On Tue, Dec 10, 2013 at 04:10:50PM +0100, Ingo Molnar wrote:
>
> * 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 ;-)
There certainly has been in the past. ;-)
> It doesn't hurt to outline our expectations in any case, agreed?
Fair enough, I will leave it as is.
Thanx, Paul
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