[jdk19] RFR: 8288949: serviceability/jvmti/vthread/ContStackDepthTest/ContStackDepthTest.java failing [v2]

[Ron Pressler]

On Sat, 25 Jun 2022 01:23:47 GMT, Ron Pressler <rpressler at openjdk.org> wrote:

>> Please review the following bug fix:
>> 
>> `Continuation.enterSpecial` is a generated special nmethod (albeit not a Java method), with a well-known frame layout that calls `Continuation.enter`.
>> 
>> Because it is compiled, it resolves the call to `Continuation.enter` to its compiled version, if available. But this results in the compiled `Continuation.enter` being called even when the thread is in interp_only_mode.
>> 
>> This change does three things:
>> 
>> 1. When entering interp_only_mode, `Continuation::set_cont_fastpath_thread_state` will clear enterSpecial's resolved callsite to Continuation.enter.
>> 2. In interp_only_mode, `SharedRuntime::resolve_static_call_C` will return `Continuation.enter`'s c2i entry rather than `verified_code_entry`.
>> 3. In interp_only_mode, the c2i stub will not patch the callsite.
>> 
>> This fix isn't perfect, because a different thread, not in interp_only_mode, might patch the call. A longer-term solution is to create an "interpreted" version of `enterSpecial` and supporting an ad-hoc deoptimization. See https://bugs.openjdk.org/browse/JDK-8289128
>> 
>> 
>> Passes tiers 1-4 and Loom tiers 1-5.
>
> Ron Pressler has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert "Remove outdated comment"
>   
>   This reverts commit 8f571d76e34bc64ceb31894184fba4b909e8fbfe.

Trying a new approach of having another entry into `enterSpecial`, used only when in interp-only-mode, and where the call to `Continuation.enter` always resolves to its interpreted version. This requires more platform-specific code, and also makes the frame appear not `frame::safe_for_sender` when at that callsite, but losing an async poll when in interp_only_mode doesn't seem to be a big issue, and the problem can be easily fixed as JFR is too eager to call `frame::safe_for_sender`.

Passes tiers 1-4 as well as Loom tiers 1-5.

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PR: https://git.openjdk.org/jdk19/pull/66