[ASDAlexander77]

Summary

@cppvtable classes so far only described C++ objects that C++ creates, so tslang could call them. With this PR, a @cppvtable class whose methods have bodies implements a C++ interface: tslang creates the object and C++ calls it. The first use is nvrhi::IMessageCallback, NVRHI's message sink:

@cppvtable
class IMessageCallback {               // nvrhi::IMessageCallback (MSVC)
    _destructor(): void;               // slot 0
    message(severity: u8, text: string): void;
}

@cppvtable
class MessageLog extends IMessageCallback {
    errors = 0;
    message(severity: u8, text: string): void {
        if (severity >= 2) { this.errors++; }
    }
}

const log = new MessageLog();          // hand <Opaque>log to C++

How it works

• ClassInfo::isCppVTableImplementation: set for a @cppvtable class with a method that has a body. Such a class gets:

  • a default constructor;
  • .new;
  • the .size static;
  • a vtable definition.

  It still gets no .instanceOf or RTTI, so the vtable is exactly the C++ slots.

• Vtable slots: a method with a body overrides the base's slot of the same name, and a constructor never takes a slot. A slot left to a method of the declared interface (no body, such as a destructor) is null, as a pure virtual's would be.

• Object layout: new stores the vtable's address in the object's first word, where C++ expects its vtable pointer. The interface's fields and then the class's own fields follow.

• Narrow results: methods with bodies are marked ts.cpp_method. LowerToLLVM gives their narrow results (boolean, u8, ...) zeroext/signext, because C++ callers rely on the callee widening them. Other definitions are unchanged.

• Errors:

  • a method with a body returning a struct (not supported yet: the body would have to write through the hidden pointer);
  • an exported implementation (an importer would get the methods but not the vtable).

  Statics, accessors, implements, generics and rc/own stay errors as before. cppvtable/method_body.ts, whose error a body no longer triggers, now tests the struct-return error.

Lifetime and limits

These are documented in docs/cppvtable-classes.md, "Implementing C++ methods":

• C++ keeps a pointer the collector can't see, so the object must be held in tslang, for example in a global, while C++ may call it.
• A string argument is C++'s buffer, valid during the call only.
• There is no RTTI: C++ must not dynamic_cast or typeid the object.
• Don't throw from a method C++ calls.

Tests

• test-jit-cppvtable-abi: cppvtable/abi_host.cpp gains a C++ Callback interface and functions that call it. Counter in abi_msvc.ts and abi_itanium.ts implements it, with a constructor and fields. C++ calls message() twice (checking the text), count(), and isEven() for a bool result; tslang then calls it through the interface type too.
  • Passes under MSVC (local).
  • Under Itanium, abi_itanium.ts linked with a g++-built abi_host.cpp passes in WSL Ubuntu.
• test-compile-cppvtable-itanium-implementation-<triple> (x86-64 Linux, AArch64 Android): checks the vtable is { null, null, message, count, isEven } and that isEven is zeroext i1.
• Error tests: cppvtable-method_body (struct return) and cppvtable-export_implementation.
• Full suite (Windows): 4012/4012.
• Real NVRHI: native_ts_graphics' nvrhi_direct implements nvrhi::IMessageCallback this way.
  • On Windows with -debug, D3D12 and Vulkan both receive a provoked validation error and pass.
  • On the Android x86_64 emulator, NVRHI holds the object, but message() isn't exercised there, since there's no validation layer.

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