RFR: 8372513: Shenandoah: ShenandoahMaxRegionSize can produce an unaligned heap alignment

[Stefan Karlsson]

While rewriting some of the heap size initialization code we hit a corner-case where the setting of `ShenandoahMaxRegionSize` to something that isn't a power-of-2 will hit an assert in `max_heap_for_compressed_oops`.

When running with:

java -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahMaxRegionSize=33m -version


 The following code:

  size_t displacement_due_to_null_page = align_up(os::vm_page_size(),
                                                  _conservative_max_heap_alignment)

triggers:

# assert(is_power_of_2(alignment)) failed: must be a power of 2: 34603008

because `_conservative_max_heap_alignment` is not a power-of-2.

This happens because Shenandoah's `conservative_max_heap_alignment()`
 returns a potentially unaligned `ShenandoahMaxRegionSize` value.


size_t ShenandoahArguments::conservative_max_heap_alignment() {
  size_t align = ShenandoahMaxRegionSize;
  if (UseLargePages) {
    align = MAX2(align, os::large_page_size());
  }
  return align;
}


I propose a small fix to adjust `align` to be a power-of-2. I've also added an earlier assert about this in `set_conservative_max_heap_alignment` and added an additional test-case in TestRegionSizeArgs.java

WDYT, is this an OK fix for this corner-case?

-------------

Commit messages:
 - 8372513: Shenandoah: ShenandoahMaxRegionSize can produce an unaligned heap alignment

Changes: https://git.openjdk.org/jdk/pull/28492/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28492&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372513
  Stats: 14 lines in 3 files changed: 13 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28492.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28492/head:pull/28492

PR: https://git.openjdk.org/jdk/pull/28492