Add an ArgumentLoc data type.

This will be used to amend function signatures with ABI lowering
information.

lib/cretonne/src/ir/valueloc.rs

@@ -22,3 +22,37 @@ impl Default for ValueLoc {
         ValueLoc::Unassigned
     }
 }
+
+/// Function argument location.
+///
+/// The ABI specifies how arguments are passed to a function, and where return values appear after
+/// the call. Just like a `ValueLoc`, function arguments can be passed in registers or on the
+/// stack.
+///
+/// Function arguments on the stack are accessed differently for the incoming arguments to the
+/// current function and the outgoing arguments to a called external function. For this reason,
+/// the location of stack arguments is described as an offset into the array of function arguments
+/// on the stack.
+///
+/// An `ArgumentLoc` can be translated to a `ValueLoc` only when we know if we're talking about an
+/// incoming argument or an outgoing argument.
+///
+/// - For stack arguments, different `StackSlot` entities are used to represent incoming and
+///   outgoing arguments.
+/// - For register arguments, there is usually no difference, but if we ever add support for a
+///   register-window ISA like SPARC, register arguments would also need to be translated.
+#[derive(Copy, Clone, Debug)]
+pub enum ArgumentLoc {
+    /// This argument has not been assigned to a location yet.
+    Unassigned,
+    /// Argument is passed in a register.
+    Reg(RegUnit),
+    /// Argument is passed on the stack, at the given byte offset into the argument array.
+    Stack(u32),
+}
+
+impl Default for ArgumentLoc {
+    fn default() -> Self {
+        ArgumentLoc::Unassigned
+    }
+}