RFR: JDK-8314890: Reduce number of loads for Klass decoding in static code [v12]

[Andrew Haley]

On Thu, 11 Jan 2024 19:43:17 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Thomas Stuefe has updated the pull request incrementally with three additional commits since the last revision:
>> 
>>  - Update src/hotspot/share/oops/compressedKlass.cpp
>>    
>>    Co-authored-by: Aleksey Shipilëv <shipilev at amazon.de>
>>  - Update src/hotspot/share/oops/compressedKlass.cpp
>>    
>>    Co-authored-by: Aleksey Shipilëv <shipilev at amazon.de>
>>  - Update src/hotspot/share/oops/compressedKlass.hpp
>>    
>>    Co-authored-by: Aleksey Shipilëv <shipilev at amazon.de>
>
> Looks good to me! Thank you!
> (And regarding the bitfield discussion - I *would* like to handle object header fields as bitfield, eventually. Not sure if it makes sense though or if we ever get there...)

> @rkennke
> 
> > (And regarding the bitfield discussion - I would like to handle object header fields as bitfield, eventually. Not sure if it makes sense though or if we ever get there...)
> 
> Yes. I wonder whether this would result in better code generated by the C++ compiler,

I doubt it very much. FYI, I believe that every compiler recognizes the canonical bitfield extract `(x << n) >> m` and some e.g. AArch64 have a single-instruction for that.

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PR Comment: https://git.openjdk.org/jdk/pull/15389#issuecomment-1895522679