RFR: JDK-8323497: On x64, use 32-bit immediate moves for narrow klass base if possible [v3]

[Aleksey Shipilev]

On Wed, 21 Feb 2024 07:35:23 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> src/hotspot/cpu/x86/compressedKlass_x86.cpp line 44:
>> 
>>> 42:       result = reserve_address_space_for_zerobased_encoding(size, aslr);
>>> 43:     }
>>> 44:   } else {
>> 
>> This should be:
>> 
>>     if (result == nullptr) {
>>         // If we cannot use zero-based encoding (when CDS is enabled), optimizing for an
>>         ...
>>     }
>
> I don't think so. Why?

I think the point is to reflow this into the series of ifs:


char* result = nullptr;
if (optimize_for_zero_base) {
   result = ...
}

if (result == nullptr) {
   result = ...
}


Probably good for future maintenance if we want to add more cases in this block.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17340#discussion_r1497302551