pythongh-104615: don't make unsafe swaps in apply_static_swaps

Lib/test/test_compile.py

@@ -1168,6 +1168,24 @@ def foo(param, lambda_exp):
         """)
         compile(code, "<test>", "exec")
 
+    def test_apply_static_swaps(self):
+        def f(x, y):
+            a, a = x, y
+            return a
+        self.assertEqual(f("x", "y"), "y")
+
+    def test_apply_static_swaps_2(self):
+        def f(x, y, z):
+            a, b, a = x, y, z
+            return a
+        self.assertEqual(f("x", "y", "z"), "z")
+
+    def test_apply_static_swaps_3(self):
+        def f(x, y, z):
+            a, a, b = x, y, z
+            return a
+        self.assertEqual(f("x", "y", "z"), "y")
+
 
 @requires_debug_ranges()
 class TestSourcePositions(unittest.TestCase):

Lib/test/test_listcomps.py

@@ -484,6 +484,12 @@ def test_nested_listcomp_in_lambda(self):
         """
         self._check_in_scopes(code, {"z": 1, "out": [(3, 2, 1)]})
 
+    def test_assign_to_comp_iter_var_in_outer_function(self):
+        code = """
+            a = [1 for a in [0]]
+        """
+        self._check_in_scopes(code, {"a": [1]}, scopes=["function"])
+
 
 __test__ = {'doctests' : doctests}
 

Python/flowgraph.c

@@ -1293,6 +1293,11 @@ swaptimize(basicblock *block, int *ix)
      (opcode) == STORE_FAST_MAYBE_NULL || \
      (opcode) == POP_TOP)
 
+#define STORES_TO(instr) \
+    (((instr).i_opcode == STORE_FAST || \
+      (instr).i_opcode == STORE_FAST_MAYBE_NULL) \
+     ? (instr).i_oparg : -1)
+
 static int
 next_swappable_instruction(basicblock *block, int i, int lineno)
 {
@@ -1344,6 +1349,23 @@ apply_static_swaps(basicblock *block, int i)
                 return;
             }
         }
+        // The reordering is not safe if the two instructions to be swapped
+        // store to the same location, or if any intervening instruction stores
+        // to the same location as either of them.
+        int store_j = STORES_TO(block->b_instr[j]);
+        int store_k = STORES_TO(block->b_instr[k]);
+        if (store_j >= 0 && store_k >= 0) {
+            if (store_j == store_k) {
+                return;
+            }
+            for (int idx = j; idx < k; idx++) {
+                int store_idx = STORES_TO(block->b_instr[idx]);
+                if (store_idx == store_j || store_idx == store_k) {
+                    return;
+                }
+            }
+        }
+
         // Success!
         INSTR_SET_OP0(swap, NOP);
         cfg_instr temp = block->b_instr[j];