Function call_indirect_host

pub fn call_indirect_host<E>(into: &mut E, id: impl Into<u8>)
where
    E: Extend<u8>,

Available on crate feature encode only.

A special opcode to halt interpreter execution and yield control back to the host.

This opcode results in DoneReason::CallIndirectHost where the id here is shepherded along to the embedder. It’s up to the embedder to determine what to do with the id and the current state of registers and the stack.

In Wasmtime this is used to implement interpreter-to-host calls. This is modeled as a call instruction where the first parameter is the native function pointer to invoke and all remaining parameters for the native function are in following parameter positions (e.g. x1, x2, …). The results of the host call are then store in x0.

Handling this in Wasmtime is done through a “relocation” which is resolved at link-time when raw bytecode from Cranelift is assembled into the final object that Wasmtime will interpret.