RFR: 8334060: Implementation of Late Barrier Expansion for G1 [v10]

[Kim Barrett]

On Tue, 27 Aug 2024 07:30:46 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> This changeset implements JEP 475 (Late Barrier Expansion for G1), including support for the x64 and aarch64 platforms. See the [JEP description](https://openjdk.org/jeps/475) for further detail.
>> 
>> We aim to integrate this work in JDK 24. The purpose of this pull request is double-fold:
>> 
>> - to allow maintainers of the arm (32-bit), ppc, riscv, s390, and x86 (32-bit) ports to contribute a port of these platforms in time for JDK 24; and
>> - to allow reviewers to review the platform-independent, x64 and aarch64, and test changes in parallel with the porting work.
>> 
>> ## Summary of the Changes
>> 
>> ### Platform-Independent Changes (`src/hotspot/share`)
>> 
>> These consist mainly of:
>> 
>> - a complete rewrite of `G1BarrierSetC2`, to instruct C2 to expand G1 barriers late instead of early;
>> - a few minor changes to C2 itself, to support removal of redundant decompression operations and to address an OopMap construction issue triggered by this JEP's increased usage of ADL `TEMP` operands; and
>> - temporary support for porting the JEP to the remaining platforms.
>> 
>> The temporary support code (guarded by the pre-processor flag `G1_LATE_BARRIER_MIGRATION_SUPPORT`) will **not** be part of the final pull request, and hence does not need to be reviewed.
>> 
>> ### Platform-Dependent Changes (`src/hotspot/cpu`)
>> 
>> These include changes to the ADL instruction definitions and the `G1BarrierSetAssembler` class of the x64 and aarch64 platforms.
>> 
>> #### ADL Changes
>> 
>> The changeset uses ADL predicates to force C2 to implement memory accesses tagged with barrier information using G1-specific, barrier-aware instruction versions (e.g. `g1StoreP` instead of the GC-agnostic `storeP`). These new instruction versions generate machine code accordingly to the corresponding tagged barrier information, relying on the G1 barrier implementations provided by the `G1BarrierSetAssembler` class. In the aarch64 platform, the bulk of the ADL code is generated from a higher-level version using m4, to reduce redundancy.
>> 
>> #### `G1BarrierSetAssembler` Changes
>> 
>> Both platforms basically reuse the barrier implementation for the bytecode interpreter, with the different barrier tests and operations refactored into dedicated functions. Besides this, `G1BarrierSetAssembler` is extended with assembly-stub routines that implement the out-of-line, slow path of the barriers. These routines include calls from the barrier into the JVM, which require support for saving and restoring live ...
>
> Roberto Castañeda Lozano has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Rename g1XChgX to g1GetAndSetX for consistency with Ideal operation names

I've only looked at the changes in gc directories (shared and cpu-specific).

src/hotspot/share/gc/g1/c2/g1BarrierSetC2.cpp line 160:

> 158:  * To reduce the number of updates to the remembered set, the post-barrier
> 159:  * filters out updates to fields in objects located in the Young Generation, the
> 160:  * same region as the reference, when the null is being written, or if the card

s/the null/null/

src/hotspot/share/gc/g1/c2/g1BarrierSetC2.cpp line 166:

> 164:  * post-barrier completely, if it is possible during compile time to prove the
> 165:  * object is newly allocated and that no safepoint exists between the allocation
> 166:  * and the store.

It might be worth saying explicitly that this is a compile-time version of the above mentioned young
generation filter.

src/hotspot/share/gc/g1/c2/g1BarrierSetC2.cpp line 229:

> 227: }
> 228: 
> 229: void refine_barrier_by_new_val_type(Node* n) {

This function should probably be `static`.

-------------

Changes requested by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19746#pullrequestreview-2259069811
PR Review Comment: https://git.openjdk.org/jdk/pull/19746#discussion_r1734167614
PR Review Comment: https://git.openjdk.org/jdk/pull/19746#discussion_r1734196887
PR Review Comment: https://git.openjdk.org/jdk/pull/19746#discussion_r1734207820