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

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Message-ID: <4CE1968D.3050706@xs4all.nl>
Date:	Mon, 15 Nov 2010 21:22:37 +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 08:51 PM, Jakub Jelinek wrote:
> On Mon, Nov 15, 2010 at 11:21:30AM -0800, Linus Torvalds wrote:
>> On Mon, Nov 15, 2010 at 11:12 AM, Jakub Jelinek <jakub@...hat.com> wrote:
>>>
>>> Ah, the problem is that memory_identifier_string is only initialized in
>>> ipa-reference.c's initialization, so it can be (and is in this case) NULL in
>>> ipa-pure-const.c.
>>
>> Ok. And I guess you can verify that all versions of gcc do this
>> correctly for "asm volatile"?
> 
> Yes, reading 4.1/4.2/4.3/4.4/4.5/4.6 code ipa-pure-const.c handles
> asm volatile correctly, in each case the function is no longer assumed to be
> pure or const in the discovery (of course, user can still say the
> function is const or pure). 4.0 and earlier didn't have ipa-pure-const.c.
> 
> Using the simplified
> 
> extern void abort (void);
> 
> __attribute__((noinline)) int
> foo (int *p)
> {
>   int r;
>   asm ("movl $6, (%1)\n\txorl %0, %0" : "=r" (r) : "r" (p) : "memory");
>   return r;
> }
> 
> int
> main (void)
> {
>   int p = 8;
>   if ((foo (&p) ? : p) != 6)
>     abort ();
>   return 0;
> }
> 
> testcase shows that in 4.1/4.2/4.3/4.4 this is miscompiled only when using
> -fno-ipa-reference, in 4.5 it is miscompiled always when optimizing
> unless -fno-ipa-pure-const (as 4.5 added local-pure-const pass which is run
> before ipa-reference) and in 4.6 this has been fixed by Honza when
> doing ipa cleanups.
> 
>> Because since we'll have to work around this problem in the kernel, I
>> suspect the simplest solution is to remove the "+m" that causes
>> register pressure problems, and then use "asm volatile" to work around
>> the const-function bug.
> 
> Yes.
> 
> 	Jakub
> 

Linus,

In case you didn't already fixed this, here's the follow-up patch.
---

The fix to work around the gcc miscompiling i8k.c to add "+m (*regs)"
caused register pressure problems. Changing the 'asm' statement to
'asm volatile' instead should prevent that and works around the gcc
bug as well.

Signed-off-by: Jim Bos <jim876@...all.nl>


View attachment "PATCH2.i8k.c" of type "text/plain" (917 bytes)