Roadmap: Support StickyImmix

[wks]

This issue tracks the progress of supporting StickyImmix, a generational collector.

• Write barrier
  • Routing RB_OBJ_WRITE and RB_OBJ_WRITTEN to MMTk barrier implementation
  • Map rb_gc_writebarrier_remember so that MMTk remembers the object
  • bulk array access
    • Use rb_gc_writebarrier_remember to remember parent array.
    • Change to region barrier if possible.
• WB-unprotected objects
  • Registering
  • Scanning
  • Checking if an object is WB-unprotected
  • Unregistering Should't be possible. If an object becomes WB-unprotected, it remains so for the rest of its life
• Testing
  • Tests pass with object movement disabled
  • Tests pass with object movement enabled
  • Test with Liquid benchmark
• Optimising
  • fstring_table: During nursery collection, only update entries added since the last GC.
    • Other global weak tables can be optimised similarly if needed.
  • Write barrier fast paths: Implement the write barrier fast path as C function that can be inlined at call sites, and eliminate the call into mmtk-core.
  • Load balance: Further parallelise weak table processing
  • T_DATA is the majority of obj_free candidates.
    • Embedded data with default free function are not obj_free candidates.
    • If we can't free things in parallel, resurrect the idea of zombies and resurrect objects for obj_free.
  • Allocate Hash buffers in the GC heap, removing Hash from the set of obj_free candidates.