cfg: fix dead block detection by accounting for fall-through edges

Author: arusuki

doc/changelog.rst

@@ -1,6 +1,14 @@
 ChangeLog
 =========
 
+2025-04-14: Version 0.16.2
+--------------------------
+
+Bugfixes:
+
+- fix ControlFlowGraph dead block detection by accounting for fall-through
+  edges. PR #161
+
 2025-01-21: Version 0.16.1
 --------------------------
 

src/bytecode/cfg.py

@@ -731,12 +731,18 @@ def get_dead_blocks(self) -> List[BasicBlock]:
             if id(block) in seen_block_ids:
                 continue
             seen_block_ids.add(id(block))
+            fall_through = True
             for i in block:
-                if isinstance(i, Instr) and isinstance(i.arg, BasicBlock):
-                    stack.append(i.arg)
+                if isinstance(i, Instr):
+                    if isinstance(i.arg, BasicBlock):
+                        stack.append(i.arg)
+                    if i.is_final():
+                        fall_through = False
                 elif isinstance(i, TryBegin):
                     assert isinstance(i.target, BasicBlock)
                     stack.append(i.target)
+            if fall_through and block.next_block:
+                stack.append(block.next_block)
 
         return [b for b in self if id(b) not in seen_block_ids]
 

tests/test_cfg.py

@@ -712,6 +712,20 @@ def test_get_block_index(self):
         other_block = BasicBlock()
         self.assertRaises(ValueError, blocks.get_block_index, other_block)
 
+    def test_get_dead_blocks(self):
+        def condition():
+            pass
+
+        def test():
+            if condition():
+                print("1")
+            else:
+                print("2")
+
+        bytecode = Bytecode.from_code(test.__code__)
+        cfg = ControlFlowGraph.from_bytecode(bytecode)
+        assert len(cfg.get_dead_blocks()) == 0
+
 
 class CFGStacksizeComputationTests(TestCase):
     def check_stack_size(self, func):