On Wed, 17 Dec 2025 10:00:06 GMT, Joel Sikström <[email protected]> wrote:
>> Hello, >> >> ZGC optimizes initialization by performing segmented clearing for objArrays, >> which reduces time-to-safepoint. In the lworld branch, we currently opt out >> of segmented clearing if the objArray being initialized is a flatArray. This >> prevents us from taking advantage of shorter time-to-safepoints when the >> flatArray could be cleared in segments. >> >> The main question is: which types of flatArrays should support segmented >> clearing? Since ZGC only supports 64-bit atomic operations, flatArrays >> containing oops are not possible without relying on internal-only features >> like loose-consistency and null-restriction. A value object containing an >> oop and the added null-marker will always exceed 64 bits with ZGC, and >> therefore such objects will not be flattened in practice due to the 64-bit >> atomicity constraint. >> >> Given this, we are currently missing the opportunity to use segmented >> clearing for flatArrays that contain only primitive types, which we should >> add support for. Support for flatArrays containing oops can be considered in >> the future, once features like loose-consistency and null-restriction are >> available to the user. >> >> Testing: >> * hotspot_valhalla, jdk_valhalla, tier1-4, with `-XX:+UseZGC` >> >> * Some sanity testing in lldb to see if I get segmented clearing for any >> flatArrays, and I can see several flatArrays containing java/lang/Integer or >> java/lang/Character being cleared in segmentes. > > Joel Sikström has updated the pull request incrementally with one additional > commit since the last revision: > > Offline review feedback Marked as reviewed by stefank (Author). ------------- PR Review: https://git.openjdk.org/valhalla/pull/1811#pullrequestreview-3586932044
