RFR: 8287788: Implement a better allocator for downcalls [v16]

[Maurizio Cimadamore]

On Thu, 23 Jan 2025 08:33:23 GMT, Matthias Ernst <duke at openjdk.org> wrote:

>> Certain signatures for foreign function calls (e.g. HVA return by value) require allocation of an intermediate buffer to adapt the FFM's to the native stub's calling convention. 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 at 25-ea/Native Method)
>> ...
>> 	at jdk.internal.foreign.abi.SharedUtils.newBoundedArena(java.base at 25-ea/SharedUtils.java:386)
>> 	at jdk.internal.foreign.abi.DowncallStub/0x000001f001084c00.invoke(java.base at 25-ea/Unknown Source)
>> ...
>> 	at java.lang.invoke.Invokers$Holder.invokeExact_MT(java.base at 25-ea/Invokers$Holder)
>> 
>> 
>> To alleviate this, this PR implements a per carrier-thread stacked allocator.
>> 
>> Performance (MBA M3):
>> 
>> 
>> Before:
>> Benchmark                    Mode  Cnt   Score   Error  Units
>> CallOverheadByValue.byPtr    avgt   10   3.333 ? 0.152  ns/op
>> CallOverheadByValue.byValue  avgt   10  33.892 ? 0.034  ns/op
>> 
>> After:
>> Benchmark                    Mode  Cnt  Score   Error  Units
>> CallOverheadByValue.byPtr    avgt   30  3.311 ? 0.034  ns/op
>> CallOverheadByValue.byValue  avgt   30  6.143 ? 0.053  ns/op
>> 
>> 
>> `-prof gc` also shows that the new call path is fully scalar-replaced vs 160 byte/call before.
>
> Matthias Ernst has updated the pull request incrementally with four additional commits since the last revision:
> 
>  - test deep linker stack
>  - Merge remote-tracking branch 'origin/mernst/cache-segments' into mernst/cache-segments
>  - topOfStack
>  - (c)

src/java.base/share/classes/jdk/internal/foreign/abi/BufferStack.java line 65:

> 63:         @ForceInline
> 64:         public Arena pushFrame(long size, long byteAlignment) {
> 65:             boolean needsLock = Thread.currentThread().isVirtual() && !lock.isHeldByCurrentThread();

@minborg please check this -- you have discovered some cases where `isVirtual` is not enough (e.g. because virtual threads use carrier in the common pool, which can also be used for non-virtual thread stuff)

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PR Review Comment: https://git.openjdk.org/jdk/pull/23142#discussion_r1926906087