RFR: 8371667: Shenandoah: Re-design alloc request type enum for better efficiency and cleaner code

[Xiaolong Peng]

Current alloc request type enum:

  enum Type {
    _alloc_shared, // Allocate common, outside of TLAB
    _alloc_shared_gc, // Allocate common, outside of GCLAB/PLAB
    _alloc_cds, // Allocate for CDS
    _alloc_tlab, // Allocate TLAB
    _alloc_gclab, // Allocate GCLAB
    _alloc_plab, // Allocate PLAB
    _ALLOC_LIMIT
  };

With current design, we have to use switch statement resulting in unnecessary branches, for instance the function is_mutator_alloc:


  inline bool is_mutator_alloc() const {
    switch (_alloc_type) {
      case _alloc_tlab:
      case _alloc_shared:
      case _alloc_cds:
        return true;
      case _alloc_gclab:
      case _alloc_plab:
      case _alloc_shared_gc:
        return false;
      default:
        ShouldNotReachHere();
        return false;
    }
  }



In PR, I have re-designed the enum to make the function like is_mutator_alloc much simpler by making the values of the enum follow two simple rules:
1. Smaller value for mutator alloc, large value for gc alloc
2. Odd for lab, even number for non-lab

Three functions have been simplified to one-line impl w/o branches in machine code:


  inline bool is_mutator_alloc() const {
    return _alloc_type <= _alloc_shared;
  }

  inline bool is_gc_alloc() const {
    return _alloc_type >= _alloc_shared_gc;
  }

  inline bool is_lab_alloc() const {
    return (_alloc_type & 1) == 1;
  }


Test:
- [x] TEST=hotspot_gc_shenandoah
- [ ] Tier 1

-------------

Commit messages:
 - touch-up
 - Merge branch 'openjdk:master' into JDK-8371667
 - re-shuffle the enum values and add comments
 - 8371667: Shenandoah: Re-design alloc request type enum for better efficiency and cleaner code

Changes: https://git.openjdk.org/jdk/pull/28247/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28247&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371667
  Stats: 56 lines in 1 file changed: 6 ins; 41 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/28247.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28247/head:pull/28247

PR: https://git.openjdk.org/jdk/pull/28247