[foreign-abi] RFR: 8248331: Intrinsify downcall handles in C2

[Paul Sandoz]

That’s some nice work across the VM and JDK. I can follow the MH bits, not totally the C2 integration.

I see the switch between trivial and non-trivial invocation (working my way through the code from bottom up) is triggered by a boolean attribute on a function descriptor. For the moment Is that just for experimental purposes or is it also intended for end users?

Paul.

> On Jun 29, 2020, at 4:59 AM, Jorn Vernee <jvernee at openjdk.java.net> wrote:
> 
> Hi,
> 
> This patch adds intrinsification of down call handles.
> 
> This is done through a new method handle intrinsic called linkToNative. We create a NativeMethodHandle that calls this
> intrinsic, which then replaces the leaf method handle in ProgrammableInvoker::getBoundMethodHandle, basically replacing
> the call to invokeMoves. Before C2 kicks in, this intrinsic will call a fallback method handle that we pass to it. The
> handle that is passed is a handle that points to ProgrammableInvoker::invokeMoves, thus simulating the current
> behaviour.
> 
> However, when a call to linkToNative is inlined, C2 will instead generate either a direct call to the target function,
> or a call to a wrapper stub (which is generated on demand) that also does the thread state transitions needed for long
> running native functions. Information about ABI, and which registers to use are captured in a so-called 'appendix
> argument' of the type NativeEntryPoint, which is passed as the last argument. This captures all the information needed
> to generate the call in C2 (note that previously in the patch shared in the discussion thread this information was
> split over several classes, but I've condensed the info into just NativeEntryPoint in order to reduce the amount of
> code needed in the vm to be able to access the information).
> 
> With this, the overhead for downcalls is on par or slightly lower than with JNI for calls that need to do thread state
> transitions, and it is even lower when the thread state transitions are omitted (see that *_trivial runs).
> 
> Benchmark                                            Mode  Cnt    Score    Error  Units
> CallOverhead.jni_blank                               avgt   30    8.461 □  0.892  ns/op
> CallOverhead.jni_identity                            avgt   30   12.585 □  0.066  ns/op
> CallOverhead.panama_blank                            avgt   30    8.562 □  0.029  ns/op
> CallOverhead.panama_blank_trivial                    avgt   30    1.372 □  0.008  ns/op
> CallOverhead.panama_identity                         avgt   30   11.813 □  0.073  ns/op
> CallOverhead.panama_identity_trivial                 avgt   30    6.042 □  0.024  ns/op
> Finished running test 'micro:CallOverhead'
> 
> Thanks,
> Jorn
> 
> -------------
> 
> Commit messages:
> - Rebase fixes
> - Remove RegionPtr
> - re-write RegionPtr without using std::equal
> - Add missing debug.hpp includes for files using assert
> - Patch cleanup mostly updating copyright headers
> - Reduce VM interface
> - Implement down call intrinsics
> 
> Changes: https://git.openjdk.java.net/panama-foreign/pull/219/files
> Webrev: https://webrevs.openjdk.java.net/panama-foreign/219/webrev.00
>  Issue: https://bugs.openjdk.java.net/browse/JDK-8248331
>  Stats: 1848 lines in 73 files changed: 1741 ins; 28 del; 79 mod
>  Patch: https://git.openjdk.java.net/panama-foreign/pull/219.diff
>  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/219/head:pull/219
> 
> PR: https://git.openjdk.java.net/panama-foreign/pull/219