Re: no-op delay loops

[Andy Shevchenko]

On Fri, Nov 27, 2015 at 11:04 AM, yalin wang <yalin.wang2010@xxxxxxxxx> wrote:
>> On Nov 27, 2015, at 16:53, Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> wrote:

>> 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 .

Might be, but Rasmus as I can see asked about *existing* code.


-- 
With Best Regards,
Andy Shevchenko
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