RFR: 8361376: Regressions 1-6% in several Renaissance in 26-b4 only MacOSX aarch64

[Thomas Schatzl]

On Sat, 19 Jul 2025 01:39:12 GMT, Dean Long <dlong at openjdk.org> wrote:

> This PR removes the recently added lock around set_guard_value, using instead Atomic::cmpxchg to atomically update bit-fields of the guard value.  Further, it takes a fast-path that uses the previous direct store when at a safepoint.  Combined, these changes should get us back to almost where we were before in terms of overhead.  If necessary, we could go even further and allow make_not_entrant() to perform a direct byte store, leaving 24 bits for the guard value.

Afaics the `NMethodEntryBarrier_lock` declaration/definition can also be removed.

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PR Comment: https://git.openjdk.org/jdk/pull/26399#issuecomment-3113913189