RFR: 8340205: Native linker allows MemoryLayout consisting of only PaddingLayout [v4]

[Maurizio Cimadamore]

On Mon, 23 Sep 2024 13:54:52 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR prevents sequence layout with padding to be used with the Linker.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add to specification and refine detection of PL GLs

src/java.base/share/classes/java/lang/foreign/Linker.java line 264:

> 262:  * <li>{@code G} does not contain padding other than what is strictly required to align
> 263:  *      its non-padding layout elements, or to satisfy (2), and</li>
> 264:  * <li>{@code G} is {@code G.memberLayouts().isEmpty()} or (at the same time) not all

I think @JornVernee mentioned that empty layouts are ok and/or accepted by C compilers. We just don't know what to do with padding-only groups.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21041#discussion_r1771591756