What the JVM needs

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