gh-160858: Use the list freelist in list()

list() allocated its result with PyType_GenericAlloc(), bypassing the
freelist that PyList_New() and the [] display use.  For an exact list,
allocate with PyList_New(0) instead.  The object handed to
list.__init__() is in the same state either way: empty, with no item
array, and tracked by the GC.  Subclasses still use
PyType_GenericAlloc().

pyperf, PGO+LTO, base 19270d3, two interleaved rounds per side
(a third round, disturbed by machine load, was dropped):

| Benchmark      | main    | branch                |
|----------------|:-------:|:---------------------:|
| list()         | 21.2 ns | 15.3 ns: 1.38x faster |
| list(d)        | 36.1 ns | 27.4 ns: 1.32x faster |
| list(t)        | 30.3 ns | 23.9 ns: 1.27x faster |
| list(l)        | 32.0 ns | 25.7 ns: 1.24x faster |
| list(r)        | 43.6 ns | 36.0 ns: 1.21x faster |
| x + 1          | 10.4 ns | 9.70 ns: 1.07x faster |
| len(l)         | 9.87 ns | 9.53 ns: 1.03x faster |
| l[0]           | 8.24 ns | 8.00 ns: 1.03x faster |
| L(t)           | 48.2 ns | 47.4 ns: 1.02x faster |
| tuple(l)       | 15.4 ns | 16.0 ns: 1.04x slower |
| Geometric mean | (ref)   | 1.13x faster          |

Not significant: [].  x + 1, len(l), l[0], L(t) and tuple(l) don't
run the changed code; their movement is layout noise, within the
up-to-1.07x spread between two runs of the same main build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: cmaloney

Misc/NEWS.d/next/Core_and_Builtins/2026-10-28-17-00-22.gh-issue-160858.JPGbOK.rst

@@ -0,0 +1,2 @@
+Speed up :class:`list` construction by reusing list objects from the free
+list, as ``[]`` already does.

Objects/listobject.c

@@ -3600,7 +3600,7 @@ static int
 list___init___impl(PyListObject *self, PyObject *iterable)
 /*[clinic end generated code: output=0f3c21379d01de48 input=b3f3fe7206af8f6b]*/
 {
-    /* Verify list invariants established by PyType_GenericAlloc() */
+    /* Verify list invariants */
     assert(0 <= Py_SIZE(self));
     assert(Py_SIZE(self) <= self->allocated || self->allocated == -1);
     assert(self->ob_item != NULL ||
@@ -3632,7 +3632,10 @@ list_vectorcall(PyObject *type, PyObject * const*args,
         return NULL;
     }
 
-    PyObject *list = PyType_GenericAlloc(_PyType_CAST(type), 0);
+    /* use PyList_New() to make exact lists from the freelist. */
+    PyObject *list = (type == (PyObject *)&PyList_Type
+                      ? PyList_New(0)
+                      : PyType_GenericAlloc(_PyType_CAST(type), 0));
     if (list == NULL) {
         return NULL;
     }