Date: Sun, 9 Feb 2003 08:22:01 -0600 From: "Jacques A. Vidrine" <nectar@FreeBSD.org> To: Terry Lambert <tlambert2@mindspring.com>, David Schultz <dschultz@uclink.berkeley.edu>, Ray Kohler <ataraxia@cox.net>, freebsd-current@FreeBSD.ORG Subject: Re: Compiling with high optimization? Message-ID: <20030209142200.GH67612@opus.celabo.org> In-Reply-To: <20030209141711.GA35708@falcon.midgard.homeip.net> References: <20030208173756.GA56030@arkadia.nv.cox.net> <20030208232724.GA20435@HAL9000.homeunix.com> <3E459BF3.BB3FC381@mindspring.com> <20030209002542.GA20812@HAL9000.homeunix.com> <3E45AD75.47C80368@mindspring.com> <20030209140357.GB67612@opus.celabo.org> <20030209141711.GA35708@falcon.midgard.homeip.net>
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On Sun, Feb 09, 2003 at 03:17:12PM +0100, Erik Trulsson wrote:
> On Sun, Feb 09, 2003 at 08:03:57AM -0600, Jacques A. Vidrine wrote:
> > On Sat, Feb 08, 2003 at 05:23:01PM -0800, Terry Lambert wrote:
> > > The compiler
> > > didn't complain when he checked it before committing it because
> > > optimization was off by default; it should have complained, e.g.:
> > ^^^^^^^^^^^^
> > Is that really what you meant? I don't believe it has anything to
> > do with optimization; rather, it is to do with lack of `warning'
> > flags. For example, if you build libc with WARNS=5 (so as to get the
> > `-Wuninitialized' flag), then you get this warning.
> >
> > > "x.c:9:warning: `foo' might be used uninitialized in this function"
>
> Some warnings are not generated unless you compile with optimization
> on. The reason for this is that to generate some of the warnings (for
> example about uninitialized variables) you need to do some dataflow
> analysis and gcc only does this when optimizing.
>
> Take for example this little program:
>
> #include <stdio.h>
> int main(void)
> {
> int a;
> printf("%d\n",a);
> return 0;
> }
>
> When compiled using 'gcc -O0 -Wall' no warnings are generated. When
> compiled with 'gcc -O1 -Wall' you get a warning that 'a' might be used
> uninitalized. (This is the case for gcc 2.95.x at least. I believe the
> situation is the same with gcc 3.x)
Ah, I see. Yes, it is the case with gcc 3.x.
cc1: warning: -Wuninitialized is not supported without -O
I don't think I ever knew that.
I should have tried it before posting, but the comment that the
problem was that `optimization was off by default' threw me --- it is
ON by default.
Cheers,
--
Jacques A. Vidrine <nectar@celabo.org> http://www.celabo.org/
NTT/Verio SME . FreeBSD UNIX . Heimdal Kerberos
jvidrine@verio.net . nectar@FreeBSD.org . nectar@kth.se
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