[foreign-memaccess+abi] RFR: 8287516: Implement fallback Linker [v6]

[Jorn Vernee]

On Fri, 20 Jan 2023 15:43:32 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> src/java.base/share/classes/jdk/internal/foreign/abi/fallback/FFIType.java line 141:
>> 
>>> 139:                     }
>>> 140:                 }
>>> 141:                 expectedOffset += element.byteSize();
>> 
>> I think the offset should only be added for structs, not for unions
>
> We can never encounter a union layout in this code. In libffi they are represented as a single element struct.

Support for unions is not implemented in the fallback linker at the moment [1](https://github.com/openjdk/panama-foreign/pull/770/files#diff-b2d5d47f7117501ef2f8d7c497f0e2d999ac4374ae0a17aacaf12590cb2fc111R116). I wanted to do that as part of: https://bugs.openjdk.org/browse/JDK-8300294 when we have some tests, and there might be issues on other linkers as well that need fixing.

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PR: https://git.openjdk.org/panama-foreign/pull/770