RFR: 8323497: On x64, use 32-bit immediate moves for narrow klass base if possible [v5]

[Thomas Stuefe]

On Thu, 22 May 2025 13:14:58 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>>> Some code density estimates:
>>> 
>>> ```
>>> $ for I in 1 2 3 4; do build/linux-x86_64-server-release/images/jdk/bin/java -XX:TieredStopAtLevel=${I} -Xcomp -XX:+CITime Hello.java 2>&1 | grep "Tier${I}" | cut -d' ' -f 3,23-; done
>>> 
>>> # Before
>>> Tier1: nmethods_code_size: 7468288 bytes
>>> Tier2: nmethods_code_size: 7947176 bytes
>>> Tier3: nmethods_code_size: 17979088 bytes
>>> Tier4: nmethods_code_size: 6113728 bytes
>>> 
>>> # After
>>> Tier1: nmethods_code_size: 7421344 bytes    ; -0.6%
>>> Tier2: nmethods_code_size: 7900176 bytes    ; -0.6%
>>> Tier3: nmethods_code_size: 17805232 bytes   ; -1.0%
>>> Tier4: nmethods_code_size: 6102368 bytes    ; -0.2%
>>> ```
>> 
>> Well, every little bit counts.
>> 
>> Yes, please review. I will be running tests before pushing but otherwise I think this is fine as it is.
>
>> Well, every little bit counts.
> 
> `0.6..1.0%` improvement in code density is great for such a small change. This is why I want it sooner :)

Thank you @shipilev. I will wait for a second review and for @vnkozlov to finish his tests.

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PR Comment: https://git.openjdk.org/jdk/pull/17340#issuecomment-2904832945