RFR: 8350607: Consolidate MethodHandles::zero into MethodHandles::constant [v2]

[Claes Redestad]

On Mon, 24 Feb 2025 23:45:37 GMT, Chen Liang <liach at openjdk.org> wrote:

>> LF editor spins classes, this avoids the spinning overhead and should speed up non-capturing lambdas too.
>> 
>> There may need to be additional intrinsic work for MH combinator lf bytecode generation.
>
> Chen Liang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   We no longer load DelegateMH as we no longer rebind

Just got back home. Some comments inline, will need to run some tests and mull this over before approval.

src/java.base/share/classes/java/lang/invoke/MethodHandleImpl.java line 2241:

> 2239:         var form = constantForm(basicType);
> 2240: 
> 2241:         if (type.isPrimitive()) {

I think you could simplify this using `Wrapper.forBasicType`; all variants should be able to use `wrapper.convert`

src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 4874:

> 4872:         Objects.requireNonNull(type);
> 4873:         return type.isPrimitive() ? primitiveZero(Wrapper.forPrimitiveType(type))
> 4874:                 : MethodHandleImpl.makeConstantReturning(type, null);

Not sure if it's important but the existing impl would cache `zero(Object.class)` while this new impl won't. Behaviorally neutral for any other reference type, though.

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PR Review: https://git.openjdk.org/jdk/pull/23706#pullrequestreview-2642528847
PR Review Comment: https://git.openjdk.org/jdk/pull/23706#discussion_r1970580771
PR Review Comment: https://git.openjdk.org/jdk/pull/23706#discussion_r1970592504