Date: Sat, 24 Feb 2001 13:33:14 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Kris Kennaway <kris@obsecurity.org> Cc: Alexander Leidinger <Alexander@Leidinger.net>, arch@FreeBSD.ORG Subject: Re: cvs commit: src/share/mk sys.mk Message-ID: <Pine.BSF.4.21.0102241315540.25623-100000@besplex.bde.org> In-Reply-To: <20010223010514.A41551@mollari.cthul.hu>
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On Fri, 23 Feb 2001, Kris Kennaway wrote:
> On Thu, Feb 22, 2001 at 10:53:52PM +0100, Alexander Leidinger wrote:
> > > Hmm, that's not what's documented in the gcc info docs.
> >
> > The docs are wrong.
> >
> > > `-march=CPU TYPE'
> > > Generate instructions for the machine type CPU TYPE. The choices
> > > for CPU TYPE are the same as for `-mcpu'. Moreover, specifying
> > > `-march=CPU TYPE' implies `-mcpu=CPU TYPE'.
> >
> > That's not true, test it on your own (see my previous mail with my test
> > program) if you think the doc is right (or have a look at "man gcc" on a
> > system with gcc 2.95.3 (e.g. a recent -current)).
No, the docs are correct here.
Don't look at "man gcc". It is not maintained except by non-FSF
volunteers (see the WARNING section on the first page of it). In
practice, this means that it hasn't changed significantly since
rev.1.1.1.1 in 1994.
> Okay, I've built several binaries and libraries with -march=pentiumpro
> and -march=pentiumpro -mcpu=pentiumpro, and verified with diff -b that
> the output in both cases is indeed identical. It looks like the docs
> are correct.
ITYM "incorrect".
This just shows that your tests didn't test the small areas where
-march makes a difference (other than implying -mcpu). Examination
of gcc.295/config/i386/i386.{md,h} shows that it it only mainly (only?)
affects areas related to the i686 fcmov instruction. The following
program uses these areas, so compiling it with -march=pentiumpro and
-mcpu=pentiumpro gives different results:
---
double d;
int i;
int
main(void)
{
return (d == i);
}
---
Bruce
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