tree 23aabbbf39cde20a448554090f0eea55d8242d07
parent a999d2182c2f3f2fbceee96d616639c345d45108
author Daco Harkes <dacoharkes@google.com> 1647945018 +0000
committer Commit Bot <commit-bot@chromium.org> 1647945018 +0000

[vm] Implement `Finalizer`

This CL implements the `Finalizer` in the GC.

(This CL does not yet implement `NativeFinalizer`.)

The GC is specially aware of two types of objects for the purposes of
running finalizers.

1) `FinalizerEntry`
2) `Finalizer` (`FinalizerBase`, `_FinalizerImpl`)

A `FinalizerEntry` contains the `value`, the optional `detach` key, and
the `token`, and a reference to the `finalizer`.
An entry only holds on weakly to the value, detach key, and finalizer.
(Similar to how `WeakReference` only holds on weakly to target).

A `Finalizer` contains all entries, a list of entries of which the value
is collected, and a reference to the isolate.

When a the value of an entry is GCed, the enry is added over to the
collected list.
If any entry is moved to the collected list, a message is sent that
invokes the finalizer to call the callback on all entries in that list.

When a finalizer is detached by the user, the entry token is set to the
entry itself and is removed from the all entries set.
This ensures that if the entry was already moved to the collected list,
the finalizer is not executed.

To speed up detaching, we use a weak map from detach keys to list of
entries. This ensures entries can be GCed.

Both the scavenger and marker tasks process finalizer entries in
parallel.
Parallel tasks use an atomic exchange on the head of the collected
entries list, ensuring no entries get lost.
The mutator thread is guaranteed to be stopped when processing entries.
This ensures that we do not need barriers for moving entries into the
finalizers collected list.
Dart reads and replaces the collected entries list also with an atomic
exchange, ensuring the GC doesn't run in between a load/store.

When a finalizer gets posted a message to process finalized objects, it
is being kept alive by the message.
An alternative design would be to pre-allocate a `WeakReference` in the
finalizer pointing to the finalizer, and send that itself.
This would be at the cost of an extra object.

Send and exit is not supported in this CL, support will be added in a
follow up CL. Trying to send will throw.

Bug: https://github.com/dart-lang/sdk/issues/47777

TEST=runtime/tests/vm/dart/finalizer/*
TEST=runtime/tests/vm/dart_2/isolates/fast_object_copy_test.dart
TEST=runtime/vm/object_test.cc

Change-Id: I03e6b4a46212316254bf46ba3f2df333abaa686c
Cq-Include-Trybots: luci.dart.try:vm-kernel-reload-rollback-linux-debug-x64-try,vm-kernel-reload-linux-debug-x64-try,vm-ffi-android-debug-arm64c-try,dart-sdk-mac-arm64-try,vm-kernel-mac-release-arm64-try,pkg-mac-release-arm64-try,vm-kernel-precomp-nnbd-mac-release-arm64-try,vm-kernel-win-debug-x64c-try,vm-kernel-win-debug-x64-try,vm-kernel-precomp-win-debug-x64c-try,vm-kernel-nnbd-win-release-ia32-try,vm-ffi-android-debug-arm-try,vm-precomp-ffi-qemu-linux-release-arm-try,vm-kernel-mac-debug-x64-try,vm-kernel-nnbd-mac-debug-x64-try,vm-kernel-nnbd-linux-debug-ia32-try,benchmark-linux-try,flutter-analyze-try,flutter-frontend-try,pkg-linux-debug-try,vm-kernel-asan-linux-release-x64-try,vm-kernel-gcc-linux-try,vm-kernel-optcounter-threshold-linux-release-x64-try,vm-kernel-precomp-linux-debug-simarm_x64-try,vm-kernel-precomp-obfuscate-linux-release-x64-try,vm-kernel-precomp-linux-debug-x64-try,vm-kernel-precomp-linux-debug-x64c-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/229544
Reviewed-by: Lasse Nielsen <lrn@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>