RFR: JDK-8277178: Reduce the priority of data dependent nodes when OptoScheduling enabled [v6]

[SUN Guoyun]

On Fri, 14 Jan 2022 17:52:33 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> My tier1-3 testing passed clean (x86 and Aarch64). But performance variations on x86 is significant. I am rerunning it again to confirm it. If it is confirmed (may be next day) can you consider apply changes only when `OptoScheduling` is on:
> 
> ```
>    bool block_size_threshold_ok = (recalc_pressure_nodes != NULL) && (block->number_of_nodes() > 10);
> +  Node* last_n = C->do_scheduling() ? block->get_node(--sched_slot) : nullptr;
> ```
> 
> Also please check `nullptr` instead of implicit null check:
> 
> ```
>      if (last_n != nullptr) {
> ```

Yes, you are right. MachNode latency in lcm will be 0 when OptoScheduling is false. so add ` Node* last_n = C->do_scheduling() ? block->get_node(--sched_slot) : nullptr;` is reasonable.

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PR: https://git.openjdk.java.net/jdk/pull/6407