Roadmap to a fully functional Ruby VM with MMTk

[wks]

This is a very high level roadmap.

• Step 1: Support NoGC
• Step 2: Support MarkSweep and non-copying Immix
  • Tracking issue: Roadmap to proper mark-sweep support #3
• Step 3: Support copying Immix
  • Tracking issue: Support copying GC #18
• Intermediate steps: Fix obvious performance issues.
  • Tracking issue: GC performance issues for MMTk Ruby #25
  • Remove excessive obj_free
    • Tracking issue: Make obj_free unnecessary for common types #11
    • We have not eliminated all obj_free candidates (it is impossible), but it is no longer the biggest performance problem that prevent us from moving forward. It still has room for improvement, though.
  • Load-balancing: Handle extra-large arrays.
  • Specialising object-scanning fast paths
  • (Optional) Use in-header forwarding bits.
• Step 4: Support StickyImmix, a generational collector
  • Tracking issues: Roadmap: Support StickyImmix #57
• Intermediate steps: Stablising
  • Re-examine disabled test cases and enable them if possible.
  • Revisit Ractor support.
• Step 5: Implement allocation and write barrier fast paths with YJIT