RFR: 8329032: C2 compiler register allocation support for APX EGPRs [v6]

[Jatin Bhateja]

On Tue, 18 Jun 2024 17:07:27 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

>> Jatin Bhateja has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:
>> 
>>  - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8329032
>>  - 32-bit build fix.
>>  - Review comments resolutions.
>>  - jvmci test failures fixes
>>  - 32-bit build fixes.
>>  - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8329032
>>  - Changes to skip over stack alignment gaps while popping registers using POP2 after comment from sviswa7
>>  - 32 bit build fix and enforced stack alignment constraints.
>>  - Support new PUSH2/POP2 instructions along with Push-Pop Acceleration (PPX) to optimize register save/restore operation.
>>  - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8329032
>>  - ... and 4 more: https://git.openjdk.org/jdk/compare/6f860f8f...8db22672
>
> src/hotspot/cpu/x86/assembler_x86.cpp line 14315:
> 
>> 14313:     push2p(rbx, rdx);
>> 14314:     // To maintain 16 byte alignment after rcx is pushed.
>> 14315:     subq(rsp, 8);
> 
> Just curious, why do we need to maintain 16 byte alignment here? It looks to me that the subq(rsp, 8) is not required here. The next push is not a push2.

Its a safety padding inserted to ensure 16 byte stack alignment constraints are preserved for subsequent consumers of stack after pusha.

> src/hotspot/cpu/x86/assembler_x86.cpp line 14327:
> 
>> 14325:     // from the value of rsp immediately after pusha (rsp + 16 * wordSize).
>> 14326:     // FIXME: For APX any such direct access should also consider EGPR size
>> 14327:     // during address compution.
> 
> This comment could be removed.

Its added in context of above comment which talks about computing the original RSP prior to pushing GPRs state on stack.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19042#discussion_r1645481972
PR Review Comment: https://git.openjdk.org/jdk/pull/19042#discussion_r1645482023