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

[Jorn Vernee]

On Fri, 13 Oct 2023 13:02:24 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Not sure how `jdouble` would be used. JNI doesn't support C structures and the double alignment is only an issue in structures. Do we support embedding `jdouble` in structures? I guess changing it would probably be better?
>> Note that we will need something which maps to the 8-Byte aligned `double`. Otherwise we get an Exception when passing a `JAVA_DOUBLE` as normal argument:
>> 
>> IllegalArgumentException: Unsupported layout: D8
>>         at java.base/jdk.internal.foreign.abi.AbstractLinker.checkSupported(AbstractLinker.java:244)
>>         at java.base/jdk.internal.foreign.abi.AbstractLinker.checkLayoutRecursive(AbstractLinker.java:185)
>>         at java.base/jdk.internal.foreign.abi.AbstractLinker.checkLayout(AbstractLinker.java:179)
>>         at java.base/java.lang.Iterable.forEach(Iterable.java:75)
>>         at java.base/jdk.internal.foreign.abi.AbstractLinker.checkLayouts(AbstractLinker.java:171)
>>         at java.base/jdk.internal.foreign.abi.AbstractLinker.downcallHandle0(AbstractLinker.java:98)
>>         at java.base/jdk.internal.foreign.abi.AbstractLinker.downcallHandle(AbstractLinker.java:85)
>>         at TestDowncall.<clinit>(TestDowncall.java:127)
>
>> I guess changing it would probably be better?
> 
> Yeah, I think so.
> 
>> Note that we will need something which maps to the 8-Byte aligned double. Otherwise we get an Exception when passing a JAVA_DOUBLE as normal argument.
> 
> Okay, so it sounds like Java double on AIX is still 8-byte aligned, but the native double is 4-byte aligned?
> 
> In that case, I'd say that passing `JAVA_DOUBLE` as an argument resulting in an exception, is expected behavior. Using `JAVA_DOUBLE` to link native functions works more or less by coincidence, since it has the same layout as the native `double`, on all other platforms. The AIX issue with double is not dissimilar to using JAVA_LONG to link against a function taking a native `long` on Windows, in which case the size of the type doesn't match.
> 
> The philosophy here is: a client is responsible for passing the right FunctionDescriptor/layouts, that matches the native function declaration. On AIX, JAVA_DOUBLE is just never the right layout, and the linker detects this and throws an exception.
> 
> I understand that some of the existing tests might be lazy and use JAVA_DOUBLE to link functions. It is fine to update those tests, e.g. to use C_DOUBLE defined in NativeTestHelper.

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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16179#discussion_r1358279022