RFR: 8323497: On x64, use 32-bit immediate moves for narrow klass base if possible [v6]

[Aleksey Shipilev]

On Thu, 22 May 2025 16:36:28 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fix COH
>
> src/hotspot/cpu/x86/compressedKlass_x86.cpp line 41:
> 
>> 39:   result = reserve_address_space_X(0, nth_bit(32), size, Metaspace::reserve_alignment(), aslr);
>> 40: 
>> 41:   if (result == 0 && optimize_for_zero_base) {
> 
> Should be `result == nullptr`.

Plus, should we really drop `reserve_address_space_for_unscaled_encoding`? Looks to me we are trying additional things for `+COH` there.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17340#discussion_r2103020685