gh-92914: Round the allocated size for lists up to the even number (GH-92915)

1 files changed, 6 insertions, 0 deletions

diff --git a/Objects/listobject.c b/Objects/listobject.c
index b50623e..0a99ec9 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -94,6 +94,12 @@ list_preallocate_exact(PyListObject *self, Py_ssize_t size)
     assert(self->ob_item == NULL);
     assert(size > 0);
 
+    /* Since the Python memory allocator has granularity of 16 bytes on 64-bit
+     * platforms (8 on 32-bit), there is no benefit of allocating space for
+     * the odd number of items, and there is no drawback of rounding the
+     * allocated size up to the nearest even number.
+     */
+    size = (size + 1) & ~(size_t)1;
     PyObject **items = PyMem_New(PyObject*, size);
     if (items == NULL) {
         PyErr_NoMemory();