RFR: 8317545: AIX PPC64: Implementation of Foreign Function & Memory API [v2]

[Jorn Vernee]

On Fri, 13 Oct 2023 14:03:34 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>>> Okay, so it sounds like Java double on AIX is still 8-byte aligned, but the native double is 4-byte aligned?
>> 
>> Correct within structures. I believe they are always 8-byte aligned except in structures which are packed more densely.
>> 
>>> In that case, I'd say that passing `JAVA_DOUBLE` as an argument resulting in an exception, is expected behavior. 
>> 
>> Hmm... Should we disallow to pass 8-byte aligned double values? Fixing the tests is a good thing. But, I have a different concern: I guess many developers may use them without testing on AIX. This would cause problems which we could avoid by supporting 8-byte aligned doubles in addition.
>> 
>> What about adding
>> 
>> layouts.put("jdouble", ValueLayout.JAVA_DOUBLE.withByteAlignment(4));
>> layouts.put("aligneddouble", ValueLayout.JAVA_DOUBLE); // allowed for usage outside of structures
>> 
>> ?
>
>> Just to clarify, is the ABI equal to what is described in this table: https://www.ibm.com/docs/en/xl-c-and-cpp-aix/16.1?topic=data-using-alignment-modes ?
>> 
>> This shows `double` having 8-byte alignment, but being 4-byte aligned when not the first member of an aggregate.
> 
> Interesting! This matches my understanding. I can see that doubles are 4-byte aligned within structures. The 8-byte alignment of the first member is already established by the allocation alignment.
> 
> I have only read the ABI documentation for linux. IBM will have to cover AIX specific details if there are more questions.

Discussed this a bit with Maurizio. There are a few things to consider:

- `cannonicalLayouts` allows for just a single mapping from type name to layout. I think `double` should map to the 8-byte aligned layout. (the same alignment you get from `_Alignof(double)` in C).
- We have to relax the checking done by the linker on AIX. Maybe change `AbstractLinker::checkLayoutRecursive` call some `checkStructMemberLayout` method, that by default just calls `checkLayoutRecursive`, but which the AIX linker can override to implement it's special rules for doubles.
- We need a small spec update to allow for struct field layouts that are not canonical layouts. I'll take a stab at that, and then get back to you.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16179#discussion_r1358344403