Scan _isAscii through a byteArrayViewVarHandle instead of Unsafe.getLong

[merlimat]

Motivation

String materialization (readString) decides between the zero-copy LATIN1 fast path and full UTF-8 decoding by scanning the copied bytes with _isAscii, 8 bytes at a time via sun.misc.Unsafe.getLong. Since JDK 24 (JEP 498) every sun.misc.Unsafe memory-access call executes beforeMemoryAccess() with an acquire-load, so the scan pays a fence per 8 bytes on every materialized string of 8+ chars — and those calls are also on the eventual-removal track.

Changes

_isAscii now scans through a MethodHandles.byteArrayViewVarHandle(long[].class, nativeOrder()) view: plain unfenced 8-byte loads on a supported API. The 0x8080808080808080 mask test is byte-order independent. The now-unused MH_GET_LONG plumbing is removed.

Results

No effect on the existing benchmarks — the only string they materialize is 3 chars, which never enters the 8-byte loop. On a temporary benchmark materializing a 57-char topic-style name (persistent://my-tenant/my-namespace/my-topic-partition-42, parse + getName(), 3 forks per side, JDK 26):

	ops/µs
Unsafe.getLong scan	35.69 ± 1.67
VarHandle scan	37.28 ± 0.70 (+4.5%)

Longer strings benefit proportionally more. This is also one step of the planned migration off sun.misc.Unsafe memory-access methods before their removal.

286 tests green.

[Copilot]

Pull request overview

This PR updates LightProto’s string materialization fast path to avoid sun.misc.Unsafe.getLong fences on JDK 24+ by switching ASCII scanning in _isAscii to use a MethodHandles.byteArrayViewVarHandle(long[].class, nativeOrder()) for plain (unfenced) 8-byte loads, and removes the now-unused MH_GET_LONG MethodHandle plumbing.

Changes:

• Replace the _isAscii 8-byte scan implementation from Unsafe.getLong (via MH_GET_LONG) to a VarHandle byte[]→long view using plain get.
• Remove MH_GET_LONG declarations/initialization now that it’s unused.
• Simplify _isAscii by removing the MethodHandle invokeExact try/catch wrapper that was only needed for Throwable.

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