gh-119396: Optimize unicode_decode_utf8_writer()

[vstinner]

Take the ascii_decode() fast-path even if dest is not aligned on size_t bytes.

• Issue: Optimize _PyUnicodeWriter implementation #119396

[vstinner]

Benchmark (C part, _testcapimodule.c):

staticPyObject*bench(PyObject*Py_UNUSED(module), PyObject*args)
{
constchar*str;
Py_ssize_tloops;
if (!PyArg_ParseTuple(args, "ny", &loops, &str)) {
returnNULL;
}
PyTime_tt1, t2;
(void)PyTime_PerfCounterRaw(&t1);
for (Py_ssize_ti=0; i<loops; i++) {
PyObject*obj=PyUnicode_FromFormat("=%s", str);
if (obj==NULL) {
returnNULL;
}
Py_DECREF(obj);
}
(void)PyTime_PerfCounterRaw(&t2);
returnPyFloat_FromDouble(PyTime_AsSecondsDouble(t2-t1));
}

Benchmark (Python part):

importpyperfimport_testcapirunner=pyperf.Runner()
arg=b'x'*10runner.bench_time_func("FromFormat('x'*10)", _testcapi.bench, arg)
arg=b'x'*1_000runner.bench_time_func("FromFormat('x'*1_000)", _testcapi.bench, arg)

Result:

+-----------------------+---------+-----------------------+
| Benchmark | ref | change |
+=======================+=========+=======================+
| FromFormat('x'*10) | 75.9 ns | 68.7 ns: 1.11x faster |
+-----------------------+---------+-----------------------+
| FromFormat('x'*1_000) | 200 ns | 191 ns: 1.05x faster |
+-----------------------+---------+-----------------------+