RFR: 8341893: AArch64: Micro-optimize compressed ptr decoding

[Andrew Haley]

On Thu, 10 Oct 2024 07:57:14 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See the bug for full description. I think there is an accidental inefficiency in decoding: we don't have to shift when shift is zero, but we are tripped by `d != s` check. Other architectures seem to do this right. This fix surgically fixes the decoding. I have a variant of a more comprehensive MacroAssembler fix that could cover this shift-to-mov translation wholesale, but that fix is riskier.
> 
> Additional testing:
>  - [x] Linux AArch64 server fastdebug, `tier{1,2,3}`

Marked as reviewed by aph (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/21443#pullrequestreview-2361007920