On Sat, 28 Mar 2026 02:24:14 GMT, Yasumasa Suenaga <[email protected]> wrote:
> On the review for [JDK-8380955](https://bugs.openjdk.org/browse/JDK-8380955) > (#30443), `Linker` and `FunctionDescriptor` do not need to be value based > class because they would not be treated as "value". > `Linker` is defined as providing a way to look up the canonical layouts > associated with the data types used by the ABI. `FunctionDescriptor` > represents the signature of a foreign function. They are not "value". > > Actually they and their child (final) classes do not have `@ValueBased`, thus > `javac` and `-XX:DiagnoseSyncOnValueBasedClasses` cannot identify if they are > used in anti-pattern of value based class. Thus this change does not change > behavior, just documentation updates. @mcimadamore I left the reply for https://github.com/openjdk/jdk/pull/30443#issuecomment-4161141979 here. > For instance, the Linker uses FD as keys in maps Can you show some example(s)? I checked JDK source with `grep -nr FunctionDescriptor * | grep Map`, but I couldn't find out. (I know that might not be enough) I think the class should be labeled "value based class" depending on the use case. In `MemoryLayout`, the user might want to branch the process like `instanceof` like following. So it is ok to be labeld "value based class". (I know we cannot use `==` for value based class, but I use it here for explanation because "value based" would be like "value" in the future) void process(MemoryLayout layout, MemorySegment data) { if (layout == LAYOUT_FOR_A) { procA(data); } else if (layout == LAYOUT_FOR_B) { procB(data); } } In `FunctionDescriptor`, it might be used like in above, but I feel it is tricky because the caller have to know return type and argument types when calling. I think `MethodHandle` is equivalent with function pointer, and `FunctionDescriptor` is equivalent with `typedef` for the function in C. In that context, I feel it is no (or rare) use case to treat `FunctionDescriptor` as "value". It is the reason why I proposed to remove "value-based" from `FunctionDescriptor`. I agree it can be "value" because it is composed by value-based fields, but it is not a result of evaluation for the class. ------------- PR Comment: https://git.openjdk.org/jdk/pull/30481#issuecomment-4167407320
