RFR: 8254693: Add Panama feature to pass heap segments to native code [v2]

[Maurizio Cimadamore]

On Wed, 18 Oct 2023 04:44:26 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 incrementally with one additional commit since the last revision:
> 
>   drop unused in_reg_spiller

src/java.base/share/classes/jdk/internal/foreign/abi/Binding.java line 205:

> 203:                    LoadFunc loadFunc, SegmentAllocator allocator);
> 204: 
> 205:     private static void checkType(Class<?> type) {

For another day: I wonder if we might be better served by more specific "check" routines. E.g. I suppose `Object` is not valid always... and maybe in other cases only certain primitives are ok.

src/java.base/share/classes/jdk/internal/foreign/abi/Binding.java line 281:

> 279:     }
> 280: 
> 281:     static SegmentOffset segmentOffsetAllowHeap() {

I suppose `heapSegmentOffset` is not good because this binding applies to both native and heap segments, right?

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PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1363547594
PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1363552900