RFR: JDK-8314890: Reduce number of loads for Klass decoding in static code [v9]

[Thomas Stuefe]

On Wed, 8 Nov 2023 10:36:59 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:
>> 
>>  - Renamed _combo
>>  - Merge branch 'master' into optimize-narrow-klass-decoding-in-c++
>>  - simplify assert
>>  - add comment
>>  - Update src/hotspot/share/oops/compressedKlass.hpp
>>    
>>    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.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>
>>  - Merge branch 'openjdk:master' into optimize-narrow-klass-decoding-in-c++
>>  - Merge branch 'openjdk:master' into optimize-narrow-klass-decoding-in-c++
>>  - ... and 6 more: https://git.openjdk.org/jdk/compare/9864951d...56cde2a9
>
> src/hotspot/share/oops/compressedKlass.cpp line 45:
> 
>> 43:   assert(theshift == 0 || theshift == LogKlassAlignmentInBytes, "invalid shift for klass ptrs");
>> 44:   _base = thebase;
>> 45:   _shift = theshift;
> 
> Do we even need `_base` and `_shift` as separate fields after this change then?

No, I don't think so. Lets remove it.

> src/hotspot/share/oops/compressedKlass.hpp line 67:
> 
>> 65:   // - Bit  [0-7]   shift
>> 66:   // - Bit  8       UseCompressedClassPointers
>> 67:   // - Bits [16-64] the base.
> 
> Suggestion:
> 
>   // - Bit  [0-7]   shift
>   // - Bit  8       UseCompressedClassPointers
>   // - Bits [16-64] heap base

Let's call it encoding base and encoding shift. It is not the heap base either.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15389#discussion_r1394144917
PR Review Comment: https://git.openjdk.org/jdk/pull/15389#discussion_r1394146454