RFR: JDK-8314890: Reduce number of loads for Klass decoding in static code [v6]

[Thomas Stuefe]

On Fri, 6 Oct 2023 17:18:01 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> A few stylistic comments.
> 
> What is confusing to me is that combo flag initialization is basically conditional on `UseCompressedClassPointers` (i.e. assert in new `set_base_and_shift`). But at the same time, we ask for `CompressedKlassPointers::use_compressed_klass_pointers()` in `oop`methods. This works "only" because the -UseCCP generates the same combo as the initial value of `0`? Seems fragile. I wonder if we want to initialize combo unconditionally.

I know. I played around with a different version before but ended up relying on the static initialization since it seemed the most simple and robust one. I added a comment to explain this better.

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PR Comment: https://git.openjdk.org/jdk/pull/15389#issuecomment-1755445730