RFR: JDK-8331859 : [PPC64] Remove support for Power7 and older [v16]

[David Linus Briemann]

On Thu, 22 May 2025 04:11:30 GMT, Suchismith Roy <sroy at openjdk.org> wrote:

>> JBS Issue: [JDK-8331859](https://bugs.openjdk.org/browse/JDK-8331859) 
>> Linux PPC64le requires Power8 since the beginning.
>> AIX requires Power8 with the new OpenXL based build ([JDK-8307520](https://bugs.openjdk.org/browse/JDK-8307520)). The old build has been removed in JDK 23 ([JDK-8327701](https://bugs.openjdk.org/browse/JDK-8327701)).
>> Linux PPC64 Big Endian is no longer officially supported (only kept alive for development, debugging and testing purposes).
>> 
>> The following checks for old processors are no longer needed:
>> 8: VM_Version::has_lqarx()
>> 7: VM_Version::has_popcntw()
>> 6: VM_Version::has_cmpb()
>> 5: VM_Version::has_popcntb()
>> These ones and some more checks for old instructions are no longer needed. All code which is no longer reachable when removing them should also get removed.
>> Checks like "PowerArchitecturePPC64 >= 8" (or older) can be removed.
>> 
>> Atomic::PlatformCmpxchg<1>::operator() can be simplified by using sub-word instructions (lharx, lbarx).
>> 
>> Temp registers can be removed from cmpxchgb and cmpxchgh.
>> 
>> Build flags "-mcpu=powerpc64 -mtune=power5" for Big Endian linux should get replaced by "-mcpu=power8 -mtune=power8" as already used for linux PPC64le.
>
> Suchismith Roy has updated the pull request incrementally with one additional commit since the last revision:
> 
>   comment remove and extra %s

Nice cleanup. I have some small suggestions.

src/hotspot/cpu/ppc/c1_LIRAssembler_ppc.cpp line 551:

> 549:       }
> 550:       __ fcfid(rdst, rdst);
> 551:       break;

Would it make sense to split this code into the two separate switch cases `_i2d` and `l2d`, now that the code has fewer lines?

src/hotspot/cpu/ppc/c1_LIRAssembler_ppc.cpp line 562:

> 560:         __ mtfprd(rdst, src->as_register_lo());
> 561:       }
> 562:       __ fcfids(rdst, rdst);

Same as above. Could be split into separate switch cases.

src/hotspot/cpu/ppc/stubGenerator_ppc.cpp line 1531:

> 1529:     __ dcbt(R3_ARG1, 0);
> 1530: 
> 1531:     // If supported set DSCR pre-fetch to deepest.

The `If supported` can be removed from the comment.

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PR Review: https://git.openjdk.org/jdk/pull/20262#pullrequestreview-2863752098
PR Review Comment: https://git.openjdk.org/jdk/pull/20262#discussion_r2104164132
PR Review Comment: https://git.openjdk.org/jdk/pull/20262#discussion_r2104167860
PR Review Comment: https://git.openjdk.org/jdk/pull/20262#discussion_r2104185204