Runtime checks of call_ref about function-references

[rossberg]

That's a validation error.

[Horcrux7]

How can an validation error occur if there are no runtime checks?

[rossberg]

I'm afraid I don't understand the question. Validation happens before compilation and execution.

[Horcrux7]

How should validation occur before compilation? Can be that I does not have understand the concept.

My understand is:

• I can pass a variable of func.ref to any location.
• At runtime there can be in a func.ref variable a pointer to any function.
• The call_ref instruction hat no type information like call_indrect.
• At compile time, there is no knowledge of the number and type of function parameters.
• At compile time, there is no knowledge of the number and type of return values.

Of course, the Wasm file compiler has the necessary knowledge about the parameter types. But this not help the Wasm engine.

[rossberg]

Please see the Summary and following sections in the Overview doc. The central novelty in this proposal is the introduction of typed references (ref $ty), which provide the validator with the necessary type information. The func.ref instruction creates such a typed function reference and the call_ref instruction can only be applied to such references (thus the name of the proposal).

[Horcrux7]

Ok, thank you. This important point I have oversee. Now all is clearer.