RFR: 8322692: ZGC: avoid over-unrolling due to hidden barrier size

[Roberto Castañeda Lozano]

On Thu, 11 Jan 2024 08:47:41 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

> This changeset refines the C2 loop unrolling heuristic by including an estimation of the final size of (Generational) ZGC barriers in the loop size computation. These are not exposed in C2's intermediate representation and thus currently ignored by the heuristic, which can lead to over-unrolling.
> 
> #### Testing
> 
> - tier1-5, stress test, fuzzing (windows-x64, linux-x64, linux-aarch64, macosx-x64, macosx-aarch64).
> - tier6-9 (windows-x64, linux-x64, linux-aarch64, macosx-x64, macosx-aarch64, ZGC-specific tests only).
> 
> #### Performance and code size evaluation
> 
> - DaCapo, SPECjvm2008, SPECjbb2015 (linux-x64 with `-XX:+UseZGC -XX:+ZGenerational`). The changeset reduces slightly the size of the C2-generated code (around 0.5% fewer bytes per compiled bytecode for the `fop` and `luindex` DaCapo benchmarks) and has no overall significant performance effect.

Switching back to draft mode to address some offline comments from Erik Österlund.

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PR Comment: https://git.openjdk.org/jdk/pull/17367#issuecomment-1889288008