RFR: 8229258: Make markOopDesc AllStatic

[Stefan Karlsson]

Hi all,

Please review this patch to make markOopDesc AllStatic.

https://cr.openjdk.java.net/~stefank/8229258/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8229258

 From the RFE:

I want to change/rename markOop and markOopDesc

Today markOopDescs inherits from oopDesc even though they are not 
oopDescs (Java objects):
  `class markOopDesc : public oopDesc {`

This is very confusing to new-comers (and probably to old-timers as well).

A simple fix would be to break the inheritance and do the following rename:
markOopDesc -> MarkWord
markOop -> markWord

However, there are more dubious code in this area. markOopDescs are 
created and used like this:

```
class oopDesc {
...
   volatile markOop _mark;
...
markOop  oopDesc::mark_raw()  const { return _mark; }
...
// Usage
obj->mark_raw()->is_marked()
...
// Implementation
   bool is_marked()   const {
     return (mask_bits(value(), lock_mask_in_place) == marked_value);
   }
...
// Implementation
   bool is_being_inflated() const { return (value() == 0); }
...
// Implementation of markOopDesc::value()
   uintptr_t value() const { return (uintptr_t) this; }
```

Remember, _mark is an arbitrary bit pattern describing the object. We 
treat it as if it were a pointer to a markOopDesc object, but it's not 
pointing to such an object. While using that (weird) pointer we call 
value() and extract it's bit pattern to be used for further bit pattern 
checking functions. AFAICT, this is also undefined behavior. At least 
is_being_inflated() const is effectively 'return (uintptr_t) this == 
NULL'. This UB was recently discussed here:
  https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-July/038704.html
  https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-July/038712.html

I propose that we change MarkWord (markOopDesc) to be an AllStatic class 
instead and get rid of this (ab)use of the markWord (markOop) pointer.

The patch is large but mostly straight forward. However, there is some 
Java code that would need some extra scrutiny: SA and JVMCI.

Tested with tier1-3.

Coleen has asked for file rename from markOop.hpp to markWord.hpp. I'd 
like to do that as a separate RFE when/if this patch gets accepted.

Thanks,
StefanK