gh-100146: Steal references from stack when building a list (#100147)

When executing the BUILD_LIST opcode, steal the references from the stack,
in a manner similar to the BUILD_TUPLE opcode.  Implement this by offloading
the logic to a new private API, _PyList_FromArraySteal(), that works similarly
to _PyTuple_FromArraySteal().

This way, instead of performing multiple stack pointer adjustments while the
list is being initialized, the stack is adjusted only once and a fast memory
copy operation is performed in one fell swoop.

1 files changed, 21 insertions, 0 deletions

diff --git a/Objects/listobject.c b/Objects/listobject.c
index b093f88..6629775 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -2565,6 +2565,27 @@ PyList_AsTuple(PyObject *v)
     return _PyTuple_FromArray(((PyListObject *)v)->ob_item, Py_SIZE(v));
 }
 
+PyObject *
+_PyList_FromArraySteal(PyObject *const *src, Py_ssize_t n)
+{
+    if (n == 0) {
+        return PyList_New(0);
+    }
+
+    PyListObject *list = (PyListObject *)PyList_New(n);
+    if (list == NULL) {
+        for (Py_ssize_t i = 0; i < n; i++) {
+            Py_DECREF(src[i]);
+        }
+        return NULL;
+    }
+
+    PyObject **dst = list->ob_item;
+    memcpy(dst, src, n * sizeof(PyObject *));
+
+    return (PyObject *)list;
+}
+
 /*[clinic input]
 list.index