* Paul E. McKenney <[email protected]> wrote:
> On Thu, Dec 05, 2013 at 10:05:47AM -0800, Paul E. McKenney wrote:
> > On Thu, Dec 05, 2013 at 10:50:42AM +0100, Ingo Molnar wrote:
> > >
> > > * Paul E. McKenney <[email protected]> wrote:
> > >
> > > > + (*) The compiler is within its rights to reload a variable, for
> > > > example,
> > > > + in cases where high register pressure prevents the compiler from
> > > > + keeping all data of interest in registers. The compiler might
> > > > + therefore optimize the variable tmp out of our previous example:
> > > > +
> > > > + while (tmp = a)
> > > > + do_something_with(tmp);
> > > > +
> > > > + This could result in the following code, which is perfectly safe
> > > > in
> > > > + single-threaded code, but can be fatal in concurrent code:
> > > > +
> > > > + while (a)
> > > > + do_something_with(a);
> > > > +
> > > > + For example, the optimized version of this code could result in
> > > > + passing a zero to do_something_with() in the case where the
> > > > variable
> > > > + a was modified by some other CPU between the "while" statement and
> > > > + the call to do_something_with().
> > >
> > > Nit: I guess references to variable names such as 'a' should be quoted
> > > (same for 'tmp', 'b', etc):
> > >
> > > For example, the optimized version of this code could result in
> > > passing a zero to do_something_with() in the case where the
> > > variable
> > > 'a' was modified by some other CPU between the "while" statement
> > > and
> > > the call to do_something_with().
> > >
> > > which makes reading it less ambiguous and more fluid IMO. This
> > > observation applies to the whole document as 'a' is used in many
> > > places.
> >
> > Good point, fixed.
>
> Which reminds me -- the thing that makes me most nervous about
> prohibiting speculative stores is the bit about having to anticipate
> all compiler optimizations that might get rid of the needed
> conditionals.
>
> Thoughts?
As long as current compiler versions behave I don't the potential of
future problems is a problem that can (or should) be solved via
documentation - there will always be a colorful tension between
specification and reality, both at the hardware, the code and the
compiler level ;-)
It doesn't hurt to outline our expectations in any case, agreed?
Thanks,
Ingo
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