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  1. JDK
  2. JDK-8229258

Rework markOop and markOopDesc into a simpler mark word value carrier

    Details

    • Type: Enhancement
    • Status: Resolved
    • Priority: P4
    • Resolution: Fixed
    • Affects Version/s: 14
    • Fix Version/s: 14
    • Component/s: hotspot
    • Subcomponent:
      gc
    • Resolved In Build:
      b11

      Description

      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

      Revoked: "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."
      Revised: "I propose that we change markWord (markOopDesc) to contain an uintptr_t _value member and stop using the bit pattern of the this pointer"

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              • Assignee:
                stefank Stefan Karlsson
                Reporter:
                stefank Stefan Karlsson
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                • Created:
                  Updated:
                  Resolved: