RFR: JDK-8331859 : [PPC64] Remove support for Power7 and older [v15]

[Martin Doerr]

On Wed, 21 May 2025 07:59:36 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 two additional commits since the last revision:
> 
>  - emptyu lines
>  - empty lines

make/autoconf/flags-cflags.m4 line 735:

> 733:       if test "x$FLAGS_CPU" = xppc64; then
> 734:         # -mminimal-toc fixes `relocation truncated to fit' error for gcc 4.1.
> 735:         # Use ppc64 instructions, but schedule for power5

Would be nice to remove this outdated comment.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/20262#discussion_r2099733311