[foreign-memaccess] [Rev 01] RFR: 8245459: Add support for complex filter value var handle adaptation

[Maurizio Cimadamore]

On Thu, 21 May 2020 19:45:22 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add more comments on LambdaFormEditor
>
> src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 5496:
> 
>> 5495:      */
>> 5496:     /* package */ static MethodHandle collectReturnValue(MethodHandle target, MethodHandle filter) {
>> 5497:         MethodType targetType = target.type();
> 
> Do you need to check that the type of last parameter of the filter function is equal to the return type of the target?
> Filter parameters size constraints > 0
> Can the filter return void?

yeah - ideally I'd need these checks - in reality the checks are performed on the VH side for now, and this is not
exposed publicly, but I can add all required checks.

As for void, yes, for consistency with filterReturnValue we should eventually support that too - but it wasn't the goal
of this patch to provide a full blown MH adapter; I have implemented what was necessary for the VH adaptation. I can
add the other parts (but would prefer to deal with that separately).

> src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 5478:
> 
>> 5477:     /**
>> 5478:      * Filter the return value of a target method handle with a filter function. The filter function is
>> 5479:      * applied to the return value of the original handle; if the filter specifies more than one parameters,
> 
> Suggested improvement to JavaDoc:
> Adapts a target method handle by post-processing
> its return value and additional arguments (if any) with a filter
> (another method handle).
> The result of the filter is returned from the adapter.
> 
> If the target returns a value, the filter must accept that value as
> as the trailing argument, with the additional arguments proceeding that.
> If the target returns void, the filter must accept only the additional arguments.
> 
> The return type of the filter
> replaces the return type of the target
> in the resulting adapted method handle.
> 
> The trailing argument type of the filter (if any) must be identical to the
> return type of the target.
> 
> The additional argument types of the filter (if any) are appended (in order)
> to the argument types of the target method handle and become the argument
> types of the resulting adapted method handle.
> 
> Here is pseudocode for the resulting adapter. In the code,
> {@code T}/{@code t} represent the result type and value of the
> {@code target}; {@code V}, the result type of the {@code filter};
> {@code A}/{@code a}, the types and values of the parameters and arguments
> of the {@code target} as well as the resulting adapter.
> {@code B}/{@code b}, the types and values of the additional parameters and arguments
> of the {@code filter} as well as appended to the resulting adapter.
> ...

Thanks - I'll see how much work it is to add support of the void case - hopefully it won't be too bad.

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

PR: https://git.openjdk.java.net/panama-foreign/pull/179