RFR: 8300237: Minor improvements in MethodHandles

[Claes Redestad]

On Tue, 17 Jan 2023 10:11:49 GMT, Rémi Forax <forax at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 6754:
>> 
>>> 6752:                         filter(t -> t.parameterCount() > skipSize).
>>> 6753:                         map(MethodType::ptypes).
>>> 6754:                         reduce((p, q) -> p.length >= q.length ? p : q).orElse(EMPTY);
>> 
>> Could avoid the need to introduce `EMPTY` if you make the stream expression return the longest list directly:
>> 
>>                 reduce((p, q) -> {
>>                     var longest = (p.size() >= q.size()) ? p : q;
>>                     return List.of(Arrays.copyOfRange(longest, skipSize, longest.size()); // checking isEmpty() is redundant here since we filter on `t.parameterCount() > skipSize`
>>                 }).orElse(List.of());
>
> Using lambdas inside MethodHandles is quite dangerous given that lambdas are initialized using method handles. It may work now because longuestParameterList() is not called when initializing a lambda but it may make any changes in the implementation of lambdas painfull in the future.

Precious little method handle use in lambda bootstrap since JDK 11. Though I agree with the sentiment - having fixed a number of bootstrap issues in the past - `MethodHandles` is a small step up the abstraction ladder and the code in particular already uses a number of method refs and lambdas.

-------------

PR: https://git.openjdk.org/jdk/pull/12025