RFR: Fix Minimal VM build failures

[Serguei Spitsyn]

On Mon, 27 Sep 2021 10:57:07 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Minimal VM configs have no C2 and no JVMTI. New Loom code fails the Minimal VM builds because of this. See for example GHA runs: https://github.com/shipilev/loom/runs/3717293969?check_suite_focus=true
> 
> C2 needs just a little bit of protection when reaching to `DerivedPointersTable`.
> 
> JVMTI needs a bit more work. `jvmtiExport` and `jvmtiThreadState` headers are usually included even with JVMTI turned off, but `.cpp` would not be compiled without JVMTI. Therefore, non-trivial implementations should go into `.cpp`. Plus, some of the paths that call the actual methods declared but not implemented without JVMTI should be protected with `INCLUDE_JVMTI`.
> 
> Additional testing:
>  - [x] Linux x86_64 server, `tier1_loom` still passes (includes JVMTI tests)
>  - [x] Linux x86_64 minimal now builds, runs `tier1_loom` (cannot pass JVMTI tests without JVMTI)

Aleksey,
I've inlined some comments.
Other than that it looks okay to me.
In fact, it is not easy to identify what else has to be under `#INCLUDE_JVMTI`.
Thanks,
Serguei

src/hotspot/share/runtime/thread.cpp line 1790:

> 1788: #if INCLUDE_JVMTI
> 1789:   assert(JvmtiVTMTDisabler::VTMT_count() == 0, "must be 0");
> 1790: #endif

All body needs to be under `#if INCLUDE_JVMTI ... #endif` for functions:
  `set_is_in_VTMT` and `set_is_VTMT_disabler` .
Is there any specific reason not to do that?
Also, it makes sense to add this fragment as well:

#else
  ShouldNotReachHere();
  return NULL;
#endif

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Marked as reviewed by sspitsyn (Committer).

PR: https://git.openjdk.java.net/loom/pull/67