[foreign-memaccess+abi] RFR: 8308293: A linker should expose the layouts it supports [v4]

[ExE Boss]

On Fri, 16 Jun 2023 14:17:29 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This is another stab at https://github.com/openjdk/jdk/pull/14037
>> 
>> I believe, after some offline discussion, that we have found a more satisfying solution to the problem of JAVA_CHAR being exposed. Jorn suggested that linkers should also provide mappings for JNI types such as `jint`, `jshort` and such (which are aliases for our layout constants anyway). I think that's a great way to bring `JAVA_CHAR` back into the fold.
>> 
>> For now, I decided not to specify support for JNI canonical layouts (but I could do so, if that's preferred). I think the highest priority is to provide some stable mappings for C builtin types (+ `size_t`) as that's what 99% of developers will be struggling with.
>> 
>> API-wise, we just expose a map. In the preovious PR there were questions as to whether the map should be split into two methods. In general I see the following options:
>> 
>> 1. Just expose a map (that's the primitive, other things can derived from it)
>> 2. Expose a map, plus a method to get a canonical layout from a type name (that's the `Charset` approach, which has both `availableCharsets` *and* `forName`)
>> 3. Expose a method to get canonical layout from name, plus a method that returns the set of supported canonical layout names
>> 
>> My (not so strong) preference would be for either (1) or (2).
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add support for `wchar_t`

Changes requested by ExE-Boss at github.com (no known OpenJDK username).

src/java.base/share/classes/jdk/internal/foreign/abi/fallback/FallbackLinker.java line 299:

> 297:     @Override
> 298:     public Map<String, MemoryLayout> canonicalLayouts() {
> 299:         return CANONICAL_LAYOUTS;

`CANONICAL_LAYOUTS` is set to a `HashMap`, which is not unmodifiable.
Suggestion:

        return Collections.unmodifiableMap(CANONICAL_LAYOUTS);

-------------

PR Review: https://git.openjdk.org/panama-foreign/pull/839#pullrequestreview-1556754886
PR Review Comment: https://git.openjdk.org/panama-foreign/pull/839#discussion_r1280478696