8370850: Shenandoah: Simplify collector allocation to save unnecessary region iteration

[pengxiaolong]

To allocate an object in Collector/OldCollector partition, current implementation may traverse the regions in the partition twice:

1. fast path: traverse regions between left most and right most in the partition, and try to allocate in an affiliated region in the partition;
2. if fails in fast path, traverse regions between left most empty and right most empty in the partition, and try try to allocate in a FREE region.

2 can be saved if we also remember the first FREE region seem in 1.

The PR makes the code much cleaner, and more efficient(although the performance impact may not be measurable, I have run some dacapo benchmarks and didn't see meaningful difference)

Test:

• hotspot_gc_shenandoah

---

Progress

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Issue

• JDK-8370850: Shenandoah: Simplify collector allocation to save unnecessary region iteration (Enhancement - P4)

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[openjdk]

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• hotspot-gc
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[mlbridge]

Webrevs

• 02: Full (8e01d691)
• 01: Full - Incremental (2c73b9f7)
• 00: Full (81cd8ab0)

[earthling-amzn]

Are we losing the check here? Before this change, the region had to be FREE and the generation affiliated with the request had to have free_unaffiliated_regions.

[pengxiaolong]

I was confused about this part, if we can find a FREE region in the partition, why we still need to test if the generation must have free_unaffiliated_regions, I can add can_allocate_in_new_region back to retain the original behavior.

[earthling-amzn]

The young and old collector reserves both have FREE regions, but we need to make sure we don't exceed the reserves for one or the other.

[pengxiaolong]

I have added the can_allocate_in_new_region check back, it should have same behavior now.

[pengxiaolong]

I have removed can_allocate_in_new_region again after merging @kdnilsen's change unifying accountings, since we don't have consistency in accountings anymore.

[earthling-amzn]

We still need to enforce that requests for a young evacuation don't take memory that was reserved for old evacuations. can_allocate_in_new_region isn't about consistency between freeset and generation accounting, it's used to maintain old/young collector reserves.