RFR: heapbase register can be allocated in compressed mode

[Pengfei Li]

Hi Xin,

> I read JDK-8234794 but I don't understand why that change involves in r27
> and CompressedOop.

JDK-8234794 is the metaspace reservation fix. It also simplifies the encoding/decoding of compressed class pointers. Before that patch, r27 is used for both compressed oops and compressed class pointers. At that time we have to consider if r27 is allocatable if compressed class pointers is on. But after that patch, r27 is for compressed oops only. That's why I could simplify my JDK-8233743 patch after JDK-8234794 was merged.

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Thanks,
Pengfei