gh-132108: Add Buffer Protocol support to int.from_bytes to improve performance (#132109)

Speed up conversion from `bytes-like` objects like `bytearray` while
keeping conversion from `bytes` stable.

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 3 people

Misc/NEWS.d/next/Core_and_Builtins/2025-04-04-20-38-29.gh-issue-132108.UwZIQy.rst

@@ -0,0 +1,2 @@
+Speed up :meth:`int.from_bytes` when passed object supports :ref:`buffer
+protocol <bufferobjects>`, like :class:`bytearray` by ~1.2x.

Objects/longobject.c

@@ -6476,14 +6476,33 @@ int_from_bytes_impl(PyTypeObject *type, PyObject *bytes_obj,
         return NULL;
     }
 
-    bytes = PyObject_Bytes(bytes_obj);
-    if (bytes == NULL)
-        return NULL;
-
-    long_obj = _PyLong_FromByteArray(
-        (unsigned char *)PyBytes_AS_STRING(bytes), Py_SIZE(bytes),
-        little_endian, is_signed);
-    Py_DECREF(bytes);
+    /* Fast-path exact bytes. */
+    if (PyBytes_CheckExact(bytes_obj)) {
+        long_obj = _PyLong_FromByteArray(
+            (unsigned char *)PyBytes_AS_STRING(bytes_obj), Py_SIZE(bytes_obj),
+            little_endian, is_signed);
+    }
+    /* Use buffer protocol to avoid copies. */
+    else if (PyObject_CheckBuffer(bytes_obj)) {
+        Py_buffer view;
+        if (PyObject_GetBuffer(bytes_obj, &view, PyBUF_SIMPLE) != 0) {
+            return NULL;
+        }
+        long_obj = _PyLong_FromByteArray(view.buf, view.len, little_endian,
+            is_signed);
+        PyBuffer_Release(&view);
+    }
+    else {
+        /* fallback: Construct a bytes then convert. */
+        bytes = PyObject_Bytes(bytes_obj);
+        if (bytes == NULL) {
+            return NULL;
+        }
+        long_obj = _PyLong_FromByteArray(
+            (unsigned char *)PyBytes_AS_STRING(bytes), Py_SIZE(bytes),
+            little_endian, is_signed);
+        Py_DECREF(bytes);
+    }
 
     if (long_obj != NULL && type != &PyLong_Type) {
         Py_SETREF(long_obj, PyObject_CallOneArg((PyObject *)type, long_obj));