[foreign-abi] RFR: 8254156: Simplify ABI classification logic

[Maurizio Cimadamore]

On Thu, 8 Oct 2020 09:49:17 GMT, Chris Vest <github.com+7993+chrisvest at openjdk.org> wrote:

>> This patch reverts recent changes to introduce a public CValueLayout class; the realization here is that, after all,
>> CValueLayout exposes a public `Type` enum which is used by ABI classification. So, instead of using subclassing (which
>> is messy and tedious with immutable data structures with covariant overrides), let's just double down on layout
>> attributes, get rid of CValueLayout and simply expose the Type enum (now renamed to TypeKind).
>
> src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/CLinker.java line 749:
> 
>> 747:          * A kind corresponding to the a C pointer type
>> 748:          */
>> 749:         POINTER(false);
> 
> Pointer or pointer-sized? We'd also use this one for `size_t` for instance, right?

This is pointer; e.g. values of this layout will be associated with the MemoryAddress.class carrier. So, no, for
size_t, you need something different (either LONG or LONGLONG, depending on the platform). We could probably add a
SIZE_T in there, since it's frequent enough.

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PR: https://git.openjdk.java.net/panama-foreign/pull/371