diff -r 14f042958c32 Include/sliceobject.h
--- a/Include/sliceobject.h	Tue Aug 30 02:12:43 2016 -0500
+++ b/Include/sliceobject.h	Tue Aug 30 20:41:26 2016 +0300
@@ -40,9 +40,18 @@ PyAPI_FUNC(int) _PySlice_GetLongIndices(
 #endif
 PyAPI_FUNC(int) PySlice_GetIndices(PyObject *r, Py_ssize_t length,
                                   Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step);
+PyAPI_FUNC(int) _PySlice_Unpack(PyObject *r,
+                                Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step);
+PyAPI_FUNC(int) _PySlice_EvalIndices(Py_ssize_t *start, Py_ssize_t *stop,
+                                     Py_ssize_t step, Py_ssize_t length,
+                                     Py_ssize_t *slicelength);
 PyAPI_FUNC(int) PySlice_GetIndicesEx(PyObject *r, Py_ssize_t length,
-				    Py_ssize_t *start, Py_ssize_t *stop,
-				    Py_ssize_t *step, Py_ssize_t *slicelength);
+                                     Py_ssize_t *start, Py_ssize_t *stop,
+                                     Py_ssize_t *step, Py_ssize_t *slicelength);
+#define PySlice_GetIndicesEx(r, length, start, stop, step, slicelength) (   \
+    _PySlice_Unpack((r), (start), (stop), (step)) < 0 ||                    \
+    _PySlice_EvalIndices((start), (stop), *(step), (length), (slicelen)) < 0 ? \
+    -1 : 0)
 
 #ifdef __cplusplus
 }
diff -r 14f042958c32 Objects/sliceobject.c
--- a/Objects/sliceobject.c	Tue Aug 30 02:12:43 2016 -0500
+++ b/Objects/sliceobject.c	Tue Aug 30 20:41:26 2016 +0300
@@ -191,15 +191,12 @@ PySlice_GetIndices(PyObject *_r, Py_ssiz
 }
 
 int
-PySlice_GetIndicesEx(PyObject *_r, Py_ssize_t length,
-                     Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step,
-                     Py_ssize_t *slicelength)
+_PySlice_Unpack(PyObject *_r,
+                Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step)
 {
     PySliceObject *r = (PySliceObject*)_r;
     /* this is harder to get right than you might think */
 
-    Py_ssize_t defstart, defstop;
-
     if (r->step == Py_None) {
         *step = 1;
     }
@@ -219,45 +216,78 @@ PySlice_GetIndicesEx(PyObject *_r, Py_ss
             *step = -PY_SSIZE_T_MAX;
     }
 
-    defstart = *step < 0 ? length-1 : 0;
-    defstop = *step < 0 ? -1 : length;
-
     if (r->start == Py_None) {
-        *start = defstart;
+        *start = *step < 0 ? PY_SSIZE_T_MAX-1 : 0;;
     }
     else {
         if (!_PyEval_SliceIndex(r->start, start)) return -1;
-        if (*start < 0) *start += length;
-        if (*start < 0) *start = (*step < 0) ? -1 : 0;
-        if (*start >= length)
-            *start = (*step < 0) ? length - 1 : length;
     }
 
     if (r->stop == Py_None) {
-        *stop = defstop;
+        *stop = *step < 0 ? -PY_SSIZE_T_MAX : PY_SSIZE_T_MAX;
     }
     else {
         if (!_PyEval_SliceIndex(r->stop, stop)) return -1;
-        if (*stop < 0) *stop += length;
-        if (*stop < 0) *stop = (*step < 0) ? -1 : 0;
-        if (*stop >= length)
-            *stop = (*step < 0) ? length - 1 : length;
-    }
-
-    if ((*step < 0 && *stop >= *start)
-        || (*step > 0 && *start >= *stop)) {
-        *slicelength = 0;
-    }
-    else if (*step < 0) {
-        *slicelength = (*stop-*start+1)/(*step)+1;
-    }
-    else {
-        *slicelength = (*stop-*start-1)/(*step)+1;
     }
 
     return 0;
 }
 
+int
+_PySlice_EvalIndices(Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t step, Py_ssize_t length,
+                     Py_ssize_t *slicelength)
+{
+    /* this is harder to get right than you might think */
+
+    assert(step != 0);
+    assert(step >= -PY_SSIZE_T_MAX);
+
+    if (*start < 0) {
+        *start += length;
+        if (*start < 0) {
+            *start = (step < 0) ? -1 : 0;
+        }
+    }
+    else if (*start >= length) {
+        *start = (step < 0) ? length - 1 : length;
+    }
+
+    if (*stop < 0) {
+        *stop += length;
+        if (*stop < 0) {
+            *stop = (step < 0) ? -1 : 0;
+        }
+    }
+    else  if (*stop >= length) {
+        *stop = (step < 0) ? length - 1 : length;
+    }
+
+    if ((step < 0 && *stop >= *start)
+        || (step > 0 && *start >= *stop)) {
+        *slicelength = 0;
+    }
+    else if (step < 0) {
+        *slicelength = (*stop-*start+1)/step+1;
+    }
+    else {
+        *slicelength = (*stop-*start-1)/step+1;
+    }
+
+    return 0;
+}
+
+#undef PySlice_GetIndicesEx
+
+int
+PySlice_GetIndicesEx(PyObject *_r, Py_ssize_t length,
+                     Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step,
+                     Py_ssize_t *slicelength)
+{
+    if (_PySlice_Unpack(_r, start, stop, step) < 0)
+        return -1;
+    return _PySlice_EvalIndices(start, stop, *step, length, slicelength);
+}
+
 static PyObject *
 slice_new(PyTypeObject *type, PyObject *args, PyObject *kw)
 {