RFR: 8351666: [PPC64] Make non-volatile VectorRegisters available for C2 register allocation [v24]

[Martin Doerr]

On Wed, 14 May 2025 08:17:19 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> Martin Doerr has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 35 additional commits since the last revision:
>> 
>>  - Improve readability of LXVX_OPCODE.
>>  - Merge remote-tracking branch 'origin' into 8351666_PPC64_nv_VRs
>>  - Add comment and clean up extra whitespaces.
>>  - Merge remote-tracking branch 'origin' into 8351666_PPC64_nv_VRs
>>  - Merge remote-tracking branch 'origin' into 8351666_PPC64_nv_VRs
>>  - Make order of MachRegisters consistent with ConcreteRegisterImpl and simplify rc_class.
>>  - Rewrite rc_class avoiding hard coded register numbers.
>>  - Add comment regarding 8-Byte aligned stack slots for VecX.
>>  - Add missing VSRs to RegisterSaver_LiveVSReg.
>>  - Merge remote-tracking branch 'origin' into 8351666_PPC64_nv_VRs
>>  - ... and 25 more: https://git.openjdk.org/jdk/compare/fbbe0ea3...e3cc8bcd
>
> src/hotspot/cpu/ppc/ppc.ad line 2013:
> 
>> 2011:       int dst_offset = ra_->reg2offset(dst_lo);
>> 2012:       if (PowerArchitecturePPC64 >= 9) {
>> 2013:         if (is_aligned(dst_offset, 16)) {
> 
> If `Matcher::_new_SP` would be aligned to 16 bytes then the offset here would always be also 16 byte aligned. Can be handled separately from this pr.

Changing to `_new_SP = OptoReg::Name(align_up(_in_arg_limit, (int)stack_alignment_in_slots()));` should solve it for PPC64.  But I don't know if such a change would be appreciated for other platforms. Some platforms have larger vectors than `StackAlignmentInBytes` and they seem to work fine with 8 Byte aligned vector stack slots. PPC64 actually also works fine with the current alignment, except that it requires an additional add instruction which is a bit ugly.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23987#discussion_r2088630777