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

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Message-ID: <4CE17098.8090000@xs4all.nl>
Date:	Mon, 15 Nov 2010 18:40:40 +0100
From:	Jim Bos <jim876@...all.nl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jakub Jelinek <jakub@...hat.com>, Andi Kleen <andi@...stfloor.org>,
	James Cloos <cloos@...loos.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andreas Schwab <schwab@...hat.com>,
	Michael Matz <matz@...e.de>,
	Dave Korn <dave.korn.cygwin@...il.com>,
	Richard Guenther <richard.guenther@...il.com>, gcc@....gnu.org
Subject: Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?

On 11/15/2010 05:04 PM, Linus Torvalds wrote:
> On Mon, Nov 15, 2010 at 3:16 AM, Jakub Jelinek <jakub@...hat.com> wrote:
>>
>> I don't see any problems on the assembly level.  i8k_smm is
>> not inlined in this case and checks all 3 conditions.
> 
> If it really is related to gcc not understanding that "*regs" has
> changed due to the memory being an automatic variable, and passing in
> "regs" itself as a pointer to that automatic variable together with
> the "memory" clobber not being sufficient, than I think it's the lack
> of inlining that will automatically hide the bug.
> 
> (Side note: and I think this does show how much of a gcc bug it is not
> to consider "memory" together with passing in a pointer to an asm to
> always be a clobber).
> 
> Because if it isn't inlined, then "regs" will be seen a a real pointer
> to some external memory (the caller) rather than being optimized to
> just be the auto structure on the stack. Because *mem is auto only
> within the context of the caller.
> 
> Which actually points to a possible simpler:
>  - remove the "+m" since it adds too much register pressure
>  - mark the i8k_smm() as "noinline" instead.
> 
> Quite frankly, I'd hate to add even more crud to that inline asm (to
> save/restore the registers manually). It's already not the prettiest
> thing around.
> 
> So does the attached patch work for everybody?
> 
>                              Linus

Hmm, that doesn't work.

[ Not sure if you read to whole thread but initial workaround was to
change the asm(..) to asm volatile(..) which did work. ]

Jim.
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