On Thu, 7 May 2026 08:27:44 GMT, Paul Hübner <[email protected]> wrote:
>> Hi all, >> >> This patch fixes some performance issues with AArch64's >> `NMethodEntryBarrier`. >> >> The purpose of a method entrypoint is to act as a barrier for potential GC >> work. Entrypoints can be "armed", such that when the method is called, a >> slow-path is triggered. This slow-path then does any necessary fixups. >> >> With the introduction of scalarized calling conventions in Valhalla, methods >> have multiple entrypoints, depending on the calling conventions that are >> used. This is due to the fact that `nmethod`s cannot be multiplexed when >> scalarized. There can be up to three entrypoints for a given method. The >> locations of these entrypoints is tracked via relocations. >> >> The problem lies within the usage of `RelocIterator`. When `static void >> set_value` is called (such as when arming entrypoints), it creates a >> `NativeNMethodBarrier` for each entrypoint. However, the `RelocIterator` >> only finds the first relocation, meaning each embodiment of >> `NativeNMethodBarrier` sees the same `_guard_addr`. Consequently, executions >> entering through the other entrypoints will not see the up-to-date armed >> value, and be unnecessarily slow-pathed. >> >> This patch introduces several improvements: >> 1. Makes `NativeNMethodBarrier` use the `ldr` instructions to determine the >> `_guard_addr`s, as opposed to relocations (contributed by @TobiHartmann). >> This gets the correct addresses for each of the guards, and avoids having to >> iterate through the `nmethod`'s relocations, which is expensive for large >> methods. >> 2. Consolidates all the entrypoints as first-class citizens of >> `NativeNMethodBarrier`, instead of having an `NativeNMethodBarrier` per >> entrypoint. This reduces object allocation, and makes the code much easier >> to read for free! >> 3. Stricter assertions. All entrypoints `ldr` instructions are verified (as >> opposed to just the scalarized ones previously). All guard addresses are >> verified to be within the `nmethod`'s instructions/stubs. >> >> Testing: tiers 1-4 and some adhoc testing with `--enable-preview`. >> >> --------- >> - [x] I confirm that I make this contribution in accordance with the >> [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai). > > Paul Hübner has updated the pull request incrementally with one additional > commit since the last revision: > > Update src/hotspot/cpu/aarch64/gc/shared/barrierSetNMethod_aarch64.cpp > > Co-authored-by: Tobias Hartmann <[email protected]> Thanks all for the reviews. ------------- PR Comment: https://git.openjdk.org/valhalla/pull/2389#issuecomment-4396816876
