JDK-8073463 : G1 does not mangle freed heap regions
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In the call to HeapRegion::hr_clear from G1CollectedHeap::free_region, true is passed for the argument "clear_space", but the region does not become mangled. There is a call to G1OffsetTableContigSpace::clear, but it does not seem to work.
ILW = Low (makes debugging harder but don't affect product), High (always), High (none) = P4
Intentionally guarded by ZapUnusedHeapArea which is only true in debug builds and not available in other builds. This is not specific to G1 either (look in Space::clear())