RFR: 8288949: serviceability/jvmti/vthread/ContStackDepthTest/ContStackDepthTest.java failing

[Dean Long]

On Fri, 24 Jun 2022 20:39:08 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.
>
> src/hotspot/share/code/compiledIC.cpp line 591:
> 
>> 589:   // Do not reset stub here:  It is too expensive to call find_stub.
>> 590:   // Instead, rely on caller (nmethod::clear_inline_caches) to clear
>> 591:   // both the call and its stub.
> 
> While at it, I noticed this comment, which appears to be out of date.

I read that comment more as a warning, in case in the future someone wondered why we don't reset the stub here, and tried to add it.  So I would leave it.  I think it's still useful.

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