RFR: 8334247: [PPC64] Consider trap based nmethod entry barriers [v5]

[Sorna Sarathi N]

On Wed, 10 Sep 2025 13:33:20 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>> We can shrink nmethod entry barriers to 4 instructions (from 8) using conditional trap instructions. Some benchmarks seem to show very small improvements. At least the code size reduction is an advantage.
>
> Martin Doerr has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:
> 
>  - Merge remote-tracking branch 'origin' into 8334247_PPC64_trap_based_nmethod_entry_barrier
>  - Merge remote-tracking branch 'origin' into 8334247_PPC64_trap_based_nmethod_entry_barrier
>  - Move nmethod entry barrier code up in the signal handler.
>  - Merge remote-tracking branch 'origin' into 8334247_PPC64_trap_based_nmethod_entry_barrier
>  - Merge remote-tracking branch 'origin' into 8334247_PPC64_trap_based_nmethod_entry_barrier
>  - 8334247: [PPC64] Consider trap based nmethod entry barriers

I carefully reviewed the proposed changes. The patch looks good to me. The code seems to be clean and well contained. I don’t see any correctness issues. 

On the performance side, the benchmark results consistently demonstrate a substantial improvement in performance compared to the baseline. Importantly, I did not see any regressions in functionality.

Overall, the code is clean and shows clear performance gains. LGTM.

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Marked as reviewed by ssarathi (Author).

PR Review: https://git.openjdk.org/jdk/pull/24135#pullrequestreview-3279016448