RFR: 8290892: C2: Intrinsify Reference.reachabilityFence [v3]

[Vladimir Ivanov]

> This PR introduces C2 support for `Reference.reachabilityFence()`.
> 
> After [JDK-8199462](https://bugs.openjdk.org/browse/JDK-8199462) went in, it was discovered that C2 may break the invariant the fix relied upon [1]. So, this is an attempt to introduce proper support for `Reference.reachabilityFence()` in C2. C1 is left intact for now, because there are no signs yet it is affected.
> 
> `Reference.reachabilityFence()` can be used in performance critical code, so the primary goal for C2 is to reduce its runtime overhead as much as possible. The ultimate goal is to ensure liveness information is attached to interfering safepoints, but it takes multiple steps to properly propagate the information through compilation pipeline without negatively affecting generated code quality.
> 
> Also, I don't consider this fix as complete. It does fix the reported problem, but it doesn't provide any strong guarantees yet. In particular, since `ReachabilityFence` is CFG-only node, nothing explicitly forbids memory operations to float past `Reference.reachabilityFence()` and potentially reaching some other safepoints current analysis treats as non-interfering. Representing `ReachabilityFence` as memory barrier (e.g., `MemBarCPUOrder`) would solve the issue, but performance costs are prohibitively high. Alternatively, the optimization proposed in this PR can be improved to conservatively extend referent's live range beyond `ReachabilityFence` nodes associated with it. It would meet performance criteria, but I prefer to implement it as a followup fix.
> 
> Another known issue relates to reachability fences on constant oops. If such constant is GCed (most likely, due to a bug in Java code), similar reachability issues may arise. For now, RFs on constants are treated as no-ops, but there's a diagnostic flag `PreserveReachabilityFencesOnConstants` to keep the fences. I plan to address it separately. 
> 
> [1] https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/ref/Reference.java#L667
> "HotSpot JVM retains the ref and does not GC it before a call to this method, because the JIT-compilers do not have GC-only safepoints."
> 
> Testing:
> - [x] hs-tier1 - hs-tier8
> - [x] hs-tier1 - hs-tier6 w/ -XX:+StressReachabilityFences -XX:+VerifyLoopOptimizations
> - [x] java/lang/foreign microbenchmarks

Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:

  renaming

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25315/files
  - new: https://git.openjdk.org/jdk/pull/25315/files/ad314e05..ca2809af

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25315&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25315&range=01-02

  Stats: 5 lines in 4 files changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/25315.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25315/head:pull/25315

PR: https://git.openjdk.org/jdk/pull/25315