JavaScript:SpiderMonkey:GC Futures

JS GC Futures

Tracking bug is bug 505308

Brain-dump of work items:

• Speed up allocator
  • remove or mitigate locking ????
  • [1]
  • Use one single GC heap chunk, avoiding frequent mmap and malloc calls
• Speed up collector
  • Allocate short-enough strings from GC heap, not malloc heap [bug 402614]
  • Allocate object dslots from GC heap, not malloc heap? Alternative: [bug NNNNNN]
  • Based on Gregor's stats consider making 32-byte and 64-byte JSObjects to cover most cases except large objects without any dslots bug 508357
• Remove byte flag per thing design [bug NNNNNN]
  • Bad cache effects of flags and things allocated from opposite ends of same arena
  • Don't want generality of different thing-types allocated from same size class
    • Can put finalizer or other index/info in the pool, not in each flag's low bits [bug NNNNNN]
    • Compress flags to mark bit in bitmap? [bug NNNNNN]
• Add new JNI-like (handles, "clothed" not "naked") global rooting API
  • But use C++ to best effect (auto storage class RAII helpers, templates)
  • Make old JS global rooting API be #ifdef JS_NAKED_GC_ROOTS (enabled at first) [bug NNNNNN]
  • Convert mozilla-central to use new global rooting API [bug NNNNNN]
  • Disable JS_NAKED_GC_ROOTS [bug NNNNNN]
• What about local roots? [bug NNNNNNN]
  • Need write barrier, efficient auto-temp-value rooting [bug NNNNNNN]
  • No naked argv[i] writes [bug NNNNNNN]
  • Conservative stack scanning to avoid temp-value rooting overheads? [bug NNNNNNN]
    • Andreas and Brendan think this is likely best (safety, clean code)
    • Equivalent to well-used (i.e. seldom-used) pinning API in effect but contained:
      • GC from shallow stack
      • clear event loop locals that might entrain garbage
    • 8/4/2009 NEEDS PROMPT INVESTIGATION
• GC safety static analysis
• Write barrier for all code including JITted code
• Write barrier in API
  • write barriers may make natives slower
• Make GC heap per thread (in JSThreadData)
  • New API for making MT-accessible objects
  • Objects are ST-accessible only without new API usage
  • Safest course for API: #ifdef JS_AUTO_MT_OBJECTS for existing API, new API entry points (C++ API, rather) for new ST vs. MT create-object methods [bug NNNNNNN]
  • Switch mozilla-central code over, turn off JS_AUTO_MT_OBJECTS [bug NNNNNNN]
• Fast generational copying allocation and collection [bug NNNNNNN]
• Go to beach