From: tomw AT tsys DOT demon DOT co DOT uk (Tom Wheeley)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: I don't understand djgpp's optimizations
Date: Mon, 07 Apr 97 23:32:35 GMT
Organization: Adventures and Diving
Message-ID: <860455955snz@tsys.demon.co.uk>
References: <33440E51 DOT 6AE AT imaginet DOT fr> <5i38mc$36q AT news DOT epcc DOT edu> <334864E0 DOT 5EDE AT cynergy DOT com DOT au> <3348B1FF DOT 167E AT gig DOT nl>
Reply-To: tw104 AT york DOT ac DOT uk
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

In article <3348B1FF DOT 167E AT gig DOT nl> bram AT gig DOT nl "Bram Stolk" writes:

> > He is doing 50,000 calculations in a 'for' loop, not pausing.
> > I'm sure this should be in the FAQ, as it's one of the
> > important aspects that everyone needs.
> 
> When benchmarking:
> 
> Make sure, that the result of the calculations is used
> afterwards (in a printf or so).
> 
> If you don't, a smart compiler will optimize it, and remove
> the calculations entirely.
> 
> This is what happened to me, after writing a very naive
> benchmark :-)
> The smart compiler was Sun's cc.

I was under the impression (can't remember where from) that gcc recognises
such loops as being (somewhat innaccurate) timing loops, and so doesn't
optimise them away.

BTW, using printf will introduce I/O into your benchmark, and that will
become the dominant factor.  Do something more useful like fill a very large
array.

-- 
:sb)