From:	G DOT DegliEsposti AT ads DOT it
To:	conserv3 AT infi DOT net
cc:	djgpp AT delorie DOT com
Message-ID:	<C12565DB.003ACF44.00@vega.ads.it>
Date:	Fri, 3 Apr 1998 12:58:39 +0200
Subject:	Re: ! Optimization in Practice
Mime-Version:	1.0

>I understand that Optimization produces code that the debugger has
>problems with for various reasons but, if -O3 produces such fast
This is not completely true: gdb can debug programs compiled with
optimization options. IIRC some problems can come up when the optimizer
swaps the order of some instructions and you can get confused because
it seems like some instruction are skipped, while they are only
executed in a different order.

>results, why not use it all the time?  Why would I choose -O1 or -O2
>over -O3?

I don't have the docs here now so I can write by my own memory:
IIRC, the -O3 level implies some space/time tradeoffs, trying to
reduce the execution time using methods thet increase the space
occupied by the programs (inlining functions, unrolling loops, ...)
Sometimes this can have the opposite effect, because if the
amount of code you get is too much then at run time it can
overflow the cpu code cache, lowering the execution speed.

If this can explain why use -O2 in stead of -O3, I agree
with you on one point: why -O1 in stead of -O2 ?
Maybe the switch is available for backward compatibility ?

ciao
  Giacomo

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