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

[Ron Pressler]

> 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  Revert "Remove outdated comment"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.

-------------

Changes:
  - all: https://git.openjdk.org/jdk19/pull/66/files
  - new: https://git.openjdk.org/jdk19/pull/66/files/fe8fe94f..4680aed2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk19&pr=66&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk19&pr=66&range=00-01

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk19/pull/66.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/66/head:pull/66

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