linux-kernel - Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?

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Message-ID: <AANLkTinAfq4HbPvEMDUM6230Qp41bJgZYPL1WtpkYHLN@mail.gmail.com>
Date:	Mon, 15 Nov 2010 11:24:48 +0100
From:	Richard Guenther <richard.guenther@...il.com>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	James Cloos <cloos@...loos.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andreas Schwab <schwab@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Michael Matz <matz@...e.de>,
	Dave Korn <dave.korn.cygwin@...il.com>, gcc@....gnu.org,
	Jim Bos <jim876@...all.nl>
Subject: Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?

On Mon, Nov 15, 2010 at 9:56 AM, Jakub Jelinek <jakub@...hat.com> wrote:
> On Sun, Nov 14, 2010 at 07:21:50PM -0800, Linus Torvalds wrote:
>> So when Richard Gunther says "a memory clobber doesn't cover automatic
>> storage", to me that very clearly spells "gcc is buggy as hell".
>> Because automatic storage with its address taken _very_ much gets
>> clobbered by things like memset etc. If the compiler doesn't
>> understand that, the compiler is just broken.
>
> I'll leave the discussion about meaning of "memory" clobber aside to
> Richard,

Of course GCC handles memset just fine.  Note that I was refering
to non-address taken automatic storage for "memory" (even though
when double-checking the current implementation GCC even thinks
that all address-taken memory is clobbered by asms as soon as
they have at least one memory operand or a "memory" clobber).

It's just that in future we might want to improve this and I think
not covering non-address taken automatic storage for "memory"
is sensible.  And I see that you don't see address-taken automatic
storage as a sensible choice to exclude from "memory", and I
have noted that.

Btw, I still haven't seen an testcase for the actual problem we are
talking about.

Richard.
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