gh-119396: Optimize %S format of PyUnicode_FromFormat()

[vstinner]

Add fast paths for str, int and float object types.

Benchmark on %S and %R formats:

+----------------+--------+----------------------+
| Benchmark      | ref    | change               |
+================+========+======================+
| str()          | 654 ns | 556 ns: 1.18x faster |
+----------------+--------+----------------------+
| repr()         | 722 ns | 627 ns: 1.15x faster |
+----------------+--------+----------------------+
| Geometric mean | (ref)  | 1.16x faster         |
+----------------+--------+----------------------+

• Issue: Optimize _PyUnicodeWriter implementation #119396

[vstinner]

Benchmark:

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index f99ebf0dde..53d2a8e5f7 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -3312,6 +3312,46 @@ function_set_warning(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
     Py_RETURN_NONE;
 }

+static PyObject *
+bench_str(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
+{
+    PyObject *abc = PyUnicode_FromString("abc");
+    assert(abc != NULL);
+    PyObject *intobj = PyLong_FromLong(123);
+    assert(intobj != NULL);
+    PyObject *floatobj = PyFloat_FromDouble(1.0);
+    assert(floatobj != NULL);
+
+    PyObject *res = PyUnicode_FromFormat(
+        "str: %S, int: %S, float: %S",
+        abc, intobj, floatobj);
+
+    Py_DECREF(abc);
+    Py_DECREF(intobj);
+    Py_DECREF(floatobj);
+    return res;
+}
+
+static PyObject *
+bench_repr(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
+{
+    PyObject *abc = PyUnicode_FromString("abc");
+    assert(abc != NULL);
+    PyObject *intobj = PyLong_FromLong(123);
+    assert(intobj != NULL);
+    PyObject *floatobj = PyFloat_FromDouble(1.0);
+    assert(floatobj != NULL);
+
+    PyObject *res = PyUnicode_FromFormat(
+        "str: %R, int: %R, float: %R",
+        abc, intobj, floatobj);
+
+    Py_DECREF(abc);
+    Py_DECREF(intobj);
+    Py_DECREF(floatobj);
+    return res;
+}
+
 static PyMethodDef TestMethods[] = {
     {"set_errno",               set_errno,                       METH_VARARGS},
     {"test_config",             test_config,                     METH_NOARGS},
@@ -3454,6 +3494,8 @@ static PyMethodDef TestMethods[] = {
     {"check_pyimport_addmodule", check_pyimport_addmodule, METH_VARARGS},
     {"test_weakref_capi", test_weakref_capi, METH_NOARGS},
     {"function_set_warning", function_set_warning, METH_NOARGS},
+    {"bench_str", bench_str, METH_NOARGS},
+    {"bench_repr", bench_repr, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };

Script:

import pyperf
import _testcapi
runner = pyperf.Runner()
runner.bench_func('str()', _testcapi.bench_str)
runner.bench_func('repr()', _testcapi.bench_repr)

[vstinner]

This change is not worth it:

• Optimize _PyUnicodeWriter implementation #119396 (comment)
• Optimize _PyUnicodeWriter implementation #119396 (comment)

I close the PR.