add List.len

compiler/gen_dev/src/generic64/mod.rs

@@ -892,6 +892,10 @@ impl<
         }
     }
 
+    fn build_list_len(&mut self, dst: &Symbol, list: &Symbol) {
+        self.storage_manager.list_len(&mut self.buf, dst, list);
+    }
+
     fn build_ptr_cast(&mut self, dst: &Symbol, src: &Symbol) {
         // We may not strictly need an instruction here.
         // What's important is to load the value, and for src and dest to have different Layouts.

compiler/gen_dev/src/generic64/storage.rs

@@ -606,6 +606,22 @@ impl<
         }
     }
 
+    // Loads the dst to be the later 64 bits of a list (its length).
+    pub fn list_len(&mut self, _buf: &mut Vec<'a, u8>, dst: &Symbol, list: &Symbol) {
+        let owned_data = self.remove_allocation_for_sym(list);
+        self.allocation_map.insert(*list, Rc::clone(&owned_data));
+        self.allocation_map.insert(*dst, owned_data);
+        let (list_offset, _) = self.stack_offset_and_size(list);
+        self.symbol_storage_map.insert(
+            *dst,
+            Stack(ReferencedPrimitive {
+                base_offset: list_offset + 8,
+                size: 8,
+                sign_extend: false,
+            }),
+        );
+    }
+
     /// Creates a struct on the stack, moving the data in fields into the struct.
     pub fn create_struct(
         &mut self,

compiler/gen_dev/src/lib.rs

@@ -550,6 +550,14 @@ trait Backend<'a> {
                 arg_layouts,
                 ret_layout,
             ),
+            LowLevel::ListLen => {
+                debug_assert_eq!(
+                    1,
+                    args.len(),
+                    "ListLen: expected to have exactly one argument"
+                );
+                self.build_list_len(sym, &args[0])
+            }
             LowLevel::StrConcat => self.build_fn_call(
                 sym,
                 bitcode::STR_CONCAT.to_string(),
@@ -685,6 +693,9 @@ trait Backend<'a> {
         arg_layout: &Layout<'a>,
     );
 
+    /// build_list_len returns the length of a list.
+    fn build_list_len(&mut self, dst: &Symbol, list: &Symbol);
+
     /// build_refcount_getptr loads the pointer to the reference count of src into dst.
     fn build_ptr_cast(&mut self, dst: &Symbol, src: &Symbol);