[foreign-memaccess+abi] RFR: 8255903: Enable multi-register return values for native invokers [v4]

[Jorn Vernee]

On Fri, 29 Oct 2021 12:20:50 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Hi,
>> 
>> This patch implements support for multi-register returns in native invokers, and removes the buffered invocation strategy for downcalls. This is achieved, essentially, by using an in memory return: the caller allocates a bit of memory, and the native invoker stub writes the values of the return register to that memory. Then, the post processing code reads the register values back from there.
>> 
>> Currently, the target address of a downcall is handled separately from the other arguments. I initially implemented passing the IMR address the same way, but I realized the removal of the buffered invocation strategy affords us a better way of doing things: we can just make the target address and the IMR address part of the normal calling sequence, and remove a bunch of special-casing code to handle these. We now just use a binding recipe to unbox these arguments and shuffle them into registers, and the ABIDescriptor tells the native invoker stub which registers to pick up these arguments from.
>> 
>> Thanks,
>> Jorn
>
> Jorn Vernee has updated the pull request incrementally with six additional commits since the last revision:
> 
>  - Benchmark fix
>  - Rename FunctionDescriptor methods again
>  - Fix tests again
>  - Rename imr -> needsReturnBuffer
>  - Address small comments
>  - Fix failing tests

I've just pushed a new iteration that basically renames `imr` to `needsReturnBuffer` everywhere (including variants like `retBuf*` instead of `imr*`).

The tests failures were a case of the newly added tests needing changes for the changes in this PR (fixed as well).

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PR: https://git.openjdk.java.net/panama-foreign/pull/603