linux-kernel - Re: [patch 2.6.19-rc6] Stop gcc 4.1.0 optimizing wait_hpet

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Date:	Wed, 29 Nov 2006 04:08:00 -0500
From:	Jakub Jelinek <jakub@...hat.com>
To:	Keith Owens <kaos@....com.au>
Cc:	Nicholas Miell <nmiell@...cast.net>, linux-kernel@...r.kernel.org
Subject: Re: [patch 2.6.19-rc6] Stop gcc 4.1.0 optimizing wait_hpet_tick away

On Wed, Nov 29, 2006 at 02:56:20PM +1100, Keith Owens wrote:
> Nicholas Miell (on Tue, 28 Nov 2006 19:08:25 -0800) wrote:
> >On Wed, 2006-11-29 at 13:22 +1100, Keith Owens wrote:
> >> Compiling 2.6.19-rc6 with gcc version 4.1.0 (SUSE Linux),
> >> wait_hpet_tick is optimized away to a never ending loop and the kernel
> >> hangs on boot in timer setup.
> >> 
> >> 0000001a <wait_hpet_tick>:
> >>   1a:   55                      push   %ebp
> >>   1b:   89 e5                   mov    %esp,%ebp
> >>   1d:   eb fe                   jmp    1d <wait_hpet_tick+0x3>
> >> 
> >> This is not a problem with gcc 3.3.5.  Adding barrier() calls to
> >> wait_hpet_tick does not help, making the variables volatile does.
> >> 
> >> Signed-off-by: Keith Owens <kaos@....com.au>
> >> 
> >> ---
> >>  arch/i386/kernel/time_hpet.c |    2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> Index: linux-2.6/arch/i386/kernel/time_hpet.c
> >> ===================================================================
> >> --- linux-2.6.orig/arch/i386/kernel/time_hpet.c
> >> +++ linux-2.6/arch/i386/kernel/time_hpet.c
> >> @@ -51,7 +51,7 @@ static void hpet_writel(unsigned long d,
> >>   */
> >>  static void __devinit wait_hpet_tick(void)
> >>  {
> >> -	unsigned int start_cmp_val, end_cmp_val;
> >> +	unsigned volatile int start_cmp_val, end_cmp_val;
> >>  
> >>  	start_cmp_val = hpet_readl(HPET_T0_CMP);
> >>  	do {
> >
> >When you examine the inlined functions involved, this looks an awful lot
> >like http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278
> >
> >Perhaps SUSE should fix their gcc instead of working around compiler
> >problems in the kernel?
> 
> Firstly, the fix for 22278 is included in gcc 4.1.0.

This actually sounds more like http://gcc.gnu.org/PR27236
And that one is broken in 4.1.0, fixed in 4.1.1.

	Jakub
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