RFR: 8271820: Implementation of JEP 416: Reimplement Core Reflection with Method Handle [v8]

[Mandy Chung]

On Wed, 1 Sep 2021 01:05:32 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> This reimplements core reflection with method handles.
>> 
>> For `Constructor::newInstance` and `Method::invoke`, the new implementation uses `MethodHandle`.  For `Field` accessor, the new implementation uses `VarHandle`.    For the first few invocations of one of these reflective methods on a specific reflective object we invoke the corresponding method handle directly. After that we spin a dynamic bytecode stub defined in a hidden class which loads the target `MethodHandle` or `VarHandle` from its class data as a dynamically computed constant. Loading the method handle from a constant allows JIT to inline the method-handle invocation in order to achieve good performance.
>> 
>> The VM's native reflection methods are needed during early startup, before the method-handle mechanism is initialized. That happens soon after System::initPhase1 and before System::initPhase2, after which we switch to using method handles exclusively.
>> 
>> The core reflection and method handle implementation are updated to handle chained caller-sensitive method calls [1] properly. A caller-sensitive method can define with a caller-sensitive adapter method that will take an additional caller class parameter and the adapter method will be annotated with `@CallerSensitiveAdapter` for better auditing.   See the detailed description from [2].
>> 
>> Ran tier1-tier8 tests.   
>> 
>> [1] https://bugs.openjdk.java.net/browse/JDK-8013527
>> [2] https://bugs.openjdk.java.net/browse/JDK-8271820?focusedCommentId=14439430&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14439430
>
> Mandy Chung has updated the pull request incrementally with one additional commit since the last revision:
> 
>   minor cleanup and more test case.

> On balance I think removing class-spinning might mean a better overall story w.r.t. footprint and maintainability. 
> : 
> So I think I'm in favor of simplifying and filing a follow-up to try and win back some of the performance we might be losing on corner-case micros without the custom class spinning.

Great!  I'll update the PR with Peter's patch + vh2mh with removal of StaticFieldAccessor/InstanceFieldAccessor inner classes [2] (no significant performance difference between [1] and [2]).

> I wish MethodHandles obtained by unreflectGetter/unreflectSetter could be less sensitive to where they are read from (constant vs. not-constant) and optimize better in non-constant case. I think we should search in this direction... to make MethodHandles obtained by unreflectGetter/unreflectSetter more optimal in non-constant case. 

Yup, that's what the performance opportunity we should explore to improve MH/VH that will benefit all clients of MH/VH as well as core reflection. 

[1] http://cr.openjdk.java.net/~mchung/jep416/vh-mh.patch
[2] http://cr.openjdk.java.net/~mchung/jep416/vh-mh-2.patch

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PR: https://git.openjdk.java.net/jdk/pull/5027