[lworld] RFR: 8373494: [lworld] markWord preservation for ParallelGC [v4]

[Coleen Phillimore]

On Fri, 12 Dec 2025 18:06:10 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

>> Yes, header_size() includes both markWord and Klass* and is 2 words for !UseCOH and 1 word for UseCOH. The header is safe to read if we have all of it, i.e. the full size. Otherwise it is not safe to read the Klass*, as it could be it the second word.
>> 
>> I'm not totally opposed to move the EnableValhalla check, but what we have right now is pretty similar to other places working with markWord's, like oopDesc::init_mark(), which I think is a plus. 
>> 
>> Also, moving the EnableValhalla check inside safe_to_read_header should give it another name, like (very explicit name) "need_to_read_header_and_safe". Keeping it the way it is allows a much easier refactor in the future once the EnableValhalla check is gone.
>
> FYI, I accidentally sent my last comment twice and I deleted the other one so the mail-link got broken :)

If you move EnableValhalla to inside safe_to_read_header(), then we'll only have to fix it once when it's only set when Arguments::is_valhalla_enabled() is set.  Right now EnablePreview is true by default.
EnableValhalla is a property that determines the answer to safe_to_read_header(), so I think it should go there.

Can you add a comment to safe_to_read_header() - the second sentence to the first paragraph above?

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1785#discussion_r2615336124