On Thu, 5 Feb 2026 14:32:44 GMT, Axel Boldt-Christmas <[email protected]> wrote:
> `IS_DEST_UNINITIALIZED ` is ment to signal to the GC that you cannot read the > contents of the field as it is uninitialized. This is used by the compiler > which have strict control over its safepoints. > > The MemAllocator in the runtime has no such guarantees, and will clear all > oop fields before handing the allocated object. These objects can have been > seen by the GC and even tenured before the call > `HeapAccess<IS_DEST_UNINITIALIZED>::value_copy`. This is unsound. ZGC for > example could miss young to old edges if the destination object had been > tenured. > > I propose we remove these and always use `HeapAccess<>::value_copy`. The > behaviour will be the same for value objects which do not contain oops, and > correct of values which contains oops. There is the potential in the future > to add something along the lines of `IS_DEST_NULL` to provide more static > information to the GC to optimise the barriers on. However the gain here is > probably negligible. Looks good to me. Thank you for the detailed explanation. Fred ------------- Marked as reviewed by fparain (Committer). PR Review: https://git.openjdk.org/valhalla/pull/2048#pullrequestreview-3759553679
