RFR: 8254693: Add Panama feature to pass heap segments to native code [v11]

[Maurizio Cimadamore]

On Sat, 21 Oct 2023 12:04:10 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Add the ability to pass heap segments to native code. This requires using `Linker.Option.critical(true)` as a linker option. It has the same limitations as normal critical calls, namely: upcalls into Java are not allowed, and the native function should return relatively quickly. Heap segments are exposed to native code through temporary native addresses that are valid for the duration of the native call.
>> 
>> The motivation for this is supporting existing Java array-based APIs that might have to pass multi-megabyte size arrays to native code, and are current relying on Get-/ReleasePrimitiveArrayCritical from JNI. Where making a copy of the array would be overly prohibitive.
>> 
>> Components of this patch:
>> 
>> - New binding operator `SegmentBase`, which gets the base object of a `MemorySegment`.
>> - Rename `UnboxAddress` to `SegmentOffset`. Add flag to specify whether processing heap segments should be allowed.
>> - `CallArranger` impls use new binding operators when `Linker.Option.critical(/* allowHeap= */ true)` is specified.
>> - `NativeMethodHandle`/`NativeEntryPoint` allow `Object` in their signatures.
>> - The object/oop + offset is exposed as temporary address to native code.
>> - Since we stay in the `_thread_in_Java` state, we can safely expose the oops passed to the downcall stub to native code, without needing GCLocker. These oops are valid until we poll for safepoint, which we never do (invoking pure native code).
>> - Only x64 and AArch64 for now.
>> - I've refactored `ArgumentShuffle` in the C++ code to no longer rely on callbacks to get the set of source and destination registers (using `CallingConventionClosure`), but instead just rely on 2 equal size arrays with source and destination registers. This allows filtering the input java registers before passing them to `ArgumentShuffle`, which is required to filter out registers holding segment offsets. Replacing placeholder registers is also done as a separate pre-processing step now. See changes in: https://github.com/openjdk/jdk/pull/16201/commits/d2b40f1117d63cc6d74e377bf88cdcf6d15ff866
>> - I've factored out `DowncallStubGenerator` in the x64 and AArch64 code to use a common `DowncallLinker::StubGenerator`.
>> - Fallback linker is also supported using JNI's `GetPrimitiveArrayCritical`/`ReleasePrimitiveArrayCritical`
>> 
>> Aside: fixed existing issue with `DowncallLinker` not properly acquiring segments in interpreted mode.
>> 
>> Numbers for the included benchmark on my machine are:
>> 
>> 
>> Benchmar...
>
> Jorn Vernee has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 46 commits:
> 
>  - Merge branch 'master' into AllowHeapNoLock
>  - bump up argument counts in TestLargeStub to their maximum
>  - s390 updates
>  - add stub size stress test for allowHeap
>  - RISC-V impl
>  - remove leftover debug log line
>  - add s390 support
>  - add PPC impl
>  - add missing file
>  - Add xor benchmark
>  - ... and 36 more: https://git.openjdk.org/jdk/compare/a876beb6...2e00beff

Looking more holistically at the Linker javadoc, there seems to be something missing in that we never say that passing heap segments to downcall is not supported? We have clarified the documentation w.r.t. by-value structs here:

https://github.com/openjdk/panama-foreign/pull/881

But I can't find anything for by-reference structs.

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PR Comment: https://git.openjdk.org/jdk/pull/16201#issuecomment-1774971391