linux-kernel - Re: [PATCH] x86-64: Prevent gcc from optimizing away venosys

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Message-ID: <20110603213713.GO27166@one.firstfloor.org>
Date:	Fri, 3 Jun 2011 23:37:13 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Dave Anderson <anderson@...hat.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, jstancek@...hat.com
Subject: Re: [PATCH] x86-64: Prevent gcc from optimizing away venosys_1()

On Fri, Jun 03, 2011 at 05:32:06PM -0400, Dave Anderson wrote:
> 
> 
> ----- Original Message -----
> > >  	return 0;
> > >  }
> > >
> > > -static long __vsyscall(3) venosys_1(void)
> > > +long __vsyscall(3) venosys_1(void)
> > 
> > Better add a __used too. Otherwise it can be optimized out again
> > in some builds e.g. when someone enables gc-sections for the linker.
> 
> Considering that all of the vsyscall prototypes have this attribute:
> 
> #define __vsyscall(nr) \
>                 __attribute__ ((unused, __section__(".vsyscall_" #nr))) notrace
> 
> How would any of them work?

unused is just to prevent warnings I think. used stops the optimizer
from throwing it away.

It would make more sense to drop the unused and add an used.  I think
used prevents the warnings too.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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