RFR: 8295842: Generational ZGC: Elide barriers on variable array accesses after array allocations

[Roberto Castañeda Lozano]

This changeset extends C2's barrier elision so that variable array accesses (array accesses with unknown offset at compile-time) are elided when they are dominated by the array's allocation and no safepoint is present in between, like in this example:


Element[] a = new Element[10]; // allocation
a[i] = e; // variable array access: the barrier is elided


This is achieved by relaxing the dominator-dominated pair test in [zBarrierSetC2.cpp](https://github.com/openjdk/zgc/blob/a460756f08df30af232fc631e94cf37c10c38352/src/hotspot/share/gc/z/c2/zBarrierSetC2.cpp#L714) so that unknown memory access offsets are accepted for array allocation dominators.

This refinement does not yield significant overall throughput changes in standard benchmark suites (DaCapo, SPECjvm2008), but has been observed to at least elide a few additional  barriers in individual benchmarks, e.g. DaCapo's H2:


Value[] keyValues = new Value[groupIndex.length]; // array allocation
// update group
for (int i = 0; i < groupIndex.length; i++) {
    int idx = groupIndex[i];
    Expression expr = expressions.get(idx);
    keyValues[i] = expr.getValue(session); // store barrier is elided by this changeset
}


#### Testing

- tier1-5 (windows-x64, linux-x64, linux-aarch64, macosx-x64, and macosx-aarch64; release and debug mode)

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Commit messages:
 - Elide barriers on variable array accesses after array allocations

Changes: https://git.openjdk.org/zgc/pull/8/files
 Webrev: https://webrevs.openjdk.org/?repo=zgc&pr=8&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295842
  Stats: 60 lines in 1 file changed: 57 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/zgc/pull/8.diff
  Fetch: git fetch https://git.openjdk.org/zgc pull/8/head:pull/8

PR: https://git.openjdk.org/zgc/pull/8