linux-kernel - Re: [RFC patch 0/4] TSC calibration improvements

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Message-ID: <alpine.LFD.1.10.0809062328160.3243@apollo.tec.linutronix.de>
Date:	Sat, 6 Sep 2008 23:30:00 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Alok Kataria <akataria@...are.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Veen <arjan@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dan Hecht <dhecht@...are.com>,
	Garrett Smith <garrett@...are.com>
Subject: Re: [RFC patch 0/4] TSC calibration improvements

On Sat, 6 Sep 2008, Linus Torvalds wrote:
> On Sat, 6 Sep 2008, Thomas Gleixner wrote:
> > 
> > One gcc does:
> > 
> >     i++;
> >     if (i >= QUICK_PIT_ITERATIONS)
> >        goto out;
> >     expect--;
> > 
> > The other one does:
> > 
> >     i++;
> >     expect--;
> >     if (i >= QUICK_PIT_ITERATIONS)
> >        goto out;
> > 
> > Don't ask me which one is correct. It's just reality :(
> 
> Show me. Because I simply don't believe you.
> 
> The first code is simply _wrong_ - except if "expect" isn't even _used_ 
> afterwards (in which case gcc can optimize away the last unused write).
> 
> And I strongly suspect that that is what you've seen. 
> 
> Because quite frankly, if what you describe is real, then your gcc is 
> incredibly buggy. So buggy that it sounds unlikely to be able to compile 
> the kernel in many other places. This is very simple and very fundamental 
> C, not something subtle or even half-way undefined.

Just checked, yes you are right. I messed that up in my tiredness to see
the obvious bug in Ingo's fixup patch.

/me resorts to bed

    tglx
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