RFR: 8331418: ZGC: generalize barrier liveness logic [v2]

[Roberto Castañeda Lozano]

On Thu, 2 May 2024 07:57:18 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> This changeset generalizes the logic to analyze, declare, and communicate which registers are live at a C2 barrier stub so that it can be used by other collectors than ZGC adopting the late barrier expansion model (including G1 in the near future, see [JEP 475](https://openjdk.org/jeps/475)).
>> 
>> The main changes are:
>> 
>> - Make it possible to compute register liveness information before (live-in) or after (live-out) each barrier, and let the collector choose by implementing `BarrierSetC2State::needs_livein_data()`.
>> 
>> - Generalize the interface with which collectors declare which registers must be additionally preserved across barrier runtime calls, adding the methods `BarrierStubC2::preserve(Register r)` and `BarrierStubC2::dont_preserve(Register r)`.
>> 
>> - Simplify the interface with which platform-specific logic computes which registers to preserve across barrier runtime calls, replacing the calls to `BarrierStubC2::result()` and `BarrierStubC2::live()` with a single call to `BarrierStubC2::preserve_set()`.
>> 
>> #### Testing
>> 
>> - tier1-5 (windows-x64, linux-x64, linux-aarch64, macosx-x64, macosx-aarch64; release and debug mode).
>> - tier1-7 (linux-x64, linux-aarch64; release and debug mode; ZGC tests only) with [an additional patch](https://github.com/openjdk/jdk/commit/4d4e743d8f4cddd5288cee1d69c70fe2b9bea066) that exercises the spilling and restoring logic by forcing ZGC read barriers to always take the slow path and clearing all general-purpose save-on-call registers upon the slow path's runtime call.
>> - Build with `make hotspot` (linux-riscv64-debug, linux-ppc64le-debug). @RealFYang, @TheRealMDoerr: could you please test and review the riscv and ppc changes? Thanks!
>
> Roberto Castañeda Lozano has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use VMReg::is_concrete for testing sub-registers

Thanks for reviewing, Martin! I reported your suggested refactoring here: https://bugs.openjdk.org/browse/JDK-8331623.

> I haven't thought about future usages of `BarrierSetC2State::needs_livein_data()`. I guess it's intended for G1.

That's correct, it is primarily intended for G1. But ZGC could also benefit, in the future, from using live-out instead of live-in data in the spilling logic. The current solution is slightly over-conservative in that it might spill some registers unnecessarily.

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

PR Comment: https://git.openjdk.org/jdk/pull/19026#issuecomment-2092395247