RFR: 8292818: replace 96-bit representation for field metadata with variable-sized streams [v3]

[Chris Plummer]

On Mon, 13 Mar 2023 17:29:28 GMT, Frederic Parain <fparain at openjdk.org> wrote:

>> src/hotspot/share/runtime/vmStructs.cpp line 2304:
>> 
>>> 2302:   declare_constant(FieldInfo::FieldFlags::_ff_generic)                    \
>>> 2303:   declare_constant(FieldInfo::FieldFlags::_ff_stable)                     \
>>> 2304:   declare_constant(FieldInfo::FieldFlags::_ff_contended)                  \
>> 
>> If there are flags that SA doesn't use, like contended, I don't think they should be included in the information that we pass to SA.
>
> The contended flag is required to be able to decode the compressed stream, because it signals the presence of an optional part of a field description. The only flag that was not required for the decoding of the stream and was not used by the SA was Stable, and I'll remove it in the next commit.

Leaving it in allows the field to be displayed if an SA user ever dumps a FieldFlags object. Generally speaking it is good to keep these structs complete, or at least complete with any info that might be useful when debugging with SA.

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PR: https://git.openjdk.org/jdk/pull/12855