RFR: 8301819: Enable continuations code by default [v3]

[Daniel D. Daugherty]

On Fri, 10 Feb 2023 09:42:03 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> The continuations code added in JDK 19 with JEP 425 is currently disabled by default and is only enabled when running with preview features enabled (`--enable-preview`). Disabled by default was the pragmatic choice at that time to reduce risk and possible performance regressions. We'd like to change this so the code is enabled by default on the platforms that have a port. Enabling this code by default helps to shake out any issues early in JDK 21 and sets things up to make virtual threads a permanent feature in the future. If something comes up in the next few weeks/months then it can be changed to be disabled by default again. This change has no impact on the builds that do not have a port.
>> 
>> Testing: tier1-tier6
>
> Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Remove include
>  - Merge
>  - Merge
>  - Update copyright date
>  - Initial commit

Marked as reviewed by dcubed (Reviewer).

src/hotspot/share/runtime/continuation.hpp line 42:

> 40: public:
> 41:   static void init();
> 42:   static bool enabled();

Based on the comment, I was expecting this line to be removed.
I'll take it that there's still a way to disable Continuations even after virtual
threads come out of "Preview"?

Update: I see the `VMContinuations` option is still present and experimental.
When will it become non-experimental or removed?

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PR: https://git.openjdk.org/jdk/pull/12459