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

[Aleksey Shipilev]

On Wed, 10 Jan 2024 13:03:39 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> That change seems to hold water on its own, saving quite a bit of code. Filed: [JDK-8323503](https://bugs.openjdk.org/browse/JDK-8323503).
>
> @shipilev I like this. I'll wait for your patch to go in.

I think it would be better to move to `movptr` here and start testing it. I suspect there are a few places in Hotspot where we count the actual instruction _size_ for some stuff, and even the `mov64` -> `movptr` rewrite without `movl` optimization would highlight it. I agree the final version should be tested when JDK-8323503 is in.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17340#discussion_r1447506558