Implement prologue weak persistent handles.

Implement prologue weak persistent handles.

Prologue weak persistent handles are similar to weak persistent
handles but exhibit different behavior during garbage collections that
invoke the prologue and epilogue callbacks.  While weak persistent
handles always weakly reference their referents, prologue weak
persistent handles weakly reference their referents only during a
garbage collection occurs that invokes the prologue and epilogue
callbacks.  During all other garbage collections, prologue weak
persistent handles strongly reference their referents.

Committed: https://code.google.com/p/dart/source/detail?r=5445

[siva, 2012-03-09 21:48:41]

lgtm

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/include/dart_a...
File runtime/include/dart_api.h (right):

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/include/dart_a...
runtime/include/dart_api.h:260: * epilogue callbacks.
I like the description you used in the gcl upload comments.

(These handles strongly reference their referent except in those garbage
collection cycles where the prolog/epilog callbacks are called).

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/include/dart_a...
runtime/include/dart_api.h:261: *
Documentation for parameters 'object, peer, callback'.
Also needs to be done for Dart_NewWeakPersistentHandle.

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/vm/dart_api_im...
File runtime/vm/dart_api_impl.cc (right):

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/vm/dart_api_im...
runtime/vm/dart_api_impl.cc:457: return
reinterpret_cast<Dart_Handle>(prologue_weak_ref);
This seems identical to Dart_NewWeakPersistentHandle maybe we could refactor
into a helper method that takes in weak_ref or prologue_weak_ref as a parameter.

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/vm/heap.cc
File runtime/vm/heap.cc (right):

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/vm/heap.cc#new...
runtime/vm/heap.cc:153: if (space == kNew) {
(space == kNew || space == kExecutable) ?

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/vm/scavenger.h
File runtime/vm/scavenger.h (right):

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/vm/scavenger.h...
runtime/vm/scavenger.h:63: void Scavenge(bool invoke_api_callbacks);
Does it make sense to use a default argument here instead of two methods?

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/vm/verifier.cc
File runtime/vm/verifier.cc (right):

https://chromiumcodereview.appspot.com/9655011/diff/36/runtime/vm/verifier.cc...
runtime/vm/verifier.cc:57: false,  // skip prologue weak handles
Why do we want to skip verification of prolog weak handles?

They are potentially reachable right.

[cshapiro, 2012-03-13 23:06:43]

PTAL

http://codereview.chromium.org/9655011/diff/36/runtime/include/dart_api.h
File runtime/include/dart_api.h (right):

http://codereview.chromium.org/9655011/diff/36/runtime/include/dart_api.h#new...
runtime/include/dart_api.h:260: * epilogue callbacks.
I replaced the comment above with the description from the check-in.  Let me
know if that is okay.

http://codereview.chromium.org/9655011/diff/36/runtime/include/dart_api.h#new...
runtime/include/dart_api.h:261: *
Done.

http://codereview.chromium.org/9655011/diff/36/runtime/vm/dart_api_impl.cc
File runtime/vm/dart_api_impl.cc (right):

http://codereview.chromium.org/9655011/diff/36/runtime/vm/dart_api_impl.cc#ne...
runtime/vm/dart_api_impl.cc:457: return
reinterpret_cast<Dart_Handle>(prologue_weak_ref);
Yes, it is pretty close.  I have factored out the allocation and initialization
into its own routine.

http://codereview.chromium.org/9655011/diff/36/runtime/vm/heap.cc
File runtime/vm/heap.cc (right):

http://codereview.chromium.org/9655011/diff/36/runtime/vm/heap.cc#newcode153
runtime/vm/heap.cc:153: if (space == kNew) {
Sure, I can make this change.  I made this choice arbitrarily as I was not
certain what the desired behavior was.  I assumed that when we implement the
collection of executable code it would follow a full GC.

http://codereview.chromium.org/9655011/diff/36/runtime/vm/scavenger.h
File runtime/vm/scavenger.h (right):

http://codereview.chromium.org/9655011/diff/36/runtime/vm/scavenger.h#newcode63
runtime/vm/scavenger.h:63: void Scavenge(bool invoke_api_callbacks);
Sorry about this.  It was meant to be a placeholder for code that heuristically
determines when to invoke the API callbacks.  I have moved the code into the .cc
file and added a corresponding TODO.

http://codereview.chromium.org/9655011/diff/36/runtime/vm/verifier.cc
File runtime/vm/verifier.cc (right):

http://codereview.chromium.org/9655011/diff/36/runtime/vm/verifier.cc#newcode57
runtime/vm/verifier.cc:57: false,  // skip prologue weak handles
We don't.  We just defer the visit until later.  More specifically, during the
visit of WeakPersistentHandles on line 61.  I'll add a matching comment next to
the "true" on line 62.

[siva, 2012-03-13 23:34:30]

lgtm

http://codereview.chromium.org/9655011/diff/8001/runtime/include/dart_api.h
File runtime/include/dart_api.h (right):

http://codereview.chromium.org/9655011/diff/8001/runtime/include/dart_api.h#n...
runtime/include/dart_api.h:271: * only during a garbage collection occurs that
invokes the prologue
garbage collection that invokes ....