RFR: 8338526: Don't store abstract and interface Klasses in class metaspace [v5]

[Coleen Phillimore]

On Fri, 6 Sep 2024 11:08:02 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add function in Metaspace to tell you if Klass pointer is in compressible space.
>
> src/hotspot/share/memory/metaspace.hpp line 165:
> 
>> 163:     return using_class_space() && (is_in_class_space(k) || is_in_shared_metaspace(k));
>> 164:   }
>> 165: 
> 
> I propose to drop this, and instead add a utility function to `CompressedKlassPointers` like this:
> 
> 
> // Returns true if p falls into the narrow Klass encoding range
> inline bool CompressedKlassPointers::is_in_encoding_range(const void* p) {
>   return _base != nullptr && p >= _base && p < (_base + _range);
> }
> 
> (Probably  the `_base != nullptr` could even be left out, since `_range==0` and `_base==nullptr` for -UseCompressedClassPointers)
> 
> And then use that function in `jfrTraceIdKlass.cpp`. That file needs to use `CompressedKlassPointers` anyway because it needs to encode the Klass*.
> 
> This avoids having to rely on the exact composition of the memory regions inside the encoding range. What counts is whether the Klass pointer points into the narrow Klass encoding range.
> 
> Essentially, with CDS, memory looks like this:
> 
> 
> encoding                                                               encoding              
> base                                                                   end
> |-----------------------------------------------------------------------|
> |----CDS---| |--------------------class space---------------------------|

oh yes that's much better.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19157#discussion_r1747241623