RFR: 8331418: ZGC: generalize barrier liveness logic

[Martin Doerr]

On Tue, 30 Apr 2024 18:43:03 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!

src/hotspot/share/gc/shared/c2/barrierSetC2.cpp line 127:

> 125:   while (OptoReg::is_reg(reg)) {
> 126:     const VMReg vm_reg = OptoReg::as_VMReg(reg);
> 127:     if (!(vm_reg->is_Register()) || vm_reg->as_Register() != r) {

This doesn't work on PPC64: We run into "assert(is_Register() && is_even(value())) failed: even-aligned GPR name" (vmreg_ppc.hpp:54). Calling `as_Register()` is only supported for the even ones.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19026#discussion_r1585549387