RFR: 8325870: Zap end padding bits for ArrayOops in non-release builds

[Stefan Karlsson]

On Thu, 15 Feb 2024 07:25:48 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> Make `ObjArrayAllocator` zap the end padding bits in not PRODUCT builds.
> 
> Issues like [JDK-8325074](https://bugs.openjdk.org/browse/JDK-8325074) / #17863 would then reproduce deterministically. Avoid future regressions.
> 
> Guarded `Copy::fill_to_bytes`, it was not clear if or how it handles `count == 0`. Given that almost all dispatch to memset, the `count == 0` would probably be alright, but as `obj_end - padding_in_bytes` may point beyond an object and thus outside the heap it seems prudent to guard it (I believe memset requires a valid pointer).

Marked as reviewed by stefank (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17864#pullrequestreview-1885076177