https://wiki.jvmlangsummit.com/api.php?action=feedcontributions&feedformat=atom&user=KreideJVMLangSummit - User contributions [en]2026-05-23T11:33:57ZUser contributionsMediaWiki 1.32.0https://wiki.jvmlangsummit.com/index.php?title=What_the_JVM_needs&diff=577What the JVM needs2010-07-28T22:47:29Z<p>Kreide: </p>
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<div>== Notes from Rich Hickey's workshop ==<br />
<br />
=== user mode limitations ===<br />
* please optimize this<br />
* fences<br />
* typestate: now immutable - especially arrays (typically encapsulated in non-writing header, a la java.lang.String)<br />
* [[Why Tailcalls]]: as a guarantee, not just an optimization<br />
* dynamic vs. inline<br />
* safepoints<br />
<br />
=== numbers ===<br />
* fixnums (say, 28-bit small ints encoded as (x >> 28) == -1) http://blogs.sun.com/jrose/entry/fixnums_in_the_vm<br />
* need a uniform entry/exit types (Object => boxing)<br />
* Hacker's delight overflow detection is OK (only for add/sub); need a static method that throws overflow-exception?<br />
* Unsigned <<br />
<br />
=== storage management ===<br />
* floating garbage foo(o, o=null); is tailcall a solution?<br />
* 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.<br />
<br />
=== limits ===<br />
* 31-bit arrays<br />
* 16-bit instructions<br />
* static data structures (array literals)<br />
<br />
=== types ===<br />
* structs (efficient pointerless element selection)<br />
* value types (below)<br />
<br />
The C#'s struct might be a source of ideas (though that's not necessarily how we want it)<br />
<br />
=== value types ===<br />
no mutability, acmp-identity, id-hashcode, monitorenter<br />
* immutable arrays == array-like value types<br />
* issue: intermediate early array construction states interact with memory model<br />
* perhaps need a typestate distinction: under construction vs. immutable</div>Kreide