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Re: [vox-tech] a comment on Pete's fast-loop question
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Re: [vox-tech] a comment on Pete's fast-loop question



On Sun 18 Jun 06,  8:13 PM, Micah J. Cowan <micah@cowan.name> said:
> On Sun, Jun 18, 2006 at 08:26:21PM -0400, Peter Jay Salzman wrote:
> > On Sun 18 Jun 06, 12:06 AM, Norm Matloff <matloff@cs.ucdavis.edu> said:
> > > Pete Salzman asked about optimizing a loop in terms of execution speed,
> > > by having the loop index go from high value to low instead of vice
> > > versa.  Some discussion ensued in terms of what machine instructions a
> > > compiler could take advantage of in this manner.
> > > 
> > > One point that you might consider, Pete, is that these considerations
> > > are kind of nickel-and-dime in comparison to things like memory
> > > hierarchy issue.  There is much better payoff potential in writing code
> > > in such a way as to minimize cache misses, which cause major time
> > > penalties, and page faults, which cause catastrophic time penalties.
> >  
> > Yeah, I know.  My company hired me for algorithmic optimizations --- my
> > knowledge of devising a set of algorithms to accomplish a given task (e.g.,
> > solving dense matrices, choosing a particular Monte Carlo for a given
> > problem).
> > 
> > I guess my attitude towards this was more along the lines of "while I'm in
> > the neighborhood, why don't I stop by and say hello".  It certainly wouldn't
> > be prudent to re-implement code to use nickel and dime optimization, but
> > while thoughts in my neurons travel down my CNS and translate into my
> > fingers tapping on keys, I might as well tap on keys in such a way that the
> > nickel and dime optimizations appear on vim's terminal.  The price of
> > admission is free, so I might as well.
> > 
> > OK.  I took that nutty metaphor as far as it can go.  Hope it made sense.
> > 
> > > There is even a book on this, I believe in the Intel Press series.
> > 
> > Definitely sounds like a book I should pick up.  I'll Google for it!
> 
> Given your explanation above, you might also find Hacker's Delight
> to be enjoyable reading (if you haven't read it already).
> 
> ...just promise me you'll document the hell out of pretty much anything
> you /use/ out of there... :-)
 
You know... a long time ago I read a /. book review of this book and it's
been in the back of my head for a long time.  I never had the money to
purchase it (I think it's still on my nerdbooks.com wishlist).  You just
reminded me of it.  Yeah, that's a book I can afford now; I think I'll take
the plunge.

I set up a wiki on www.dirac.org/programming to document everything I learn
about programming, numerics, and numerical algorithms.  Slowly but surely, a
lot of my saved vox-tech messages (like this one) are getting documented
there.  I also plan on documenting things I learn from books like Hacker's
Delight.

Thanks for reminding me of this book!

Pete
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