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gh-125196: Use PyUnicodeWriter for repr(list) #125202

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Merged
merged 5 commits into from
Oct 9, 2024
Merged

gh-125196: Use PyUnicodeWriter for repr(list) #125202

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8c33f91
gh-125196: Use PyUnicodeWriter for repr(list)
vstinner Oct 9, 2024
5fa3466
Fix error handling
vstinner Oct 9, 2024
0273017
Avoid PyUnicodeWriter_WriteUTF8()
vstinner Oct 9, 2024
8b33b0e
PyUnicodeWriter_Discard(NULL) does nothing
vstinner Oct 9, 2024
9c910b5
Revert "PyUnicodeWriter_Discard(NULL) does nothing"
vstinner Oct 9, 2024
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42 changes: 20 additions & 22 deletions Objects/listobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -522,49 +522,47 @@ list_dealloc(PyObject *self)
static PyObject *
list_repr_impl(PyListObject *v)
{
PyObject *s;
_PyUnicodeWriter writer;
Py_ssize_t i = Py_ReprEnter((PyObject*)v);
if (i != 0) {
return i > 0 ? PyUnicode_FromString("[...]") : NULL;
int res = Py_ReprEnter((PyObject*)v);
if (res != 0) {
return (res > 0 ? PyUnicode_FromString("[...]") : NULL);
}

_PyUnicodeWriter_Init(&writer);
writer.overallocate = 1;
/* "[" + "1" + ", 2" * (len - 1) + "]" */
writer.min_length = 1 + 1 + (2 + 1) * (Py_SIZE(v) - 1) + 1;
Py_ssize_t prealloc = 1 + 1 + (2 + 1) * (Py_SIZE(v) - 1) + 1;
PyUnicodeWriter *writer = PyUnicodeWriter_Create(prealloc);
if (writer == NULL) {
goto error;
}

if (_PyUnicodeWriter_WriteChar(&writer, '[') < 0)
if (PyUnicodeWriter_WriteChar(writer, '[') < 0) {
goto error;
}

/* Do repr() on each element. Note that this may mutate the list,
so must refetch the list size on each iteration. */
for (i = 0; i < Py_SIZE(v); ++i) {
for (Py_ssize_t i = 0; i < Py_SIZE(v); ++i) {
if (i > 0) {
if (_PyUnicodeWriter_WriteASCIIString(&writer, ", ", 2) < 0)
if (PyUnicodeWriter_WriteUTF8(writer, ", ", 2) < 0) {
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What is faster, PyUnicodeWriter_WriteUTF8(writer, ", ", 2) or two PyUnicodeWriter_WriteChar()?

This is an idle question. Even if the latter is marginally faster, it may still be more preferable to use the former. But if the difference is significant, this may be a question for optimization.

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What is faster, PyUnicodeWriter_WriteUTF8(writer, ", ", 2) or two PyUnicodeWriter_WriteChar()?

Two PyUnicodeWriter_WriteChar() calls is faster:

+----------------+-------------+-----------------------+
| Benchmark      | change_utf8 | change_2char          |
+================+=============+=======================+
| repr([])       | 50.2 ns     | 54.0 ns: 1.07x slower |
+----------------+-------------+-----------------------+
| repr(list3)    | 825 ns      | 802 ns: 1.03x faster  |
+----------------+-------------+-----------------------+
| repr(list4)    | 2.89 us     | 2.66 us: 1.09x faster |
+----------------+-------------+-----------------------+
| Geometric mean | (ref)       | 1.01x faster          |
+----------------+-------------+-----------------------+

goto error;
}
}

s = PyObject_Repr(v->ob_item[i]);
if (s == NULL)
goto error;

if (_PyUnicodeWriter_WriteStr(&writer, s) < 0) {
Py_DECREF(s);
if (PyUnicodeWriter_WriteRepr(writer, v->ob_item[i]) < 0) {
goto error;
}
Py_DECREF(s);
}

writer.overallocate = 0;
if (_PyUnicodeWriter_WriteChar(&writer, ']') < 0)
if (PyUnicodeWriter_WriteChar(writer, ']') < 0) {
goto error;
}

Py_ReprLeave((PyObject *)v);
return _PyUnicodeWriter_Finish(&writer);
return PyUnicodeWriter_Finish(writer);

error:
_PyUnicodeWriter_Dealloc(&writer);
if (writer != NULL) {
PyUnicodeWriter_Discard(writer);
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Should not PyUnicodeWriter_Discard(NULL) be no-op?

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Ok, I modified PyUnicodeWriter_Discard(writer) to do nothing if writer is NULL.

}
Py_ReprLeave((PyObject *)v);
return NULL;
}
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