RFR: 8346194: Improve G1 pre-barrier C2 cost estimate

[Thomas Schatzl]

Hi all,

  please review this change that modifies pre-barrier node costs for loop-unrolling to only consider the fast path. The reasoning is similar to zgc (and the new costs as well): only the part of the barrier inlined into the main code stream, as the slow path is laid out separately and does/should not directly affect performance (particularly if there is no marking going on).

There are no differences/impact in performance since the post barrier cost is still very large, which fill be fixed elsewhere.

Testing: gha, perf testing standalone (neither micros nor actual benchmarks give any difference outside of variance), testing with JDK-8342382

Hth,
  Thomas

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Commit messages:
 - 8346194

Changes: https://git.openjdk.org/jdk/pull/23862/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23862&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346194
  Stats: 11 lines in 1 file changed: 5 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23862.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23862/head:pull/23862

PR: https://git.openjdk.org/jdk/pull/23862