RFR: 8343658: Parallel: Implement block_start for Young generation

[Albert Mingkun Yang]

On Thu, 7 Nov 2024 12:46:25 GMT, Volker Simonis <simonis at openjdk.org> wrote:

>>> > this will not work in the general case, if the heap is not walkable.
>>> 
>>> True, but this is the best-effort approach used in other GCs, as far as I can tell. Is there a real use case that warrants a more sophisticated variant?
>> 
>> The **only** use case for this code during hs_err reporting for heap-addresses not pointing at the beginning of an oop. I think we should be conservative here, because a secondary crash will cut the information available in the hs_err file and will therefor do more harm then being helpful.
>
>> What would be nice would be something like `oopDesc::safe_klass_or_null()` or similar, feeding into a corresponding `oopDesc::size_given_klass_safe_or_0()`. The former would check the klass word for validity before dereferencing - `CompressedKlassPointers::is_encodable(p)` and then the load of layouthelper etc should happen with SafeFetch. Alternatively (and a bit more unsafe), check the readability of Klass* with SafeFetch beforehand, then call normal size_given_klass.
> 
> We already have [LocationPrinter::is_valid_obj()](https://github.com/openjdk/jdk/blob/ac82a8f89c7066fb1d379b12bcfd68053cb39ba4/src/hotspot/share/gc/shared/locationPrinter.cpp#L33) which uses [Klass::is_valid()](https://github.com/openjdk/jdk/blob/ac82a8f89c7066fb1d379b12bcfd68053cb39ba4/src/hotspot/share/oops/klass.cpp#L1038) to check the validity of an oop. I don't think we need `SafeFetch` here.

> I think we should be conservative here, because a secondary crash will cut the information available in the hs_err file 

Have you ever seen "a secondary crash" in practice (for other GCs)? I am a bit concerned that we might be adding complex code that is never exercised.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21919#discussion_r1836428995