bpo-27145: small_ints[x] could be returned in long_add and long_sub (GH-15716)

Author: hongweipeng

Lib/test/test_long.py

@@ -956,6 +956,14 @@ def test_huge_rshift_of_huge(self, size):
         self.assertEqual(huge >> (sys.maxsize + 1), (1 << 499) + 5)
         self.assertEqual(huge >> (sys.maxsize + 1000), 0)
 
+    @support.cpython_only
+    def test_small_ints_in_huge_calculation(self):
+        a = 2 ** 100
+        b = -a + 1
+        c = a + 1
+        self.assertIs(a + b, 1)
+        self.assertIs(c - a, 1)
+
     def test_small_ints(self):
         for i in range(-5, 257):
             self.assertIs(i, i + 0)

Misc/NEWS.d/next/Core and Builtins/2019-09-06-16-40-12.bpo-27145.njuCXU.rst

@@ -0,0 +1 @@
+int + int and int - int operators can now return small integer singletons. Patch by hongweipeng.

Objects/longobject.c

@@ -3206,7 +3206,7 @@ x_sub(PyLongObject *a, PyLongObject *b)
     if (sign < 0) {
         Py_SIZE(z) = -Py_SIZE(z);
     }
-    return long_normalize(z);
+    return maybe_small_long(long_normalize(z));
 }
 
 static PyObject *
@@ -3254,13 +3254,15 @@ long_sub(PyLongObject *a, PyLongObject *b)
         return PyLong_FromLong(MEDIUM_VALUE(a) - MEDIUM_VALUE(b));
     }
     if (Py_SIZE(a) < 0) {
-        if (Py_SIZE(b) < 0)
-            z = x_sub(a, b);
-        else
+        if (Py_SIZE(b) < 0) {
+            z = x_sub(b, a);
+        }
+        else {
             z = x_add(a, b);
-        if (z != NULL) {
-            assert(Py_SIZE(z) == 0 || Py_REFCNT(z) == 1);
-            Py_SIZE(z) = -(Py_SIZE(z));
+            if (z != NULL) {
+                assert(Py_SIZE(z) == 0 || Py_REFCNT(z) == 1);
+                Py_SIZE(z) = -(Py_SIZE(z));
+            }
         }
     }
     else {