Re: Compilers and RCU readers: Once more unto the breach!

[Ramana Radhakrishnan <ramana dot radhakrishnan at arm dot com>]

On 20/05/15 15:03, Paul E. McKenney wrote:
> On Wed, May 20, 2015 at 02:44:30PM +0100, Ramana Radhakrishnan wrote:
> > On 20/05/15 14:37, David Howells wrote:
> > > Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > >
> > > > I was thinking of "y" as a simple variable, but if it is something more
> > > > complex, then the compiler could do this, right?
> > > >
> > > > 	char *x;
> > > >
> > > > 	y;
> > > > 	x = z;
> > >
> > > Yeah.  I presume it has to maintain the ordering, though.
> >
> > The scheduler for e.g. is free to reorder if it can prove there is
> > no dependence (or indeed side-effects for y) between insns produced
> > for y and `x = z'.
>
> So for example, if y is independent of z, the compiler can do the
> following:
>
> 	char *x;
>
> 	x = z;
> 	y;
>
> But the dependency ordering is still maintained from z to x, so this
> is not a problem.


Well, reads if any of x (assuming x was initialized elsewhere) would 
need to happen before x got assigned to z.

I understood the original "maintain the ordering" as between the 
statements `x = z' and `y'.


> Or am I missing something subtle here?

No, it sounds like we are on the same page here.

regards
Ramana

> 							Thanx, Paul