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no-op delay loops

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  no-op delay loops Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-11-27 10:00 +0100
    Re: no-op delay loops yalin wang <yalin.wang2010@gmail.com> - 2015-11-27 10:10 +0100
      Re: no-op delay loops Andy Shevchenko <andy.shevchenko@gmail.com> - 2015-11-27 10:30 +0100
    Re: no-op delay loops Arnd Bergmann <arnd@arndb.de> - 2015-11-27 12:20 +0100
    Re: no-op delay loops Ralf Baechle <ralf@linux-mips.org> - 2015-11-27 12:50 +0100

#1278651 — no-op delay loops

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?

Rasmus
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#1278659

> 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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#1278669

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


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With Best Regards,
Andy Shevchenko
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#1278756

On Friday 27 November 2015 09:53:50 Rasmus Villemoes 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?
> 

I remember that gcc used to not optimize code that looked like a
delay loop such as the above, and my tests show that this was still
the case in gcc-4.0.3, but starting with gcc-4.1 it opimtized away
that loop.

	Arnd
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#1278768

On Fri, Nov 27, 2015 at 09:53:50AM +0100, Rasmus Villemoes 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?

Thanks for reporting!

GCC used to intentionally not eleminate empty loops.  This has changed a
while ago.  Using volatile for the loop variable will result in atrocious
code so should be avoided.

One problem of these open coded loops is that it's not obvious how much
delay was actually intended so when fixing this I will have to do a bit
of precission guessing ;-)

  Ralf
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