RFR: 8300237: Minor improvements in MethodHandles

[Rémi Forax]

On Tue, 17 Jan 2023 09:51:31 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> - `MethodType.ptypes()` can be used instead of `MethodType.parameterList()` when we don't need a copy
>> - comparison of two lists can be done without `Stream.reduce()`
>
> 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.

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