RFR: 8334247: [PPC64] Consider trap based nmethod entry barriers [v2]

[Martin Doerr]

> We can shrink nmethod entry barriers to 4 instructions (from 8) using conditional trap instructions. Some benchmarks seem to show very small improvements. At least the code size reduction is an advantage.

Martin Doerr has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge remote-tracking branch 'origin' into 8334247_PPC64_trap_based_nmethod_entry_barrier
 - 8334247: [PPC64] Consider trap based nmethod entry barriers

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24135/files
  - new: https://git.openjdk.org/jdk/pull/24135/files/67442d4b..dfc6aa5d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24135&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24135&range=00-01

  Stats: 552727 lines in 7278 files changed: 228189 ins; 286910 del; 37628 mod
  Patch: https://git.openjdk.org/jdk/pull/24135.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24135/head:pull/24135

PR: https://git.openjdk.org/jdk/pull/24135