RFR: 8315548: G1: Document why VM_G1CollectForAllocation::doit() may allocate without completing a GC

[Ivan Walulya]

Hi all,

Please review this trivial change to add a comment explaining why we attempt to allocate at a VM_G1CollectForAllocation safepoint before triggering garbage collection.

Thanks,
Ivan

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Commit messages:
 - add comment

Changes: https://git.openjdk.org/jdk/pull/15559/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15559&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315548
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15559.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15559/head:pull/15559

PR: https://git.openjdk.org/jdk/pull/15559