Date: Tue, 4 Nov 2003 08:57:14 +0000 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Zhang Weiwu <weiwuzhang@hotmail.com> Cc: freebsd-questions@freebsd.org Subject: Re: optimize with -O2 or -O? Message-ID: <20031104085714.GC31448@happy-idiot-talk.infracaninophile.co.uk> In-Reply-To: <3FA76159.6010807@hotmail.com> References: <3FA76159.6010807@hotmail.com>
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On Tue, Nov 04, 2003 at 04:20:41PM +0800, Zhang Weiwu wrote:
> Hello. I'm running 5.1-RELEASE.
>=20
> I read the gcc man page. It is not clear what -O implys. Is -O equal to=
=20
> -O1 or -O2? If I wish to optimize making does it make sense to add=20
> CFLAGS=3D-O2 in the make.conf(5)?
-O is the same as -O1
Ah -- the shibboleth of cranking up the optimisation level to make
things run *really fast*. First of all, for general purpose code, -O
gets you most of the available optimisation benefits without causing
any problems. You can try CFLAGS=3D-O2 in /etc/make.conf if you like,
and most things will work OK but by no means all: some code will not
compile correctly under high optimization levels. The immediate
response if you report this sort of thing as a bug will be to see if
the problem is repeatable using a lower optimisation level.
Secondly, you will achieve a much greater performance improvement by
setting the CPUTYPE variable appropriately.
Cheers,
Matthew
--=20
Dr Matthew J Seaman MA, D.Phil. 26 The Paddocks
Savill Way
PGP: http://www.infracaninophile.co.uk/pgpkey Marlow
Tel: +44 1628 476614 Bucks., SL7 1TH UK
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