[Python-checkins] cpython: Issue #27870: A left shift of zero by a large integer no longer attempts to

[mark.dickinson]

https://hg.python.org/cpython/rev/09fa42818cf8
changeset:   102941:09fa42818cf8
parent:      102939:1902e1d79e25
user:        Mark Dickinson <dickinsm at gmail.com>
date:        Mon Aug 29 19:27:06 2016 +0100
summary:
  Issue #27870: A left shift of zero by a large integer no longer attempts to allocate large amounts of memory.

files:
  Lib/test/test_long.py |  15 +++++++++++++++
  Misc/NEWS             |   3 +++
  Objects/longobject.c  |   5 +++++
  3 files changed, 23 insertions(+), 0 deletions(-)


diff --git a/Lib/test/test_long.py b/Lib/test/test_long.py
--- a/Lib/test/test_long.py
+++ b/Lib/test/test_long.py
@@ -878,6 +878,21 @@
             self.check_truediv(-x, y)
             self.check_truediv(-x, -y)
 
+    def test_lshift_of_zero(self):
+        self.assertEqual(0 << 0, 0)
+        self.assertEqual(0 << 10, 0)
+        with self.assertRaises(ValueError):
+            0 << -1
+
+    @support.cpython_only
+    def test_huge_lshift_of_zero(self):
+        # Shouldn't try to allocate memory for a huge shift. See issue #27870.
+        # Other implementations may have a different boundary for overflow,
+        # or not raise at all.
+        self.assertEqual(0 << sys.maxsize, 0)
+        with self.assertRaises(OverflowError):
+            0 << (sys.maxsize + 1)
+
     def test_small_ints(self):
         for i in range(-5, 257):
             self.assertIs(i, i + 0)
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@
 Core and Builtins
 -----------------
 
+- Issue #27870: A left shift of zero by a large integer no longer attempts
+  to allocate large amounts of memory.
+
 - Issue #25402: In int-to-decimal-string conversion, improve the estimate
   of the intermediate memory required, and remove an unnecessarily strict
   overflow check. Patch by Serhiy Storchaka.
diff --git a/Objects/longobject.c b/Objects/longobject.c
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -4281,6 +4281,11 @@
         PyErr_SetString(PyExc_ValueError, "negative shift count");
         return NULL;
     }
+
+    if (Py_SIZE(a) == 0) {
+        return PyLong_FromLong(0);
+    }
+
     /* wordshift, remshift = divmod(shiftby, PyLong_SHIFT) */
     wordshift = shiftby / PyLong_SHIFT;
     remshift  = shiftby - wordshift * PyLong_SHIFT;

-- 
Repository URL: https://hg.python.org/cpython