linux-kernel - Re: no-op delay loops

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Message-Id: <7B39C9C2-1093-49CE-9A1E-5059A57C298A@gmail.com>
Date:	Fri, 27 Nov 2015 17:04:43 +0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org
Subject: Re: no-op delay loops


> On Nov 27, 2015, at 16:53, Rasmus Villemoes <linux@...musvillemoes.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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