RFR: 8256406: G1 x86 C1/Interpreter post write barrier always uses 32 bit to access variable sized PtrQueue::_index

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that fixes benign issues with the G1 C1/interpreter barriers that on x64 only used the lower 32 bit word of the 64 bit PtrQueue::_index member?

Testing: tier1, linux-x86 builds

Thanks,
  Thomas

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Commit messages:
 - Initial import

Changes: https://git.openjdk.java.net/jdk/pull/1257/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1257&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256406
  Stats: 19 lines in 2 files changed: 7 ins; 6 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1257.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1257/head:pull/1257

PR: https://git.openjdk.java.net/jdk/pull/1257