RFR: 8351666: [PPC64] Make non-volatile VectorRegisters available for C2 register allocation [v4]

[Richard Reingruber]

On Wed, 23 Apr 2025 13:20:11 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>> The comments in HotSpot make it clear that VMRegs are 4 bytes wide on all platforms, and stack slots are the size of VMRegs, and physical registers are modelled as some number of VMRegs. I never knew how PPC worked before.
>> 
>>> Hm, is it really necessary to model the vector registers as 4 32-bit parts?
>
> Seems like pretending that only one 4 Byte slot was used even though a VSX register occupies 16 Bytes has worked for the register allocation. That may have strange implications because some code doesn't expect that.

It might be that currently in master only VSR32, VSR36, VSR40, ... are actually used.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23987#discussion_r2056144796