RFR: 8271820: Implementation of JEP 416: Reimplement Core Reflection with Method Handle [v3]

[Claes Redestad]

On Sat, 21 Aug 2021 22:37:05 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:
> 
>   Remove separate accessor for static vs instance method
>   
>   There is no effective difference when using MethodHandle::dropArgument for static method.   Removing Static*Accessor and Instance*Accessor simplifies the implementation.

Right, early on splitting instance and static accessor implementations looked like a win for at least the instance accessor on micros, but it seems much of this win gets eaten up in more complex benchmarks since the hot call-site in `Method.invoke` sees more implementations and will be penalized by becoming bi- or megamorphic. 

Merging the split implementations back and accepting the need for a `dropArguments` combinator in the static case appears to be the right choice for maintainability and peak performance. A bit more overhead setting up - especially when reflecting on static methods (which is all the cold start benchmark does) - but I think that's the right trade-off in this case.

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