RFR: 8229258: Make markOopDesc AllStatic

[Kim Barrett]

> On Aug 8, 2019, at 7:01 AM, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> class oopDesc {
>  volatile markWord _mark;
> }
> ...
> _mark = prototype(); <-- complains here that the assignment operator discards volatile qualifier
> 
> I'm unsure what performance effects it will have if I provide a hand-written assignment operator, since the class will not be a trivial class anymore. Because of that, I backed away from such a change.

[Not a review, just a comment about the above.]

There are some other places where that problem arises in our code base.
They’ve been dealt with by casting away the volatile, e.g. here would be

  // discard volatile post-assignment read.
  (void)const_cast<markWord&>(_mark = prototype());

To find some other examples (with some false positives):
egrep “const_cast<[^>]+&>”

taskqueue has several, with others scattered here and there.  There
are enough of them (more than I expected, over a dozen), and they are
long and ugly and obscure enough, that maybe there should be a helper
function for this.  I think something like (completely untested)

template<typename T>
ALWAYSINLINE void discard_volatile(T volatile& x) {
  static_cast<void>(const_cast<T&>(x));
}