> Certain signatures for foreign function calls require allocation of an
> intermediate buffer to adapt the FFM's to the native stub's calling
> convention ("needsReturnBuffer"). In the current implementation, this buffer
> is malloced and freed on every FFM invocation, a non-negligible overhead.
>
> Sample stack trace:
>
> java.lang.Thread.State: RUNNABLE
> at jdk.internal.misc.Unsafe.allocateMemory0(java.base@25-ea/Native
> Method)
> at
> jdk.internal.misc.Unsafe.allocateMemory(java.base@25-ea/Unsafe.java:636)
> at
> jdk.internal.foreign.SegmentFactories.allocateMemoryWrapper(java.base@25-ea/SegmentFactories.java:215)
> at
> jdk.internal.foreign.SegmentFactories.allocateSegment(java.base@25-ea/SegmentFactories.java:193)
> at
> jdk.internal.foreign.ArenaImpl.allocateNoInit(java.base@25-ea/ArenaImpl.java:55)
> at
> jdk.internal.foreign.ArenaImpl.allocate(java.base@25-ea/ArenaImpl.java:60)
> at
> jdk.internal.foreign.ArenaImpl.allocate(java.base@25-ea/ArenaImpl.java:34)
> at
> java.lang.foreign.SegmentAllocator.allocate(java.base@25-ea/SegmentAllocator.java:645)
> at
> jdk.internal.foreign.abi.SharedUtils$2.<init>(java.base@25-ea/SharedUtils.java:388)
> at
> jdk.internal.foreign.abi.SharedUtils.newBoundedArena(java.base@25-ea/SharedUtils.java:386)
> at
> jdk.internal.foreign.abi.DowncallStub/0x000001f001084c00.invoke(java.base@25-ea/Unknown
> Source)
> at
> java.lang.invoke.DirectMethodHandle$Holder.invokeStatic(java.base@25-ea/DirectMethodHandle$Holder)
> at
> java.lang.invoke.LambdaForm$MH/0x000001f00109a400.invoke(java.base@25-ea/LambdaForm$MH)
> at
> java.lang.invoke.Invokers$Holder.invokeExact_MT(java.base@25-ea/Invokers$Holder)
>
>
> When does this happen? A fairly easy way to trigger this is through returning
> a small aggregate like the following:
>
> struct Vector2D {
> double x, y;
> };
> Vector2D Origin() {
> return {0, 0};
> }
>
>
> On AArch64, such a struct is returned in two 128 bit registers v0/v1.
> The VM's calling convention for the native stub consequently expects an 32
> byte output segment argument.
> The FFM downcall method handle instead expects to create a 16 byte result
> segment through the application-provided SegmentAllocator, and needs to
> perform an appropriate adaptation, roughly like so:
>
> MemorySegment downcallMH(SegmentAllocator a) {
> MemorySegment tmp = SharedUtils.allocate(32);
> try {
> nativeStub.invoke(tmp); // leaves v0, v1 in tmp
> MemorySegment result = a.allocate(16);
> result.setDouble(0, tmp.getDouble(0));
> result.setDouble(8, tmp.getDouble(16));
> return result;
> ...
Matthias Ernst has updated the pull request incrementally with one additional
commit since the last revision:
Implementation notes.
-------------
Changes:
- all: https://git.openjdk.org/jdk/pull/23142/files
- new: https://git.openjdk.org/jdk/pull/23142/files/4a2210df..35a3a156
Webrevs:
- full: https://webrevs.openjdk.org/?repo=jdk&pr=23142&range=01
- incr: https://webrevs.openjdk.org/?repo=jdk&pr=23142&range=00-01
Stats: 5 lines in 2 files changed: 4 ins; 0 del; 1 mod
Patch: https://git.openjdk.org/jdk/pull/23142.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/23142/head:pull/23142
PR: https://git.openjdk.org/jdk/pull/23142
