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Subject: Re: no-op delay loops
From: yalin wang <yalin.wang2010@gmail.com>
In-Reply-To: <87si3rbz6p.fsf@rasmusvillemoes.dk>
Date: Fri, 27 Nov 2015 17:04:43 +0800
Cc: Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org,
        linux-kernel@vger.kernel.org
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References: <87si3rbz6p.fsf@rasmusvillemoes.dk>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 27, 2015, at 16:53, Rasmus Villemoes <linux@rasmusvillemoes.dk> wrote:
> 
> Hi,
> 
> It seems that gcc happily compiles
> 
> for (i = 0; i < 1000000000; ++i) ;
> 
> into simply
> 
> i = 1000000000;
> 
> (which is then usually eliminated as a dead store). At least at -O2, and
> when i is not declared volatile. So it would seem that the loops at
> 
> arch/mips/pci/pci-rt2880.c:235
> arch/mips/pmcs-msp71xx/msp_setup.c:80
> arch/mips/sni/reset.c:35
> 
> actually don't do anything. (In the middle one, i is 'register', but
> that doesn't change anything.) Is mips compiled with some special flags
> that would make gcc actually emit code for the above?
> 
you can try to declare i as  volatile int i;
may gcc will not optimize it .

Thanks

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