Date: Tue, 13 May 1997 09:42:21 -0400
From: Bryan Murphy <bryan AT hcst DOT com>
Subject: Re: Any tips on optimizing C code?
To: jon <quacci AT vera DOT com>
cc: djgpp AT delorie DOT com
In-Reply-To: <33775c59.19219875@news.cis.yale.edu>
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> I'm interested in understanding what can be done to speed up straight
> C code. In the specific thing I am writing, I've already done the
> obvious things, like switched most calcs from FP to integer, using bit
> shifting wherever possible for multiplying and dividing, etc. But is
> there a complied source of information on just
> what-is-faster-than-what? Like, does running a "for" loop by
> decrementing rather than incrementing actually save a cycle? or does a
> "case" command actually beat a series of "if"s? Do global variable
> speed things up? I figure there must be something out there that has
> the low-down on just this sort of nitty-gritty info.
> 
> DJGPP is my compiler of choice, if that makes a difference.

Have you profiled your code with gprof yet?  That will give you the 
biggest insights.  Optimizing is too big a topic and too dependant on
your own coding style.  If you run gprof, it'll let you know where the
slow parts of your program are.  Knowing what it is you have to 
optimize is the first most important step, then just look for short
cuts or other ways around things.

Here are a few tricks I've picked up along the way that really help:

(1) Unroll For Loops (though I think DJGPP might already do this).
(2) Use look up tables
(3) Multiply by 1/numbers in stead of dividing in critical areas

etc. etc.

Just make sure you know what it is that needs optimization.  There is
no point in optimizing something that takes up only 2% of the cpu's 
power, when you are skipping something that takes up 90%.