      Re: question about symmertic evaluation Gerd Isenberg Wed May 23, 2007 8:46 pm
            Re: question about symmertic evaluation Uri Blass Wed May 23, 2007 9:04 pm
            Re: question about symmertic evaluation Robert Hyatt Wed May 23, 2007 9:51 pm
                  Re: question about symmertic evaluation Uri Blass Wed May 23, 2007 10:04 pm
                  Re: question about symmertic evaluation Gerd Isenberg Wed May 23, 2007 10:09 pm
                        Re: question about symmertic evaluation Robert Hyatt Thu May 24, 2007 12:01 am
                              Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 7:37 am
                                    Re: question about symmertic evaluation H.G.Muller Thu May 24, 2007 8:06 am
                                          Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 9:24 am
                                                Re: question about symmertic evaluation H.G.Muller Thu May 24, 2007 10:39 am
                                                Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 11:36 am
                                                      Re: question about symmertic evaluation H.G.Muller Thu May 24, 2007 11:55 am
                                                Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 7:46 pm
                                          Re: question about symmertic evaluation Robert Hyatt Thu May 24, 2007 8:18 pm
                                                Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 8:55 pm
                                                      Re: question about symmertic evaluation Uri Blass Fri May 25, 2007 10:44 am
                                                            Re: question about symmertic evaluation Gerd Isenberg Fri May 25, 2007 6:28 pm
                                                                  Re: question about symmertic evaluation Uri Blass Fri May 25, 2007 7:42 pm
                                                                        Re: question about symmertic evaluation Gerd Isenberg Fri May 25, 2007 8:00 pm
                                    Re: question about symmertic evaluation Robert Hyatt Thu May 24, 2007 8:13 pm
                                          Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 8:33 pm
                                                Re: question about symmertic evaluation Robert Hyatt Sat May 26, 2007 1:05 am
                                                      Re: question about symmertic evaluation Gerd Isenberg Sat May 26, 2007 7:18 am
                                                            Re: question about symmertic evaluation Robert Hyatt Sat May 26, 2007 5:08 pm
                                                                  Re: question about symmertic evaluation Robert Hyatt Sat May 26, 2007 5:25 pm
                                                                  Re: question about symmertic evaluation Gerd Isenberg Sat May 26, 2007 6:23 pm
                                                                        Re: question about symmertic evaluation Robert Hyatt Sun May 27, 2007 2:48 pm
                  Re: question about symmertic evaluation Steven Edwards Thu May 24, 2007 3:42 am
                        Re: question about symmertic evaluation Robert Hyatt Thu May 24, 2007 8:20 pm
      Re: question about symmertic evaluation Robert Hyatt Wed May 23, 2007 9:50 pm
      Re: question about symmertic evaluation William H. Rogers Fri May 25, 2007 7:48 pm
            Re: question about symmertic evaluation Uri Blass Sat May 26, 2007 11:38 am
                  Re: question about symmertic evaluation H.G.Muller Sat May 26, 2007 2:07 pm
      Re: question about symmertic evaluation Vincent Diepeveen Sat May 26, 2007 7:27 pm
            Re: question about symmertic evaluation Gerd Isenberg Sat May 26, 2007 8:20 pm
            Re: question about symmertic evaluation Uri Blass Sat May 26, 2007 9:29 pm
Post new topic    TalkChess.com Forum Index -> Computer Chess Club: Programming and Technical Discussions

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum




Powered by phpBB © 2001, 2005 phpBB Group
Enhanced with Moby Threads