Re: [gnugo-devel] engine/influence.c (and DFA)

[Marco Scheurer]

On Tuesday, September 3, 2002, at 06:11  pm, Arend Bayer wrote:
> On Tue, 3 Sep 2002, Marco Scheurer wrote:
>
> > Speaking of micro optimizations and speed/space trade offs, I've been
> > wondering about these:
> >
> > - There are many bi-dimensional arrays. Would it make sense to replace
> > them by pointers to pointers? Two indirections are supposed to be 
> > faster
> > than the arithmetic needed to access elements in a bi-directional 
> > array.
> > The syntax to read an element is the same, but some initialization code
> > is needed.
>
> I think in all performance relevant spots (maybe with the exception of
> influence.c), we are using one-dimensional arrays since the
> 1D-transition by Gunnar and Inge early in 3.1.x.

influence.c still has bidimensional arrays, I think, and even with the 
1D transition you still have a multiplication, don't you? Based on what 
I read a (probably too long) while ago in Numerical Recipes in C, double 
indirection was faster than multiply, at least on Pentium.

> Also, isn't chasing pointers supposed to be the worst possible you
> can do the the cache? (I.e., most likely to result in cache misses.)

Yes, this could be an issue, and it probably depends, again, on 
architectures and compilers. This makes me think that all these 
optimizations (loop unrolling, inlining, etc) should probably be left 
out.

On the other hand, I've also been wondering what the PPC Altivec could 
do to the pattern matching code...

Marco Scheurer
Sen:te, Lausanne, Switzerland    http://www.sente.ch