RFR: 8334247: [PPC64] Consider trap based nmethod entry barriers [v3]

[Martin Doerr]

On Wed, 2 Jul 2025 22:32:01 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 incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - 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

We'll probably get back to this after [JDK-8360540](https://bugs.openjdk.org/browse/JDK-8360540) is resolved.

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PR Comment: https://git.openjdk.org/jdk/pull/24135#issuecomment-3049185284