What the JVM needs

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Contents

Notes from Rich Hickey's workshop

user mode limitations

  • please optimize this
  • fences
  • typestate: now immutable - especially arrays (typically encapsulated in non-writing header, a la java.lang.String)
  • Why Tailcalls: as a guarantee, not just an optimization
  • dynamic vs. inline
  • safepoints

numbers

  • fixnums (say, 28-bit small ints encoded as (x >> 28) == -1) http://blogs.sun.com/jrose/entry/fixnums_in_the_vm
  • need a uniform entry/exit types (Object => boxing)
  • Hacker's delight overflow detection is OK (only for add/sub); need a static method that throws overflow-exception?
  • Unsigned <

storage management

  • floating garbage foo(o, o=null); is tailcall a solution?
  • concurrent collection of PermGen (only in HotSpot). This is especially important for big server applications (large heap) which uses concurrent GC to avoid long GC pauses; when PermGen fills up a Full GC has to occur which can pause the VM for 10-30 seconds or more which can be unacceptably long for interactive requests.

limits

  • 31-bit arrays
  • 16-bit instructions
  • static data structures (array literals)

types

  • structs (efficient pointerless element selection)
  • value types (below)

The C#'s struct might be a source of ideas (though that's not necessarily how we want it)

value types

no mutability, acmp-identity, id-hashcode, monitorenter

  • immutable arrays == array-like value types
  • issue: intermediate early array construction states interact with memory model
  • perhaps need a typestate distinction: under construction vs. immutable
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