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

[Claes Redestad]

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.

I'm not sure how to assess how minor the "Var" case really is. I wouldn't be surprised if reflection-heavy frameworks hold `Method`s etc in some collection, which means they wouldn't be rooted in a way that allows the JIT to fold through. Thus leaning only on MH customization could be adding some performance risks. Off-list Vladimir Ivanov suggested the "Var" micros have some issues with inlining that make them look worse than they should, though.

On balance I think removing class-spinning might mean a better overall story w.r.t. footprint and maintainability. Had we started this review using a patch that looked more like what Peter is suggestion and someone had suggested we spin classes to get a performance edge in a subset of cases I think we'd not be looking favorably at that and instead tried reaching for narrowing those performance gaps via other less intrusive means. 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.

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