RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v2]

[Kelvin Nilsen]

On Tue, 19 Mar 2024 13:16:12 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> When adjusting LAB sizes, round down rather than rounding up.  Otherwise, we may violate the ShenandoahHumongousThreshold bound.
>
> Kelvin Nilsen has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Add comment to explain requirement for PLAB alignment
>  - Assure PLAB alignment by construction rather than fixup

I have made the suggested changes to ShenandoahHeap::allocate_new_plab() and ShenandoahFreeSet::allocate_aligned_plab()

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PR Comment: https://git.openjdk.org/shenandoah/pull/401#issuecomment-2007220479