[foreign-memaccess+abi] RFR: 8291639: Improve the ability to visualize a MemorySegment in human readable forms [v2]

[Glavo]

On Fri, 26 May 2023 13:06:57 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR proposes to add a "Record Mapper" that allows `MemorySegment` instances to be projected onto `Record` types given `GroupLayout` instances.
>> 
>> Here is a simple example of how to use the mapper:
>> 
>> 
>>     private static final GroupLayout POINT_LAYOUT = MemoryLayout.structLayout(
>>             JAVA_INT.withName("x"),
>>             JAVA_INT.withName("y"));
>> 
>>     MemorySegment segment =  MemorySegment.ofArray(new int[]{3, 4});
>> 
>>     Point point = POINT_LAYOUT.recordMapper(Point.class)
>>           .apply(segment); // Point[x=3, y=4]
>> 
>> 
>> I think the implementation can be improved later, for example by de-duplicating handling of arrays and maybe add recursive "un-pealing" of multidimensional arrays.
>
> Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 29 commits:
> 
>  - Merge master
>  - Fix typos
>  - Clean up toString methods
>  - Add support for MemoryAddress arrays and improve docs
>  - Improve javadoc
>  - Change to IAE if mapper fails
>  - Remove caching
>  - Add tests for initial issue examples
>  - Clean up and optimize
>  - Add support for multidimensional records
>  - ... and 19 more: https://git.openjdk.org/panama-foreign/compare/bee4503b...1d7db589

> Is it possible that we will get something like this in the future:
> 
> C Header:
> 
> ```c
> struct Point {
>     int x, y;
> };
> 
> extern void f1(struct Point p);
> extern void f2(struct Point *pp);
> ```
> 
> Java Code:
> 
> ```java
> record Point(int x, int y) {}
> 
> GroupLayout pointLayout = MemoryLayout.structLayout(
>             JAVA_INT.withName("x"),
>             JAVA_INT.withName("y"));
> 
> ValueLayout pointValueLayout = pointLayout.withCarrier(Point.class);
> ValueLayout pointPtrLayout = ADDRESS.withTargetLayout(pointLayout).withCarrier(Point.class);
> 
> MethodHandle f1 = nativeLinker.downcallHandle(lookup.find("f1").get(), FunctionDescriptor.ofVoid(pointValueLayout));
> MethodHandle f2 = nativeLinker.downcallHandle(lookup.find("f2").get(), FunctionDescriptor.ofVoid(pointPtrLayout));
> 
> f1.invoke(new Point(10, 20)); // ok
> f2.invoke(new Point(10, 20)); // ok
> ```

The conversion between record and MemorySegment involves lifecycle issues, so it may not be as useful. But I think the direct mapping between record and struct is very useful.

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

PR Comment: https://git.openjdk.org/panama-foreign/pull/833#issuecomment-1564525582