RFR: 8254693: Add Panama feature to pass heap segments to native code [v12]

[Vladimir Ivanov]

On Tue, 24 Oct 2023 15:09:57 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 two additional commits since the last revision:
> 
>  - a -> an
>  - add note to downcallHandle about passing heap segments by-reference

hotspot changes look fine.

src/hotspot/cpu/aarch64/downcallLinker_aarch64.cpp line 182:

> 180:   ArgumentShuffle arg_shuffle(filtered_java_regs, out_regs, shuffle_reg);
> 181: 
> 182: #ifndef PRODUCT

Any particular reason to exclude the logging in product builds? `ArgumentShuffle::print_on()` is unconditionally available there.

src/java.base/share/classes/java/lang/foreign/Linker.java line 792:

> 790:          * @param allowHeapAccess whether the linked function should allow access to the Java heap.
> 791:          */
> 792:         static Option critical(boolean allowHeapAccess) {

Speaking of public API, I'm surprised to see critical function property conflated with ability to perform on-heap accesses. These aspects look orthogonal to me. Any particular reason not to represent them as 2 separate `Option`s?

Even though it's straightforward to support on-heap accesses during critical function calls, object pinning would support that for non-critical function calls as well, but proposed API doesn't cover it and new API will be required. What's the plan here?

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PR Review: https://git.openjdk.org/jdk/pull/16201#pullrequestreview-1693030613
PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1387130520
PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1387341549