happy new year! (original) (raw)

Arnold Schwaighofer arnold.schwaighofer at gmail.com
Wed Jan 21 03:46:09 PST 2009

Hi everybody,

i am working on the project to bring tail calls to Hotspot.

A prototype was implemented on x86-32 and will hopefully be soon availabe on the mlvm site. The modified hotspot supports tail calls in the interpreter, client and server compiler. To mark a call as tail call the programmer emits a 'wide' byte code before the various invoke... instructions. The verifier checks that the call really is a tail call (i.e the last thing the method does before it returns etc.). Hotspot guarantees that a series of such tail calls execute in bounded stack space. That is no stackoverflow exception happens. The vm implements most tail calls by removing the calling stack frame. Some stack frames can not be removed because of java's access security mechanism. Every stack frame points to a protection domain via which permissions can be computed. On access to a guarded resource permissions are computed by intersecting all protection domains that are on the stack. If caller and callee protection domain are different we can not remove the caller's stack frame (or we would have to look whether an ancestor frame already has the same pd on every call - not very efficient). In such a case the tail call proceeds like a normal function call i.e leaving a stack frame. If a series of tail calls with different pds happen the stack grows. Eventually if the recursion is deep enough a stack overflow would happen. The implementation catches this situation and compresses the stack by removing stack frames that have the same protection domain.

The prototype is mostly finished. I am currently writing my thesis describing the changes and hope to be finished mid february. By then i hope to have cleaned up the code and will probably post it here on the mail list.

regards arnold

On Tue, Jan 20, 2009 at 10:50 AM, Patrick Wright <pdoubleya at gmail.com> wrote:

John

Thanks for the update.

5. encourage the great work that people are doing or will do on tail call, continuations, interface injection, invokedynamic, etc. (Cheers, Arnold, Lukas, Tobias, Remi.) It would be great to get an update on how these sub-projects are going, what they are (currently, realistically) aiming to achieve, etc. Perhaps the people involved can give us a brief status update? Thanks! Patrick

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