Metacircular Client Interface

[wks]

It has been proposed long ago that "everything the API can do should also be possible in the Mu IR".

This issue maintains a checklist of features not in the Mu IR. These functions should be gradually added to the IR.

Features in the API but not the IR:

• bundle loading
• stack introspection: current_func_ver, current_instruction, dump_keepalives
• OSR: pop_frame, push_frame
• Handle traps and undefined functions in Mu IR. It depends on how the Mu VM itself is implemented.
• Handle watchpoint in Mu IR: an instruction which is a no-op when disabled, but a (maybe limited kind of) Mu function call when enabled.

Things that can be done dynamically via handles in the API, but can only be done statically in the IR. I am not sure how dynamic Mu should be, or need to be, because some of the items below can be worked around with some Mu IR code, such as maintaining a hash table implemented in Mu IR, or writing wrapper functions.

• Opaque handle type: a handle that holds any Mu value. ref<T> can be a candidate for this purpose.
• Getting a Mu constant value (including constants, globals and functions) by its ID. This is a kind of introspection.
  • This can be done with a type argument: %c = GET_CONST <@T> %id, %c is a @T
  • Or return a handle: %ch = GET_CONST_H %id, %ch is a ref<void>
• Creating Mu heap objects by a type ID.
• Calling a Mu function (or constructing a Mu frame) with both the callee and the arguments as handles. This allows calling a Mu function with a run-time-determined arity and arg types.
• Dump keepalive variables.
  • This can be done with a type argument: %sixth_ka = GET_KEEPALIVE <@T> %frame 6
  • or return handle: %sixth_ka_handle = GET_KEEPALIVE_H %frame 6

[wks]

Working around some dynamism

Getting a Mu constant/global/function/expFunc by ID: maintain a HashMap<int, Callable<Object>>. For every constant ID, the Callable returns a boxed value that contains that constant. Or maintain a HashMap<int,Object> where each constant is pre-boxed and prepared for dynamic getting.

Extract value/insert value using a boxed value (handle) and a dynamically-supplied index; Creating a Mu heap object by type ID; Memory addressing/accessing via dynamic types, boxed values and dynamic indices; Pinning a ref/iref held in boxed values: Use dynamic dispatching. Either map the argument types (inside the boxed value) to accessor functions, or use SWITCH, or use virtual tables.

Calling a Mu function with a dynamic callee and a dynamic list of arguments (of run-time-known lengths and types): This is a bit tricky.

For a Mu function whose signature is @retty (@paramty1 @paramty2 ... @paramtyn), make a wrapper Mu function whose signature is ref<void> (hybrid<int<64> ref<void>>), i.e. the only parameter is a hybrid, with an int<64> as the fixed part which encodes the length of the variable part. Each element in the variable part is a ref<void>, which can refer to an object of arbitrary type. The return type is a boxed type, too. The actual implementation of this wrapper function may be:

// for simplicity, this function is written in Java
RetTy actualFunc(ParamTy1 param1, ParamTy2 param2, ..., ParamTyn paramn) { ... }

Object wrapper(Object[] args) {
    if (args.length != n) undefinedBehaviour();

    ParamTy1 arg1 = ((Ref<ParamTy1>)args[0]).load();
    ParamTy2 arg2 = ((Ref<ParamTy2>)args[1]).load();
    ...
    ParamTyn argn = ((Ref<ParamTyn>)args[n]).load();

    RetTy rv = actualFunc(arg1, arg2, ..., argn);

    return new Ref<RetTy>(rv);
}

When making a dynamic call, a list of actual arguments is supplied to the wrapper. The arity and the types are checked (if necessary) in the wrapper function.

A HashMap<int, WrapperFunction> is maintained to map each function ID to its wrapper, so that the callee itself can be dynamic.

dump keepalive variables without a type argument: For each trap ID, create a function which dumps all keep-alive variables and boxes all of them into a list.

In conclusion, the IR does not need to be dynamic. The client can always implement its own dynamism.