Mean +- std dev: [main] 9.25 ms +- 0.07 ms -> [patched] 7.68 ms +- 0.03 ms: 1.20x faster

_PyUnicodeWriter_WriteChar(writer,'"')_PyUnicodeWriter_WriteStr(writer,pystr)// original string_PyUnicodeWriter_WriteChar(writer,'"')

$./python -m pyperf compare_to with-fast-path.json use_write_ascii.json -GSlower (3):- json_dumps(ensure_ascii=False): List of 256 dicts with 1 int: 101 us +- 0 us -> 102 us +- 0 us: 1.00x slower- json_loads: Dict with 256 lists of 256 dicts with 1 int: 46.6 ms +- 0.1 ms -> 46.8 ms +- 0.5 ms: 1.00x slower- json_dumps(ensure_ascii=False): List of 256 floats: 239 us +- 1 us -> 239 us +- 1 us: 1.00x slowerFaster (10):- json_dumps(ensure_ascii=False): List of 256 strings: 303 us +- 5 us -> 279 us +- 3 us: 1.08x faster- json_dumps: List of 256 strings: 302 us +- 3 us -> 278 us +- 3 us: 1.08x faster- json_dumps(ensure_ascii=False): List of 256 booleans: 16.5 us +- 0.1 us -> 15.3 us +- 0.1 us: 1.08x faster- json_dumps: List of 256 booleans: 16.5 us +- 0.1 us -> 15.3 us +- 0.1 us: 1.07x faster- json_dumps: Complex object: 1.96 ms +- 0.01 ms -> 1.87 ms +- 0.01 ms: 1.05x faster- json_dumps(ensure_ascii=False): Complex object: 1.96 ms +- 0.01 ms -> 1.87 ms +- 0.02 ms: 1.05x faster- json_dumps: Medium complex object: 173 us +- 1 us -> 171 us +- 1 us: 1.01x faster- json_dumps(ensure_ascii=False): Medium complex object: 172 us +- 1 us -> 171 us +- 1 us: 1.01x faster- json_loads: Medium complex object: 148 us +- 1 us -> 147 us +- 1 us: 1.00x faster- json_dumps: List of 256 floats: 239 us +- 0 us -> 239 us +- 0 us: 1.00x fasterBenchmark hidden because not significant (13): json_dumps: List of 256 ASCII strings, json_dumps: List of 256 dicts with 1 int, json_dumps: Dict with 256 lists of 256 dicts with 1 int, json_dumps(ensure_ascii=False): List of 256 ASCII strings, json_dumps(ensure_ascii=False): Dict with 256 lists of 256 dicts with 1 int, json_loads: List of 256 booleans, json_loads: List of 256 ASCII strings, json_loads: List of 256 floats, json_loads: List of 256 dicts with 1 int, json_loads: List of 256 strings, json_loads: Complex object, json_loads: List of 256 stringsensure_ascii=False, json_loads: Complex objectensure_ascii=False

diff --git a/Modules/_json.c b/Modules/_json.cindex cd08fa688d3..cd57760282a 100644--- a/Modules/_json.c+++ b/Modules/_json.c@@ -351,7 +351,7 @@ write_escaped_ascii(PyUnicodeWriter *writer, PyObject *pystr)         }         if (buf_len + 12 > ESCAPE_BUF_SIZE) {-            ret = PyUnicodeWriter_WriteUTF8(writer, buf, buf_len);+            ret = _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, buf, buf_len);             if (ret) return ret;             buf_len = 0;         }@@ -359,7 +359,7 @@ write_escaped_ascii(PyUnicodeWriter *writer, PyObject *pystr)     assert(buf_len < ESCAPE_BUF_SIZE);     buf[buf_len++] = '"';-    return PyUnicodeWriter_WriteUTF8(writer, buf, buf_len);+    return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, buf, buf_len); } static int@@ -1612,13 +1612,13 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,     int rv;     if (obj == Py_None) {-      return PyUnicodeWriter_WriteUTF8(writer, "null", 4);+      return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "null", 4);     }     else if (obj == Py_True) {-      return PyUnicodeWriter_WriteUTF8(writer, "true", 4);+      return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "true", 4);     }     else if (obj == Py_False) {-      return PyUnicodeWriter_WriteUTF8(writer, "false", 5);+      return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "false", 5);     }     else if (PyUnicode_Check(obj)) {         return encoder_write_string(s, writer, obj);@@ -1779,7 +1779,7 @@ encoder_listencode_dict(PyEncoderObject *s, PyUnicodeWriter *writer,     if (PyDict_GET_SIZE(dct) == 0) {         /* Fast path */-        return PyUnicodeWriter_WriteUTF8(writer, "{}", 2);+        return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "{}", 2);     }     if (s->markers != Py_None) {@@ -1883,7 +1883,7 @@ encoder_listencode_list(PyEncoderObject *s, PyUnicodeWriter *writer,         return -1;     if (PySequence_Fast_GET_SIZE(s_fast) == 0) {         Py_DECREF(s_fast);-        return PyUnicodeWriter_WriteUTF8(writer, "[]", 2);+        return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "[]", 2);     }     if (s->markers != Py_None) {