linux-kernel - Re: kernel + gcc 4.1 = several problems

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Message-ID: <Pine.LNX.4.64.0701021349420.4473@woody.osdl.org>
Date:	Tue, 2 Jan 2007 14:01:06 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Adrian Bunk <bunk@...sta.de>
cc:	Alistair John Strachan <s0348365@....ed.ac.uk>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>, Greg KH <greg@...ah.com>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	Andrew Morton <akpm@...l.org>
Subject: Re: kernel + gcc 4.1 = several problems

On Tue, 2 Jan 2007, Adrian Bunk wrote:
> 
> My point is that we have several reported problems only visible
> with gcc 4.1.
> 
> Other bug reports are e.g. [2] and [3], but they are only present with
> using gcc 4.1 _and_ using -Os.

Traditionally, afaik, -Os has tended to show compiler problems that 
_could_ happen with -O2 too, but never do in practice. It may be that 
gcc-4.1 without -Os miscompiles some very unusual code, and then with -Os 
we just hit more cases of that.

That said, I th ink gcc-4.1.1 is very common - I know it's the Fedora 
compiler. Also, CC_OPTIMIZE_FOR_SIZE defaults to 'y' if you have 
EXPERIMENTAL on, and from all the bug-reports about other features that 
are marked EXPERIMENTAL, I know that a lot of people do seem to select for 
it. So I would expect that gcc-4.1.1 and -Os is actually a fairly common 
combination. I just checked, and it's what I use personally, for example.

Of course, my main machine is an x86-64, and it has more registers. At 
least some historical -Os bug was about bad things happening under 
register pressure, iirc, and so x86-64 would show fewer problems than 
regular 32-bit x86 (which has far fewer registers for the compiler to 
use).

It is a bit worrisome. These things seem to be about 50:50 real kernel 
bugs (just hidden by some common code generation sequence) and real 
honest-to-goodness compiler bugs. But they are hard as hell to find.

		Linus
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