RFR: 8316955: Unnecessary memory barrier for reading OopMapCacheEntry from OopMapCache

[Zhengyu Gu]

OopMapCacheEntry is installed by CAS with memory_order_conservative, there is no need a memory barrier on read side.

Test:
  hotspot_gc on MacOSX (fastdebug and release)

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Commit messages:
 - 8316955: Unnecessary memory barrier for reading OopMapCacheEntry from OopMapCache

Changes: https://git.openjdk.org/jdk/pull/15944/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15944&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316955
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15944.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15944/head:pull/15944

PR: https://git.openjdk.org/jdk/pull/15944