RFR: 8258032: Reconsider LEAF entry restrictions

Fri Dec 18 19:43:05 UTC 2020

This change allows JRT_LEAF functions to create Handles because it doesn't need them but some code inside the VM needs Handles.  There's a NoSafepointVerifier in JRT_LEAF functions.
The JNI_LEAF and JVM_LEAF functions have NoHandleMark (so you can't create handles) but the transition ThreadInVMfromNative will reenable the HandleMarks, so that all this code doesn't have to do it itself.
Tested with tier1-6.

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Commit messages:
 - 8258032: Reconsider LEAF entry restrictions

Changes: https://git.openjdk.java.net/jdk/pull/1846/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1846&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258032
  Stats: 102 lines in 17 files changed: 7 ins; 69 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1846.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1846/head:pull/1846

PR: https://git.openjdk.java.net/jdk/pull/1846