instanceof: slot 0 of every class and interface vtable is .instanceOf

[ASDAlexander77]

Asking what an object is (instanceof, casting down to a class, unboxing an any) reads .instanceOf from slot 0 of the vtable. That slot was not always there.

1. A class that implements an interface

A root class put its interface vtable pointers first, so slot 0 held an interface pointer and not .instanceOf. Unboxing such an object from any (<B>anyValue) called through the wrong pointer and crashed under every memory model. A root class now reserves slot 0 for .instanceOf before its interfaces.

2. An interface value

Every interface vtable now starts with a header slot: slot 0 is the implementer's .instanceOf. An object literal has no class to ask, so it gets a generated .instanceOf.none that returns false.

• Members keep their logical indexes. Only the vtable builders and InterfaceSymbolRef lowering add INTERFACE_VTABLE_HEADER_SLOTS (1), so the index arithmetic for extends is unchanged.

• i instanceof B on an interface was decided at compile time and was always false. It now asks the vtable.

• <B>i, i as B, and narrowing by instanceof crashed the compiler with an index assert in castFieldsToClass. They now return the very object when it is a B. When it is not a B, the cast still builds a new B from the interface's fields, but only if every field of B is in the interface. Otherwise it throws Can't cast from interface.

• <B>a, where a is an any holding an interface, threw Can't cast from any type. ___unbox now has an interface branch. An any holding an object literal throws a catchable cast error.

• own: borrowedParam sees through the interface cast and ExtractInterfaceThis, and isInstanceOfSlot accepts the interface vtable form. Interface values themselves stay rejected under -mm=own, as before.

• rc: OwnedReturnConsumptionPass reads an interface call's possible implementations from the vtable globals, and now skips the header slot too. Reading at the member's own index looked at the member before it: i.make() (member 0) asked .instanceOf and leaked every result (4.6 → 10.8 MB). New test: own/rc_interface_call_owned_result.ts (JIT, AOT, and an IR check that every interface call returning a class carries __owned_result).

ABI

Both vtable layouts change. A prebuilt DefaultLib must be rebuilt with this compiler; its StringIterator implements ClassIterator.

Tests

• New: 00any_unbox_interface_class.ts and 00interface_instanceof.ts, each compile + JIT.
• Windows Release: 3236/3236.
• Linux (WSL): 3220/3220.
• DefaultLib rebuilt with this compiler: 157/157 (release, compile + JIT).
• -mm=own corpus: unchanged at 303 compiling files.

🤖 Generated with Claude Code