[foreign-preview] RFR: 8286306: Upcall wrapper class sharing

[Jorn Vernee]

On Fri, 6 May 2022 14:50:53 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> This patch adds upcall wrapper class sharing.
> 
> I recently realized that these classes can be shared if they have the same input FunctionDescriptor, but a different target method handle.
> 
> This is implemented by generating a wrapper class that takes the target method handles as a leading argument. The target method handle is then inserted into the parameter list using `MethodHandles::insertArguments` to create the final method handle that is returned by the specializer. After MH customization kicks in, the target will be a constant again (so shouldn't affect peak performance).
> 
> Doing this doesn't effect peak performance, as verified through the Upcalls benchmark, but does significantly improve linking performance.
> 
> Before:
> 
>     Benchmark              Mode  Cnt   Score   Error  Units
>     LinkUpcall.link_blank  avgt   30  69.323 � 2.522  us/op
> 
> After:
> 
>     Benchmark              Mode  Cnt   Score   Error  Units
>     LinkUpcall.link_blank  avgt   30  11.787 � 0.744  us/op

src/java.base/share/classes/jdk/internal/foreign/abi/SharedUtils.java line 215:

> 213:             // adjust return type so it matches the inferred type of the effective
> 214:             // function descriptor
> 215:             target = target.asType(target.type().changeReturnType(Addressable.class));

Note that this is needed to prevent a situation on Windows where the effective function descriptor of `()[<some IMR struct>]` is the same as a plain `(a64)a64` function descriptor. The `target` will currently have the type `(MemoryAddress)MemoryAddress` in the former case, but `(MemoryAddress)Addressable` in the latter case. However, they share an upcall wrapper class because they have the same effective function descriptor (which is good), and this leads to a `WrongMethodTypeException` when invoking one of them.

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PR: https://git.openjdk.java.net/panama-foreign/pull/679