RFR: 8332082: Shenandoah: Use SATB active flag for C2 pre-write barrier on x86 and PPC

[William Kemper]

On Fri, 10 May 2024 16:13:51 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This is consistent with c1 and other platforms.

Both the gc-state and satb-active flags are synchronized to thread local fields on a safepoint. I expect the cost for the barrier to access either of them is similar. However, there is some additional c2 code that looks like it recognizes the barrier IR based on the presence of nodes to load the gc state: (for example, `ShenandoahBarrierSetC2::is_shenandoah_marking_if`).

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PR Comment: https://git.openjdk.org/jdk/pull/19180#issuecomment-2108547726