[foreign-memaccess+abi] RFR: 8275644: Replace VMReg in shuffling code with something more fine grained. [v2]

[Jorn Vernee]

On Wed, 10 Aug 2022 12:51:29 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> This patch replaces `VMReg` with a new `VMStorage` class in the C++ code, which can be used to represent stack arguments of different sizes, as well as allowing finer grained stack addressing.
>> 
>> The old masm code that used `VMReg` for moving values is also replaced with new code that uses `VMStorage`. And `VMReg` is changed to `VMStorage` everywhere.
>> 
>> The `vmstorageBase` class was added in C++ to mirror the `jdk.internal.foreign.VMStorage`. That class now also has an additional field to encode the size of a stack argument, or a register mask, which can also be used to encode the size of a register argument. This also leads to some stubs needing to be updated on unsupported platforms.
>> 
>> Lastly, instead of encoding stack indexes as 64-bit slots as we did before, they are now encoded as byte offsets, which allows finer grained stack addressing. (this leads to a bunch of values being multiplied by 8 in the Java code as well).
>> 
>> ---
>> 
>> This change is needed to address https://bugs.openjdk.org/browse/JDK-8275584 as well as adding support for `long double` as well as more exotic ABIs that take arguments as vectors.
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   rename index -> indexOrOffset

I'm still testing but I added some comments, and changed the name to indexOrOffset/index_or_offset on both sides. (I also got rid of the union in the C++ code, since it didn't really buy us anything)

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PR: https://git.openjdk.org/panama-foreign/pull/699