On Mon, 27 Jan 2025 22:06:58 GMT, Jorn Vernee <jver...@openjdk.org> wrote:
> Currently, to free the memory allocated in a confined arena, we keep track of > a list of 'cleanup actions', stored in linked list format in a so-called > `ResourceList`, attached to the scope of the arena. When the scope is closed, > we loop over all the entries in this resource list, and run all the cleanup > actions one by one. > > However, due to this linked list format, plus the control flow introduced by > the cleanup loop, C2's escape analysis can not keep track of the nodes of > this linked list (`ResourceList.ResourceCleanup`), and as a result, they can > not be scalar replaced. > > We can prevent just the first `ResourceCleanup` instance from escaping, by > pulling out the first element of the list into a separate field. I also tried > a setup where I had 2 separate fields for the first 2 elements, as well as a > setup with an array with a fixed set of elements. While these also worked to > prevent the first node from escaping, they were not able to provide the same > benefit for multiple resource cleanup instances. Nevertheless, avoiding the > allocation of the first element is relatively simple, and seems like a > low-hanging fruit. > > I've changed the `AllocTest` benchmark a bit so that we don't return the > `MemorySegment` in `alloc_confined`, which would make it always escape. That > way, we can use this existing benchmark to test whether there are any > allocations when calling `allocate` on a confined arena. This matches what we > were doing in the other benchmark methods in the same class. This pull request has now been integrated. Changeset: fac63d43 Author: Jorn Vernee <jver...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/fac63d4383c931ea515dcdf7a89e4285f753f41b Stats: 47 lines in 3 files changed: 30 ins; 9 del; 8 mod 8348668: Prevent first resource cleanup in confined arena from escaping Reviewed-by: liach ------------- PR: https://git.openjdk.org/jdk/pull/23321
