RFR: 8340205: Native linker allows MemoryLayout consisting of only PaddingLayout [v8]

[Per Minborg]

On Tue, 5 Nov 2024 14:20:32 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>>> Also, don't we already check the alignment before when calling `checkMemberOffset`? Why is it checked again here?
>> 
>> Good catch.
>
>> What happens here if an element is already aligned, and `padding.byteSize() == element.byteAlignment()`? e.g. `MemoryLayout.structLayout(MemoryLayout.paddingLayout(4), ValueLayout.JAVA_INT)`?
> 
> This is an error that existed before this PR. On the main branch:
> 
> 
>     @Test
>     public void alignedByInitialPadding() {
>         Linker linker = Linker.nativeLinker();
>         var struct = MemoryLayout.structLayout(
>                 MemoryLayout.paddingLayout(Integer.BYTES),
>                 JAVA_INT);
>         var fd = FunctionDescriptor.of(struct, struct, struct);
>         linker.downcallHandle(fd);
>     }
> 
> 
> would produce:
> 
> 
> test TestLinker.alignedByInitialPadding(): failure
> java.lang.IllegalArgumentException: Member layout 'i4', of '[x4i4]' found at unexpected offset: 4 != 0
> 	at java.base/jdk.internal.foreign.abi.AbstractLinker.checkMemberOffset(AbstractLinker.java:235)
> 	at java.base/jdk.internal.foreign.abi.AbstractLinker.checkLayoutRecursive(AbstractLinker.java:195)
> 	at java.base/jdk.internal.foreign.abi.AbstractLinker.checkLayout(AbstractLinker.java:176)
> 	at java.base/java.util.Optional.ifPresent(Optional.java:178)
> 	at java.base/jdk.internal.foreign.abi.AbstractLinker.checkLayouts(AbstractLinker.java:167)
> 	at java.base/jdk.internal.foreign.abi.AbstractLinker.downcallHandle0(AbstractLinker.java:98)
> 	at java.base/jdk.internal.foreign.abi.AbstractLinker.downcallHandle(AbstractLinker.java:92)
> 	at TestLinker.alignedByInitialPadding(TestLinker.java:160)
> 	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)

I've raised a separate issue for this: https://bugs.openjdk.org/browse/JDK-8343620

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PR Review Comment: https://git.openjdk.org/jdk/pull/21041#discussion_r1829445374