RFR: 8327963: [Umbrella] Incorrect result of C2 compiled code since JDK-8237581

[Quan Anh Mai]

On Mon, 14 Apr 2025 15:35:17 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> It would be great if we have union memory slices for this.
>
>> It would be great if we have union memory slices for this.
> 
> Something like that would fix it but it would be trickier to get right that this point fix, I think. Do you see any other use for it?

@rwestrel There are other places in C2 when the memory slices are not expressive enough, which leads us to wrapping those accesses in `CPUMemBar` because otherwise the memory edges would be incorrect. This would become more prominent in Valhalla as atomic flat accesses try to write multiple fields at once.

https://github.com/openjdk/valhalla/blob/6b476fa7d67bf80591ed86d291b167d3d5ee5c28/src/hotspot/share/opto/inlinetypenode.cpp#L855

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PR Comment: https://git.openjdk.org/jdk/pull/24570#issuecomment-2811880177