initialize frame values to NULL when args are known

Author: Fidget-Spinner

Lib/test/test_capi/test_opt.py

@@ -2557,8 +2557,30 @@ def testfunc(n):
         self.assertIsNotNone(ex)
         uops = get_opnames(ex)
         self.assertIn("_POP_TOP_NOP", uops)
+        self.assertIn("_PUSH_FRAME", uops)
+        self.assertLessEqual(len(matching_opnames(ex, "_POP_TOP")), 1)
+
+    def test_store_fast_refcount_elimination_when_uninitialized(self):
+        def foo():
+            # Since y is known to be
+            # uninitialized (NULL) here,
+            # The refcount is eliminated in the STORE_FAST.
+            y = 2
+            return y
+        def testfunc(n):
+            # The STORE_FAST for the range here needs a POP_TOP
+            # (for now, until we do loop peeling).
+            for _ in range(n):
+                foo()
+
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertIsNotNone(ex)
+        uops = get_opnames(ex)
+        self.assertIn("_POP_TOP_NOP", uops)
+        self.assertIn("_PUSH_FRAME", uops)
         self.assertLessEqual(len(matching_opnames(ex, "_POP_TOP")), 1)
 
+
     def test_float_op_refcount_elimination(self):
         def testfunc(args):
             a, b, n = args

Python/optimizer_symbols.c

@@ -846,8 +846,12 @@ _Py_uop_frame_new(
         frame->locals[i] = args[i];
     }
 
+    // If the args are known, then it's safe to just initialize
+    // every other non-set local to null symbol.
+    bool default_null = args != NULL;
+
     for (int i = arg_len; i < co->co_nlocalsplus; i++) {
-        JitOptRef local = _Py_uop_sym_new_unknown(ctx);
+        JitOptRef local = default_null ? _Py_uop_sym_new_null(ctx) : _Py_uop_sym_new_unknown(ctx);
         frame->locals[i] = local;
     }