Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!uwvax!mcvoy
From: mcvoy@uwvax.UUCP
Newsgroups: comp.sys.m68k
Subject: Re: Incidentally ... (was Re: Recent Motorola ad)
Message-ID: <3425@rsch.WISC.EDU>
Date: Sat, 4-Apr-87 18:14:13 EST
Article-I.D.: rsch.3425
Posted: Sat Apr  4 18:14:13 1987
Date-Received: Sun, 5-Apr-87 13:19:46 EST
References: <362@sbcs.UUCP> <1466@ncr-sd.SanDiego.NCR.COM> <580@plx.UUCP> <73@eps2.UUCP> <6134@mimsy.UUCP>
Reply-To: mcvoy@rsch.WISC.EDU (Larry McVoy)
Distribution: comp
Organization: U of Wisconsin CS Dept
Lines: 41

(Chris Torek) writes:
>>	while (wy--)  {
>>		j = wx;
>>		while (j--)  {
>
>(The above is an inner loop.)  Such loops should usually (always?)
>be written as
>
>		while (--j >= 0)
>
>---assuming j is signed.  Why?  They both do the same thing, but
> [good justification that (--j >= 0) is faster]


I'm not sure that it worth everyones time to be thinking about this.
And I speak from a certain amount of experience, to wit: last semester
I had to write a fake TCP/IP from scratch and I wanted to make a fast
implementation.  Over the course of the semester I gathered a directory
full of t.{c,s} files where I was looking at exactly this sort of
thing.  I really wanted fast code.  As it turned out, my implementation
did not gain much at all from all this extra work - mainly because it
as just not used.  The old "90% of the time 10% of the code" saw
applies.  I would have been much better off to profile my code and
rewrite the bottlenecks.

Please don't take this the wrong way - Chris is a smart guy, and he's
right in a technical sense.  And I get a warm fuzzy feeling from
writing my code in a efficient manner too.  It's just that I think it's
misleading to to appear worried about this sort of thing in general
sense - it really belongs in the "profiling code" section, not "general
programming tips".  Kernighan and Plauger say "Premature Optimization
is the root of all evil".  I think this is a bit extreme, but I agree
in principle.

Food for thought (?),

--larry
-- 
Larry McVoy 	        mcvoy@rsch.wisc.edu  or  uwvax!mcvoy

"It's a joke, son! I say, I say, a Joke!!"  --Foghorn Leghorn