On Wed, 18 Oct 2023 04:44:26 GMT, Jorn Vernee <jver...@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/java/lang/foreign/Linker.java line 792: > 790: * such as loss of performance, or JVM crashes. > 791: * <p> > 792: * Critical functions can optionally allow access to the Java > heap. This allows a client to pass heap Suggestion: * Critical functions can optionally allow access to the Java heap. This allows clients to pass heap src/java.base/share/classes/java/lang/foreign/Linker.java line 795: > 793: * memory segments as addresses, where normally only off-heap > memory segments would be allowed. The memory region > 794: * inside the Java heap is exposed through a temporary native > address that is valid for the duration of the > 795: * function call. As such, these temporary addresses, or any > addresses derived from them, should not be used The API does not expose temporary addresses - so it is not 100% clear when reading what this para refers to. I suppose you mean a native function that "captures" an object's address and then returns it, so that the client wraps it in a zero-length memory segment? I can't decide on top of my head if this is too cornery or not, even for this javadoc. src/java.base/share/classes/java/lang/foreign/Linker.java line 800: > 798: * prohibitive in terms of performance. > 799: * > 800: * @implNote As a consequence of allowing heap access, the JVM > will either lock the garbage collector, or pin the This is a very low-level comment. Which is fine per se. But it doesn't say what does it mean for the developer using this functionality. I think you want to say that GC is impacted in some way or form. If so, please say that. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1363538621 PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1363541304 PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1363544042
