gh-91247: improve performance of list and tuple repeat (with specialization for n=1)

Include/internal/pycore_list.h

@@ -56,6 +56,19 @@ _PyList_AppendTakeRef(PyListObject *self, PyObject *newitem)
     return _PyList_AppendTakeRefListResize(self, newitem);
 }
 
+// Repeat the bytes of a buffer in place
+static inline void
+_Py_memory_repeat(char* dest, Py_ssize_t len_dest, Py_ssize_t len_src)
+{
+    assert(len_src > 0);
+    Py_ssize_t copied = len_src;
+    while (copied < len_dest) {
+        Py_ssize_t bytes_to_copy = Py_MIN(copied, len_dest - copied);
+        memcpy(dest + copied, dest, bytes_to_copy);
+        copied += bytes_to_copy;
+    }
+}
+
 typedef struct {
     PyObject_HEAD
     Py_ssize_t it_index;

Include/internal/pycore_object.h

@@ -32,6 +32,16 @@ PyAPI_FUNC(void) _Py_NO_RETURN _Py_FatalRefcountErrorFunc(
 #define _Py_FatalRefcountError(message) \
     _Py_FatalRefcountErrorFunc(__func__, (message))
 
+// Increment reference count by n
+static inline void _Py_RefcntAdd(PyObject* op, Py_ssize_t n)
+{
+#ifdef Py_REF_DEBUG
+    _Py_RefTotal += n;
+#endif
+    op->ob_refcnt += n;
+}
+#define _Py_RefcntAdd(op, n) _Py_RefcntAdd(_PyObject_CAST(op), n)
+
 static inline void
 _Py_DECREF_SPECIALIZED(PyObject *op, const destructor destruct)
 {

Misc/NEWS.d/next/Core and Builtins/2022-03-22-13-12-27.bpo-47091.tJcy-P.rst

@@ -0,0 +1 @@
+Improve performance of repetition of :class:`list` and :class:`tuple` by using ``memcpy`` to copy data and performing the reference increments in one step.

Objects/listobject.c

@@ -551,47 +551,41 @@ list_concat(PyListObject *a, PyObject *bb)
 static PyObject *
 list_repeat(PyListObject *a, Py_ssize_t n)
 {
-    Py_ssize_t size;
-    PyListObject *np;
-    if (n < 0)
-        n = 0;
-    if (n > 0 && Py_SIZE(a) > PY_SSIZE_T_MAX / n)
-        return PyErr_NoMemory();
-    size = Py_SIZE(a) * n;
-    if (size == 0)
+    const Py_ssize_t input_size = Py_SIZE(a);
+    if (input_size == 0 || n <= 0)
         return PyList_New(0);
-    np = (PyListObject *) list_new_prealloc(size);
+    assert(n > 0);
+
+    if (input_size > PY_SSIZE_T_MAX / n)
+        return PyErr_NoMemory();
+    Py_ssize_t output_size = input_size * n;
+
+    PyListObject *np = (PyListObject *) list_new_prealloc(output_size);
     if (np == NULL)
         return NULL;
+
     PyObject **dest = np->ob_item;
-    PyObject **dest_end = dest + size;
-    if (Py_SIZE(a) == 1) {
+    if (input_size == 1) {
         PyObject *elem = a->ob_item[0];
-        Py_SET_REFCNT(elem, Py_REFCNT(elem) + n);
-#ifdef Py_REF_DEBUG
-        _Py_RefTotal += n;
-#endif
+        _Py_RefcntAdd(elem, n);
+        PyObject **dest_end = dest + output_size;
         while (dest < dest_end) {
             *dest++ = elem;
         }
     }
     else {
         PyObject **src = a->ob_item;
-        PyObject **src_end = src + Py_SIZE(a);
+        PyObject **src_end = src + input_size;
         while (src < src_end) {
-            Py_SET_REFCNT(*src, Py_REFCNT(*src) + n);
-#ifdef Py_REF_DEBUG
-            _Py_RefTotal += n;
-#endif
-            *dest++ = *src++;
-        }
-        // Now src chases after dest in the same buffer
-        src = np->ob_item;
-        while (dest < dest_end) {
+            _Py_RefcntAdd(*src, n);
             *dest++ = *src++;
         }
+
+        _Py_memory_repeat((char *)np->ob_item, sizeof(PyObject *)*output_size,
+                                        sizeof(PyObject *)*input_size);
     }
-    Py_SET_SIZE(np, size);
+
+    Py_SET_SIZE(np, output_size);
     return (PyObject *) np;
 }
 
@@ -743,12 +737,8 @@ PyList_SetSlice(PyObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
 static PyObject *
 list_inplace_repeat(PyListObject *self, Py_ssize_t n)
 {
-    PyObject **items;
-    Py_ssize_t size, i, j, p;
-
-
-    size = PyList_GET_SIZE(self);
-    if (size == 0 || n == 1) {
+    Py_ssize_t input_size = PyList_GET_SIZE(self);
+    if (input_size == 0 || n == 1) {
         Py_INCREF(self);
         return (PyObject *)self;
     }
@@ -759,22 +749,21 @@ list_inplace_repeat(PyListObject *self, Py_ssize_t n)
         return (PyObject *)self;
     }
 
-    if (size > PY_SSIZE_T_MAX / n) {
+    if (input_size > PY_SSIZE_T_MAX / n) {
         return PyErr_NoMemory();
     }
+    Py_ssize_t output_size = input_size * n;
 
-    if (list_resize(self, size*n) < 0)
+    if (list_resize(self, output_size) < 0)
         return NULL;
 
-    p = size;
-    items = self->ob_item;
-    for (i = 1; i < n; i++) { /* Start counting at 1, not 0 */
-        for (j = 0; j < size; j++) {
-            PyObject *o = items[j];
-            Py_INCREF(o);
-            items[p++] = o;
-        }
+    PyObject **items = self->ob_item;
+    for (Py_ssize_t j = 0; j < input_size; j++) {
+        _Py_RefcntAdd(items[j], n-1);
     }
+    _Py_memory_repeat((char *)items, sizeof(PyObject *)*output_size,
+                      sizeof(PyObject *)*input_size);
+
     Py_INCREF(self);
     return (PyObject *)self;
 }

Objects/tupleobject.c

@@ -495,52 +495,47 @@ tupleconcat(PyTupleObject *a, PyObject *bb)
 static PyObject *
 tuplerepeat(PyTupleObject *a, Py_ssize_t n)
 {
-    Py_ssize_t size;
-    PyTupleObject *np;
-    if (Py_SIZE(a) == 0 || n == 1) {
+    const Py_ssize_t input_size = Py_SIZE(a);
+    if (input_size == 0 || n == 1) {
         if (PyTuple_CheckExact(a)) {
             /* Since tuples are immutable, we can return a shared
                copy in this case */
             Py_INCREF(a);
             return (PyObject *)a;
         }
     }
-    if (Py_SIZE(a) == 0 || n <= 0) {
+    if (input_size == 0 || n <= 0) {
         return tuple_get_empty();
     }
-    if (n > PY_SSIZE_T_MAX / Py_SIZE(a))
+    assert(n>0);
+
+    if (input_size > PY_SSIZE_T_MAX / n)
         return PyErr_NoMemory();
-    size = Py_SIZE(a) * n;
-    np = tuple_alloc(size);
+    Py_ssize_t output_size = input_size * n;
+
+    PyTupleObject *np = tuple_alloc(output_size);
     if (np == NULL)
         return NULL;
+
     PyObject **dest = np->ob_item;
-    PyObject **dest_end = dest + size;
-    if (Py_SIZE(a) == 1) {
+    if (input_size == 1) {
         PyObject *elem = a->ob_item[0];
-        Py_SET_REFCNT(elem, Py_REFCNT(elem) + n);
-#ifdef Py_REF_DEBUG
-        _Py_RefTotal += n;
-#endif
+        _Py_RefcntAdd(elem, n);
+        PyObject **dest_end = dest + output_size;
         while (dest < dest_end) {
             *dest++ = elem;
         }
     }
     else {
         PyObject **src = a->ob_item;
-        PyObject **src_end = src + Py_SIZE(a);
+        PyObject **src_end = src + input_size;
         while (src < src_end) {
-            Py_SET_REFCNT(*src, Py_REFCNT(*src) + n);
-#ifdef Py_REF_DEBUG
-            _Py_RefTotal += n;
-#endif
-            *dest++ = *src++;
-        }
-        // Now src chases after dest in the same buffer
-        src = np->ob_item;
-        while (dest < dest_end) {
+            _Py_RefcntAdd(*src, n);
             *dest++ = *src++;
         }
+
+        _Py_memory_repeat((char *)np->ob_item, sizeof(PyObject *)*output_size,
+                          sizeof(PyObject *)*input_size);
     }
     _PyObject_GC_TRACK(np);
     return (PyObject *) np;