[3.6] bpo-33132: Fix reference counting issues in the compiler. (GH-6209). (GH-6321)

(cherry picked from commit a95d9860)

Python/compile.c

@@ -2599,8 +2599,7 @@ compiler_import_as(struct compiler *c, identifier name, identifier asname)
                                        PyUnicode_GET_LENGTH(name));
             if (!attr)
                 return 0;
-            ADDOP_O(c, LOAD_ATTR, attr, names);
-            Py_DECREF(attr);
+            ADDOP_N(c, LOAD_ATTR, attr, names);
             pos = dot + 1;
         }
     }
@@ -2628,8 +2627,7 @@ compiler_import(struct compiler *c, stmt_ty s)
         if (level == NULL)
             return 0;
 
-        ADDOP_O(c, LOAD_CONST, level, consts);
-        Py_DECREF(level);
+        ADDOP_N(c, LOAD_CONST, level, consts);
         ADDOP_O(c, LOAD_CONST, Py_None, consts);
         ADDOP_NAME(c, IMPORT_NAME, alias->name, names);
 
@@ -2662,9 +2660,7 @@ static int
 compiler_from_import(struct compiler *c, stmt_ty s)
 {
     Py_ssize_t i, n = asdl_seq_LEN(s->v.ImportFrom.names);
-
-    PyObject *names = PyTuple_New(n);
-    PyObject *level;
+    PyObject *level, *names;
     static PyObject *empty_string;
 
     if (!empty_string) {
@@ -2673,14 +2669,15 @@ compiler_from_import(struct compiler *c, stmt_ty s)
             return 0;
     }
 
-    if (!names)
-        return 0;
-
     level = PyLong_FromLong(s->v.ImportFrom.level);
     if (!level) {
-        Py_DECREF(names);
         return 0;
     }
+    ADDOP_N(c, LOAD_CONST, level, consts);
+
+    names = PyTuple_New(n);
+    if (!names)
+        return 0;
 
     /* build up the names */
     for (i = 0; i < n; i++) {
@@ -2691,16 +2688,12 @@ compiler_from_import(struct compiler *c, stmt_ty s)
 
     if (s->lineno > c->c_future->ff_lineno && s->v.ImportFrom.module &&
         _PyUnicode_EqualToASCIIString(s->v.ImportFrom.module, "__future__")) {
-        Py_DECREF(level);
         Py_DECREF(names);
         return compiler_error(c, "from __future__ imports must occur "
                               "at the beginning of the file");
     }
+    ADDOP_N(c, LOAD_CONST, names, consts);
 
-    ADDOP_O(c, LOAD_CONST, level, consts);
-    Py_DECREF(level);
-    ADDOP_O(c, LOAD_CONST, names, consts);
-    Py_DECREF(names);
     if (s->v.ImportFrom.module) {
         ADDOP_NAME(c, IMPORT_NAME, s->v.ImportFrom.module, names);
     }
@@ -2723,7 +2716,6 @@ compiler_from_import(struct compiler *c, stmt_ty s)
             store_name = alias->asname;
 
         if (!compiler_nameop(c, store_name, Store)) {
-            Py_DECREF(names);
             return 0;
         }
     }
@@ -3101,8 +3093,7 @@ compiler_nameop(struct compiler *c, identifier name, expr_context_ty ctx)
                             "param invalid for local variable");
             return 0;
         }
-        ADDOP_O(c, op, mangled, varnames);
-        Py_DECREF(mangled);
+        ADDOP_N(c, op, mangled, varnames);
         return 1;
     case OP_GLOBAL:
         switch (ctx) {
@@ -4552,11 +4543,11 @@ compiler_annassign(struct compiler *c, stmt_ty s)
         if (s->v.AnnAssign.simple &&
             (c->u->u_scope_type == COMPILER_SCOPE_MODULE ||
              c->u->u_scope_type == COMPILER_SCOPE_CLASS)) {
+            VISIT(c, expr, s->v.AnnAssign.annotation);
             mangled = _Py_Mangle(c->u->u_private, targ->v.Name.id);
             if (!mangled) {
                 return 0;
             }
-            VISIT(c, expr, s->v.AnnAssign.annotation);
             /* ADDOP_N decrefs its argument */
             ADDOP_N(c, STORE_ANNOTATION, mangled, names);
         }