RFR: 8343658: Parallel: Implement block_start for Young generation

[Albert Mingkun Yang]

On Wed, 6 Nov 2024 08:04:17 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple block_start implementation for Parallel young-gen. Related to https://github.com/openjdk/jdk/pull/21870
> 
> Test: tier1-3

@simonis My intention was simply to align Parallel with the existing implementations in Serial and G1, thereby removing the oddly structured `DebuggingContext` code.

While it’s possible to invest additional effort in object iteration to make failure-reporting more robust, it’s worth noting that the default GCs (Serial and G1) haven’t, to my knowledge, required such measures. Therefore, I wonder if we might be overthinking this without a specific, concrete need.

If you still think this is a step in the wrong direction, I will close the corresponding ticket.

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PR Comment: https://git.openjdk.org/jdk/pull/21919#issuecomment-2574718495