RFR: 8250269: Replace ATTRIBUTE_ALIGNED with alignas [v17]

[Roman Kennke]

On Fri, 23 Jun 2023 02:31:11 GMT, Julian Waters <jwaters at openjdk.org> wrote:

>> C++11 added the alignas attribute, for the purpose of specifying alignment on types, much like compiler specific syntax such as gcc's __attribute__((aligned(x))) or Visual C++'s __declspec(align(x)).
>> 
>> We can phase out the use of the macro in favor of the standard attribute. In the meantime, we can replace the compiler specific definitions of ATTRIBUTE_ALIGNED with a portable definition. We might deprecate the use of the macro but changing its implementation quickly and cleanly applies the feature where the macro is being used.
>> 
>> Note: With certain parts of HotSpot using ATTRIBUTE_ALIGNED so indiscriminately, this commit will likely take some time to get right
>> 
>> This will require adding the alignas attribute to the list of language features approved for use in HotSpot code. (Completed with [8297912](https://github.com/openjdk/jdk/pull/11446))
>
> Julian Waters has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 17 commits:
> 
>  - Merge branch 'openjdk:master' into alignas
>  - Merge branch 'master' into alignas
>  - Merge branch 'openjdk:master' into alignas
>  - alignas
>  - Merge branch 'openjdk:master' into alignas
>  - Merge branch 'openjdk:master' into alignas
>  - Merge branch 'openjdk:master' into alignas
>  - Merge branch 'openjdk:master' into alignas
>  - Merge branch 'openjdk:master' into alignas
>  - Merge branch 'openjdk:master' into alignas
>  - ... and 7 more: https://git.openjdk.org/jdk/compare/5a82fa3b...bb9ae391

This looks reasonable to me. Would be good to phase out the actual macro uses as well, in the future.
Also, would be great to be able to use offsetof() instead of offset_of() whereever this is possible.

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Marked as reviewed by rkennke (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/11431#pullrequestreview-1579424138