[foreign-memaccess+abi] RFR: 8274602: Generalize UpcallStub into NativeSymbol [v2]

[Jorn Vernee]

On Tue, 5 Oct 2021 14:34:55 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> It might sometimes be useful to define a custom symbol lookup which returns symbols backed by a library that can be unloaded dynamically (e.g. using dlopen).
>> We indeed have all the pieces to get there, except that symbnol lookup returns a memory address, which is not a scoped entity.
>> 
>> To address this, I propose that we add a new sealed interface/final class (`NativeSymbol`) which implements `Addressable` and is used in two places:
>> * as a return value for a symbol lookup
>> * as a proxy for an upcall stub
>> 
>> In other words, `NativeSymbol` is used to reference to some symbol in some library, where the library can have a given scope. Upcall stubs are just a special case where the upcall symbol is synthetized on the fly by the VM. With this in place, I think we can drop `CLinker.UpcallStub` and just use an "anonymous` NativeSymbol to represent upcall stubs.
>> 
>> With this, it should be possible to define a dlopen-based lookup such as the one below:
>> https://github.com/sundararajana/panama-jextract-samples/blob/master/dlopen/Dlopen.java
>> 
>> In a way that is completely *safe* - that is, so that it is not possible for a library to be unloaded while a function is being executed (even when the scope  associated to the native symbol is shared).
>
> Maurizio Cimadamore has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains ten additional commits since the last revision:
> 
>  - Merge branch 'foreign-memaccess+abi' into native_symbol
>  - Use NativeSymbol in downcallHandle
>  - Merge branch 'foreign-memaccess+abi' into native_symbol
>  - Add missing copyright headers
>  - Merge branch 'foreign-memaccess+abi' into native_symbol
>  - Fix QSort benchmark
>  - Add scope check
>    Add test
>  - Merge branch 'foreign-memaccess+abi' into native_symbol
>  - Initial push

Marked as reviewed by jvernee (Committer).

Latest changes look very good! Thanks.

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

PR: https://git.openjdk.java.net/panama-foreign/pull/589