Re: [patch] spinlocks: remove 'volatile'

[J.A. MagallÃn]

On Thu, 06 Jul 2006 14:39:43 +0200, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:

> On Thu, 2006-07-06 at 08:29 -0400, linux-os (Dick Johnson) wrote:
> > On Thu, 6 Jul 2006, Arjan van de Ven wrote:
> > 
> > > On Thu, 2006-07-06 at 07:59 -0400, linux-os (Dick Johnson) wrote:
> > >> On Thu, 6 Jul 2006, Ingo Molnar wrote:
> > >>
> > >>>
> > >>> * Linus Torvalds <torvalds@xxxxxxxx> wrote:
> > >>>
> > >>>> I wonder if we should remove the "volatile". There really isn't
> > >>>> anything _good_ that gcc can do with it, but we've seen gcc code
> > >>>> generation do stupid things before just because "volatile" seems to
> > >>>> just disable even proper normal working.
> > >>
> > >> Then GCC must be fixed. The keyword volatile is correct. It should
> > >> force the compiler to read the variable every time it's used.
> > >
> > > this is not really what the C standard says.
> > >
> > >
> > >
> > >> This is not pointless. If GCC generates bad code, tell the
> > >> GCC people. The volatile keyword is essential.
> > >
> > > no the "volatile" semantics are vague, trecherous and evil. It's a LOT
> > > better to insert the well defined "barrier()" in the right places.
> > 
> > Look at:
> > 
> >  	http://en.wikipedia.org/wiki/Volatile_variable
> > 
> > This is just what is needed to prevent the compiler from making non working
> > code during optimization.
> 
> and an entry level document at wikipedia is more important than the C
> standard ;)
> 
> > 
> > Also look at:
> > 
> >  	http://en.wikipedia.org/wiki/Memory_barrier
> > 
> > This is used to prevent out-of-order execution, not at all what is
> > necessary.
> 
> I did not talk about memory barriers. In fact, barrier() is NOT a memory
> barrier. It's a compiler optimization barrier!
> 

// Read 10 samples from 2 A/D converters.

int*	ina;
int	a[10];
int*	inb;
int	b[10];

for (int i=0; i<10; i++)
{
	a[i] = *ina;
	barrier();
	b[i] = *inb;
}

The barrier prevents the compiler of translating this to:

for (int i=0; i<10; i++)
{
	b[i] = *inb;
	a[i] = *ina;
}

or even to:

for (int i=0; i<10; i++)
	a[i] = *ina;
for (int i=0; i<10; i++)
	b[i] = *inb;

but does not prevent it to do this:

register int tmp_a = *ina;
register int tmp_b = *inb;

for (int i=0; i<10; i++)
{
	a[i] = tmp_a;
	b[i] = tmp_b;
}

because nor 'ina' nor 'inb' change under what the compiler sees inside
the loop. 'volatile' prevents the compiler of do a high level cache of
*ina or *inb.

--
J.A. Magallon <jamagallon()ono!com>     \               Software is like sex:
                                         \         It's better when it's free
Mandriva Linux release 2007.0 (Cooker) for i586
Linux 2.6.17-jam01 (gcc 4.1.1 20060518 (prerelease)) #2 SMP PREEMPT Wed
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