[foreign-memaccess+abi] RFR: 8255903: Enable multi-register return values for native invokers [v2]

[Jorn Vernee]

> 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 with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Merge branch 'foreign-memaccess+abi' into Multi_Reg_Returns
 - Fix whitespace errors
 - Remove spurious import
 - AArch64 fixes
 - Fold more data into CallingSequence
 - Pass down target and imr addr regs through ABIDescriptor
 - handle target address and imr address in callingSequence
 - Remove downcall buffered invocation strategy
 - AArch64 changes
 - Implement MH specialization for multi-reg returns
 - ... and 2 more: https://git.openjdk.java.net/panama-foreign/compare/0d35f42d...8e79856c

-------------

Changes: https://git.openjdk.java.net/panama-foreign/pull/603/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=603&range=01
  Stats: 1307 lines in 38 files changed: 422 ins; 601 del; 284 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/603.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/603/head:pull/603

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