On Tue, 20 Jan 2026 17:56:00 GMT, Joel Sikström <[email protected]> wrote:
> Hello, > > Today much of the reading of flattened values in the interpreter is > implemented both in the platform-specific templateTable_XX.cpp files, and > also in the runtime via InterpreterRuntime::read_flat_field. The reason we > have both is becuase the interpreterlet implements a fast-path for null-free > types, which attempts to allocate the buffered object inside the thread's > TLAB, before moving on to copy the src payload to the buffered objected. The > copying is done in the VM via a call_VM_leaf call, which notably does not > safepoint-poll (nor allow anything that might GC for example). > > The slow-path in the interpreterlet calls into the VM via a call_VM, which > notably safepoint-point polls upon exit. The slow-path is taken when 1) the > src payload is nullable or 2) the fast-path TLAB allocation fails. > > I propose we redesign the dance around when and how to enter the VM by having > the logic of reading a flat field exclusively inside the runtime and thus > always entering the VM. This approach allows us to have one canonical way to > read flat fields, without having effectively duplicate code in > interpreterlets (for all supported platforms) and inside the runtime. A > benefit from this is that it becomes easier for porters to port the Valhalla > changes to their platform(s) and later on maintain that port, which is a plus. > > Since all objects are NULLABLE in JEP 401 (disregarding F&F interface), all > reads of flat fields are already entering the VM via the "slow-path". This > means that this change only has a practical effect on > null-free/null-restricted fields. I think we should consider if having a > fast-path is worth it or not when that time comes, although I anticipate that > it doesn't make much difference since the copy is always done in the VM > anyway. > > As a small optimization, I've added a check for nullable and marked-as-null > before entering the VM. > > Testing: > * hotspot_valhalla, jdk_valhalla and Oracle's tier1-4 on linux-x64-debug and > linux-aarch64-debug This pull request has now been integrated. Changeset: b2f45e0f Author: Joel Sikström <[email protected]> Committer: Paul Hübner <[email protected]> URL: https://git.openjdk.org/valhalla/commit/b2f45e0f857fb32bdc36e7b93346ebc967b74736 Stats: 140 lines in 11 files changed: 11 ins; 113 del; 16 mod 8375719: [lworld] Move reads of flat fields entirely to the runtime Reviewed-by: coleenp, phubner, fparain ------------- PR: https://git.openjdk.org/valhalla/pull/1936
