RFR: 8351666: [PPC64] Make non-volatile VectorRegisters available for C2 register allocation [v6]

[Martin Doerr]

> This PR makes the non-volatile VectorRegisters available for C2's register allocation.
> 
> I had to implement the VectorRegisters properly (4 VM Regs) like on other platforms. The old version has run into assertions and looked strange.
> 
> The non-volatile VectorRegisters are now saved when entering Java: call_stub and upcall_stubs.
> I have rewritten the save and restore functions and used them for both. Then, I have removed code which has become dead. I only save and restore them if C2 uses the vector instructions (controlled by `SuperwordUseVSX`).
> I have moved the non-volatile spill area out of the entry_frame_locals because it has a variable size, now.
> 
> The stack area for all non-volatile registers has become larger than the 288 Bytes which are allowed to be used below the SP (specified by the ABI). Therefore, I had to rewrite the call_stub sequence significantly. We need to push the new frame before saving the registers, now.
> 
> Saving and restoring the FP registers is not needed in the slow signature handler which also uses the save and restore code for non-volatile registers.
> 
> On Power10, we use vector pair instructions since Commit 8. E.g. in the call stub:
> 
>   0x000072c9483c07b4:   stxvp   vs52,-224(r2)
>   0x000072c9483c07b8:   stxvp   vs54,-192(r2)
>   0x000072c9483c07bc:   stxvp   vs56,-160(r2)
>   0x000072c9483c07c0:   stxvp   vs58,-128(r2)
>   0x000072c9483c07c4:   stxvp   vs60,-96(r2)
>   0x000072c9483c07c8:   stxvp   vs62,-64(r2)
> 
> 
> 
>   0x000072c9483c0914:   lxvp    vs52,-224(r2)
>   0x000072c9483c0918:   lxvp    vs54,-192(r2)
>   0x000072c9483c091c:   lxvp    vs56,-160(r2)
>   0x000072c9483c0920:   lxvp    vs58,-128(r2)
>   0x000072c9483c0924:   lxvp    vs60,-96(r2)
>   0x000072c9483c0928:   lxvp    vs62,-64(r2)

Martin Doerr has updated the pull request incrementally with two additional commits since the last revision:

 - Improve comments.
 - Move block comments from upcallLinker to MacroAssembler.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23987/files
  - new: https://git.openjdk.org/jdk/pull/23987/files/8142a36e..82880def

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23987&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23987&range=04-05

  Stats: 24 lines in 3 files changed: 8 ins; 12 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23987.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23987/head:pull/23987

PR: https://git.openjdk.org/jdk/pull/23987