[foreign-memaccess+abi] RFR: 8291639: Improve the ability to visualize a MemorySegment in human readable forms [v2]

[Glavo]

On Fri, 26 May 2023 16:08:53 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> And, in many cases, the Linker will have to bulk-copy the struct or union somewhere else (e.g. in the stack). 

Because I only want to map the struct value to the record local variable, I don't really want to allocate a record object unless it escapes. So, I don't think this will cause any trouble on this issue.

Well, specifically...Okay, let's use this PR as an example.

I don't want to map a `struct Point *` to `Point[]`. What I want is:


private static final ValueLayout.OfRecord<Point> POINT_LAYOUT = MemoryLayout.structLayout(
                JAVA_INT.withName("x"),
                JAVA_INT.withName("y"))
        .withCarrier(Point.class);

MemorySegment segment =  MemorySegment.ofArray(new int[]{3, 4});

Point p = segment.get(POINT_LAYOUT, 0); // Point[x=3, y=4]
segment.set(POINT_LAYOUT, 0, new Point(5, 6));


I prefer this kind of thing to this PR.

Similar to the example I wrote earlier that called the C function, all records are only used locally in the method. I hope the JIT compiler can think that passing a record to an foreign MethodHandle is not an escape, and store the record object freely on the stack, passing it through registers to the C function, thereby avoiding additional overhead.

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PR Comment: https://git.openjdk.org/panama-foreign/pull/833#issuecomment-1564836555