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Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
- From: Andreas Schwab <schwab at linux-m68k dot org>
- To: Andi Kleen <andi at firstfloor dot org>
- Cc: Jim <jim876 at xs4all dot nl>, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, gcc at gcc dot gnu dot org
- Date: Sun, 07 Nov 2010 23:41:37 +0100
- Subject: Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
- References: <4CD538CA.8010901@xs4all.nl> <87wroostw3.fsf@basil.nowhere.org>
Andi Kleen <andi@firstfloor.org> writes:
> Jim <jim876@xs4all.nl> writes:
>
>> After upgrading my Dell laptop, both OS+kernel the i8k interface was giving
>> nonsensical output. As it turned out it's not the kernel but compiler
>> upgrade which broke this.
>>
>> Guys at Archlinux have found the underlying cause (but don't seem to have
>> submitted a patch yet):
>> https://bbs.archlinux.org/viewtopic.php?pid=780692#p780692
>> gcc seems to optimize the assembly statements away.
>>
>> And indeed, applying this patch makes the i8k interface work again,
>> i.e. replacing the asm(..) construct by asm volatile(..)
>
> The compiler really should not optimize the asm away, because
> it has both input and output arguments which are later used.
> "asm volatile" normally just means "don't move significantly"
The asm fails to mention that it modifies *regs.
Andreas.
--
Andreas Schwab, schwab@linux-m68k.org
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