8385592: Shenandoah: Introduce ShenandoahAllocator interface to encapsulate memory allocation

[pengxiaolong]

The memory allocation path in ShenandoahGC is currently embedded within ShenandoahFreeSet, mixing region-selection strategy with partition management and accounting. This coupling makes it difficult to experiment with alternative allocation strategies (e.g., CAS-based concurrent allocation).

This change introduces a ShenandoahAllocator interface and a concrete ShenandoahSerialAllocator implementation that extracts the allocation logic out of ShenandoahFreeSet. The serial allocator performs all allocations under the heap lock, preserving existing behavior:

• Mutator allocations from the Mutator partition with left/right bias alternation
• Collector allocations from Collector/OldCollector partitions with affiliation preference
• Overflow stealing of empty regions from Mutator for collector use
• Humongous allocations delegated to ShenandoahFreeSet::allocate_contiguous()

Partition accounting (used/capacity tracking, region retirement, affiliated region totals) remains in ShenandoahFreeSet, invoked by the allocator via notify_allocation().

ShenandoahHeap now owns the allocator instance and calls it directly, replacing the previous ShenandoahFreeSet::allocate() call site.

This is a pure refactoring — no behavioral change.

Test:

• MacOS aarch64 fastdebug, hotspot_gc_shenandoah
• Internal benchmark CI

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Progress

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Issue

• JDK-8385592: Shenandoah: Introduce ShenandoahAllocator interface to encapsulate memory allocation (Sub-task - P4)

Reviewers

• Kelvin Nilsen (@kdnilsen - Reviewer) 🔄 Re-review required (review applies to 6804dcec)

Reviewing

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[mlbridge]

Webrevs

• 03: Full - Incremental (95070326)
• 02: Full (6804dcec)
• 01: Full - Incremental (584e424a)
• 00: Full (e7bccc4a)

[kdnilsen]

Thanks for this refactoring. It establishes a good foundation for further integrations.