[foreign-jextract] RFR: 8261906: Improve jextract support for virtual functions [v4]

[Jorn Vernee]

On Wed, 3 Mar 2021 12:42:55 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> I've addressed the various comments; I ended up simplifying the code a bit and remove the Constant.MethodHandle abstraction which was, I think, a bit too convoluted. Now virtual function invokers are generated as part of JavaSourceBuilder::addVar, which I think makes the most sense from the perspective of OutputFactory. To facilitate passing multiple arguments from OutputFactory to the various builders, I've added a couple of records (VarInfo and FunctionInfo) which store various info about variables and functions (e.g. carrier, method types, layouts, descriptors). This also allowed me to reuse most of the code in OutputFactory between visitVariable/visitFunction and generateFunctionalInteraface. Now all the three methods call a factory which returns an optional FunctionInfo (if missing, it means that the function/variable/func interface cannot be generated).
>
> Question: I realized there are two ways to support function pointers: one would be to do what this patch does, e.g. add an extra accessor to "call" the underlying function pointer. The other would be to have an accessor which, given a memory address encoding a function pointer, returns an instance of the functional interface that can be used to call it. In other words, we have a choice here:
> 
> typedef void (*func)(int,float);
> 
> func f;
> 
> One option is to generate a toplevel accessor, like this patch does:
> 
> void f(int x0, float x1) { ... }
> 
> another option is to piggy back on the functional interface that jextract emits for the typedef:
> 
> func f();
> 
> Both are valid approaches. The latter seem to depend on JDK-8259054

I think both options look good. The latter is maybe a little more flexible, but the usage of the former looks more like what you would see in C:
foo->f(10, 21F)
In Java:
foo.f(segment, 10, 21F)
vs.
foo.f(segment).apply(10, 21F)

Maybe we can just do both? Otherwise, I think out of these 2 I prefer the first, since it's the more primitive variant of the 2 (i.e. no need to create an intermediate wrapper instance).

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PR: https://git.openjdk.java.net/panama-foreign/pull/456