RFR: 8341708: Optimize safepoint poll encoding with smaller poll data offset

[Vladimir Kozlov]

On Tue, 8 Oct 2024 10:42:40 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See the bug for discussion. We can optimize the encoding for safepoint polls by making sure the polling data is at small offset in `Thread`. There is already the area where we pull data like this for better encoding.
> 
> Code density improves on x86_64, can be seen with just `-Xcomp -XX:+CITime`:
> 
> 
> # Before
>   nmethod code size         :  7107136 bytes
>   nmethod code size         :  7107120 bytes
>   nmethod code size         :  7107136 bytes
> 
> # After (-0.25%)
>   nmethod code size         :  7088896 bytes
>   nmethod code size         :  7088896 bytes
>   nmethod code size         :  7088864 bytes
> 
> 
> Additional testing:
>  - [x] Ad-hoc code density checks
>  - [x] Linux x86_64 server fastdebug, `all`

Good.

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Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21405#pullrequestreview-2357741248