8369068: GenShen: Generations still aren't reconciled assertion failure

[earthling-amzn]

There are certain code paths in Shenandoah's generational mode that need to know which generation is being collected, but it is not possible to pass this information on the stack (barriers, for example). To address this, we introduced an _active_generation member in ShenandoahHeap. Over time, the usage of this field grew beyond its intended purpose and we began to have issues where not all threads would see a consistent value for this field. To address this issue, we added another field _gc_generation which was only meant to be used by GC threads. At this point, we have three different ways to determine which generation is being collected: the active generation, the gc generation, and the usual function parameters and member fields of the gc components. This PR removes _gc_generation and reduces the use of _active_generation to only those places where it is not possible to get this information from other means (barriers, mostly). All GC components that can have this information passed through function calls, now do so.

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Issue

• JDK-8369068: GenShen: Generations still aren't reconciled assertion failure (Bug - P4)

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