RFR: JDK-8300783: Consolidate byteswap implementations [v13]

[Justin King]

On Tue, 31 Jan 2023 20:05:21 GMT, Justin King <jcking at openjdk.org> wrote:

>> Deduplicate byte swapping implementations by consolidating them into `utilities/byteswap.hpp`, following `std::byteswap` introduced in C++23. Further simplification of `Bytes` will follow in https://github.com/openjdk/jdk/pull/12078.
>
> Justin King has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix copyright
>   
>   Signed-off-by: Justin King <jcking at google.com>

> > Oh, if we do not care about old XLC shall we drop it elsewhere? I assume TARGET_COMPILER_xlc is the old XLC?
> > I filed JDK-8301987 to remove `TARGET_COMPILER_xlc` and any legacy XL C/C++ specific stuff. We should be able to treat Open XL C/C++ as Clang.
> 
> TARGET_COMPILER_xlc now designates the new Clang-based compiler. It has various options compatible with the old non-Clang compiler, and things like that. Remember that it's Clang-_based_, and so far as I can tell from afar, is its own dialect. I think it's up to the aix-ppc port maintainers (the only OpenJDK port using it) to decide / address those kinds of issues.

I do not particularly care about old XLC, but new XLC is fine. It looks like it handles the builtins the same, it should generate the single instruction to store with reversal.

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PR: https://git.openjdk.org/jdk/pull/12114