Function jit_insn_set_param 

pub unsafe extern "C" fn jit_insn_set_param(
    func: jit_function_t,
    value: jit_value_t,
    offset: jit_nint,
) -> c_int

Set the parameter slot at offset in the outgoing parameter area to value. This may be used instead of jit_insn_push if it is more efficient to store directly to the stack than to push. The outgoing parameter area is allocated within the frame when the function is first entered.

You normally wouldn’t call this yourself - it is used internally by the CPU back ends to set up the stack for a subroutine call.