Runloop unrolling for exception handling

[NotFound]

The attached patch is a proof of concept implementation of a runloop unrolling for exception handling. The idea is that when a exception handler takes care and finishes the handling, it signals that decision and the control return to the runloop that established the handler, unrolling the C stack if needed.

The second attach is an example of use.

For simplicity, this implementation uses a method in the ExceptionHandler PMC. A specific opcode will simplify the usage.

It uses the ExceptionHandler object to identify the runloop, but the 'push_eh label' doesn't provide one. A variant '$P0 = push_eh label' is a possible solution.

[NotFound]

loopunroll2.patch is an improved version, with better handling of the runloops, and allowing to use the exception or the exception handler to control the rewind. As a side effect, it also clarifies the creation and destruction of runloops.

Updated the example handler.pir showing the new feature and with more comments.

[NotFound]

Third version: now uses a new opcode instead of methods. The opcode takes a PMC with the Exception or the ExceptionHandler and jumps to the next op after the urolling, no need to a label.

unroll.pir is the example for this version.

[NotFound]

Last version of the patch applied as r46696, unroll opcode marked as experimental in DEPRECATED.pod

[NotFound]

This is a post with he rationale and an example of use:  http://notfound.posterous.com/the-unroll-opcode

[NotFound]

After some discussion, renamed to 'finalize' in r47023

[whiteknight]

I brought this ticket up yesterday at #parrotsketch, and it was received with weak support. I'm going to try to put in a few more tests and raise the issue again in a few weeks.

[coke]

No longer experimental? 2 upvotes, no objections at:  http://irclog.perlgeek.de/parrotsketch/2010-11-23#i_3024695.

Note that #833 was just closed with a note that finalize is no longer experimental. If so, we can remove the note from DEPRECATED.pod and close this ticket.

[whiteknight]

As of 02de970 the finalize op is no longer experimental.