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gh-135336: Add fast path to json string encoding #133239

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930e938
Fast path for string encoding
nineteendo May 1, 2025
32eefb3
Merge branch 'main' into speedup-json-encode
nineteendo May 13, 2025
078965b
Merge branch 'main' into speedup-json-encode
nineteendo May 30, 2025
83499a4
Improve slow path
nineteendo May 30, 2025
25da453
Reduce diff
nineteendo May 30, 2025
b4a8026
Reduce diff
nineteendo Jun 10, 2025
8ee4c93
Reuse code
nineteendo Jun 10, 2025
7b055be
Fix compiler warnings
nineteendo Jun 10, 2025
660d962
📜🤖 Added by blurb_it.
blurb-it[bot] Jun 11, 2025
19f252a
Improve fast path
nineteendo Jun 12, 2025
c4feeb9
Merge branch 'main' into speedup-json-encode
serhiy-storchaka Jul 28, 2025
b12d42d
Address review
nineteendo Jul 28, 2025
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225 changes: 203 additions & 22 deletions Modules/_json.c
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Original file line number Diff line number Diff line change
Expand Up @@ -51,7 +51,7 @@ typedef struct _PyEncoderObject {
char sort_keys;
char skipkeys;
int allow_nan;
PyCFunction fast_encode;
int (*fast_encode)(PyUnicodeWriter *, PyObject *);
} PyEncoderObject;

#define PyEncoderObject_CAST(op) ((PyEncoderObject *)(op))
Expand Down Expand Up @@ -102,8 +102,10 @@ static PyObject *
_encoded_const(PyObject *obj);
static void
raise_errmsg(const char *msg, PyObject *s, Py_ssize_t end);
static PyObject *
encoder_encode_string(PyEncoderObject *s, PyObject *obj);
static int
_steal_accumulate(PyUnicodeWriter *writer, PyObject *stolen);
static int
encoder_write_string(PyEncoderObject *s, PyUnicodeWriter *writer, PyObject *obj);
static PyObject *
encoder_encode_float(PyEncoderObject *s, PyObject *obj);

Expand Down Expand Up @@ -209,6 +211,80 @@ ascii_escape_unicode(PyObject *pystr)
return rval;
}

static int
write_escaped_ascii(PyUnicodeWriter *writer, PyObject *pystr)
{
/* Take a PyUnicode pystr and return a new ASCII-only escaped PyUnicode */
Py_ssize_t i;
Py_ssize_t input_chars;
Py_ssize_t output_size;
Py_ssize_t chars;
PyObject *rval;
const void *input;
Py_UCS1 *output;
int kind;

input_chars = PyUnicode_GET_LENGTH(pystr);
input = PyUnicode_DATA(pystr);
kind = PyUnicode_KIND(pystr);

/* Compute the output size */
for (i = 0, output_size = 2; i < input_chars; i++) {
Py_UCS4 c = PyUnicode_READ(kind, input, i);
Py_ssize_t d;
if (S_CHAR(c)) {
d = 1;
}
else {
switch(c) {
case '\\': case '"': case '\b': case '\f':
case '\n': case '\r': case '\t':
d = 2; break;
default:
d = c >= 0x10000 ? 12 : 6;
}
}
if (output_size > PY_SSIZE_T_MAX - d) {
PyErr_SetString(PyExc_OverflowError, "string is too long to escape");
return -1;
}
output_size += d;
}

if (output_size == input_chars + 2) {
/* No need to escape anything */
if (PyUnicodeWriter_WriteChar(writer, '"') < 0) {
return -1;
}
if (PyUnicodeWriter_WriteStr(writer, pystr) < 0) {
return -1;
}
return PyUnicodeWriter_WriteChar(writer, '"');
}

rval = PyUnicode_New(output_size, 127);
if (rval == NULL) {
return -1;
}
output = PyUnicode_1BYTE_DATA(rval);
chars = 0;
output[chars++] = '"';
for (i = 0; i < input_chars; i++) {
Py_UCS4 c = PyUnicode_READ(kind, input, i);
if (S_CHAR(c)) {
output[chars++] = c;
}
else {
chars = ascii_escape_unichar(c, output, chars);
}
}
output[chars++] = '"';
#ifdef Py_DEBUG
assert(_PyUnicode_CheckConsistency(rval, 1));
#endif
return _steal_accumulate(writer, rval);
}

static PyObject *
escape_unicode(PyObject *pystr)
{
Expand Down Expand Up @@ -303,6 +379,111 @@ escape_unicode(PyObject *pystr)
return rval;
}

static int
write_escaped_unicode(PyUnicodeWriter *writer, PyObject *pystr)
{
/* Take a PyUnicode pystr and return a new escaped PyUnicode */
Py_ssize_t i;
Py_ssize_t input_chars;
Py_ssize_t output_size;
Py_ssize_t chars;
PyObject *rval;
const void *input;
int kind;
Py_UCS4 maxchar;

maxchar = PyUnicode_MAX_CHAR_VALUE(pystr);
input_chars = PyUnicode_GET_LENGTH(pystr);
input = PyUnicode_DATA(pystr);
kind = PyUnicode_KIND(pystr);

/* Compute the output size */
for (i = 0, output_size = 2; i < input_chars; i++) {
Py_UCS4 c = PyUnicode_READ(kind, input, i);
Py_ssize_t d;
switch (c) {
case '\\': case '"': case '\b': case '\f':
case '\n': case '\r': case '\t':
d = 2;
break;
default:
if (c <= 0x1f)
d = 6;
else
d = 1;
}
if (output_size > PY_SSIZE_T_MAX - d) {
PyErr_SetString(PyExc_OverflowError, "string is too long to escape");
return -1;
}
output_size += d;
}

if (output_size == input_chars + 2) {
/* No need to escape anything */
if (PyUnicodeWriter_WriteChar(writer, '"') < 0) {
return -1;
}
if (PyUnicodeWriter_WriteStr(writer, pystr) < 0) {
return -1;
}
return PyUnicodeWriter_WriteChar(writer, '"');
}

rval = PyUnicode_New(output_size, maxchar);
if (rval == NULL)
return -1;

kind = PyUnicode_KIND(rval);

#define ENCODE_OUTPUT do { \
chars = 0; \
output[chars++] = '"'; \
for (i = 0; i < input_chars; i++) { \
Py_UCS4 c = PyUnicode_READ(kind, input, i); \
switch (c) { \
case '\\': output[chars++] = '\\'; output[chars++] = c; break; \
case '"': output[chars++] = '\\'; output[chars++] = c; break; \
case '\b': output[chars++] = '\\'; output[chars++] = 'b'; break; \
case '\f': output[chars++] = '\\'; output[chars++] = 'f'; break; \
case '\n': output[chars++] = '\\'; output[chars++] = 'n'; break; \
case '\r': output[chars++] = '\\'; output[chars++] = 'r'; break; \
case '\t': output[chars++] = '\\'; output[chars++] = 't'; break; \
default: \
if (c <= 0x1f) { \
output[chars++] = '\\'; \
output[chars++] = 'u'; \
output[chars++] = '0'; \
output[chars++] = '0'; \
output[chars++] = Py_hexdigits[(c >> 4) & 0xf]; \
output[chars++] = Py_hexdigits[(c ) & 0xf]; \
} else { \
output[chars++] = c; \
} \
} \
} \
output[chars++] = '"'; \
} while (0)

if (kind == PyUnicode_1BYTE_KIND) {
Py_UCS1 *output = PyUnicode_1BYTE_DATA(rval);
ENCODE_OUTPUT;
} else if (kind == PyUnicode_2BYTE_KIND) {
Py_UCS2 *output = PyUnicode_2BYTE_DATA(rval);
ENCODE_OUTPUT;
} else {
Py_UCS4 *output = PyUnicode_4BYTE_DATA(rval);
assert(kind == PyUnicode_4BYTE_KIND);
ENCODE_OUTPUT;
}
#undef ENCODE_OUTPUT

#ifdef Py_DEBUG
assert(_PyUnicode_CheckConsistency(rval, 1));
#endif
return _steal_accumulate(writer, rval);
}

static void
raise_errmsg(const char *msg, PyObject *s, Py_ssize_t end)
{
Expand Down Expand Up @@ -1255,8 +1436,11 @@ encoder_new(PyTypeObject *type, PyObject *args, PyObject *kwds)

if (PyCFunction_Check(s->encoder)) {
PyCFunction f = PyCFunction_GetFunction(s->encoder);
if (f == py_encode_basestring_ascii || f == py_encode_basestring) {
s->fast_encode = f;
if (f == py_encode_basestring_ascii) {
s->fast_encode = write_escaped_ascii;
}
else if (f == py_encode_basestring) {
s->fast_encode = write_escaped_unicode;
}
}

Expand Down Expand Up @@ -1437,24 +1621,27 @@ encoder_encode_float(PyEncoderObject *s, PyObject *obj)
return PyFloat_Type.tp_repr(obj);
}

static PyObject *
encoder_encode_string(PyEncoderObject *s, PyObject *obj)
static int
encoder_write_string(PyEncoderObject *s, PyUnicodeWriter *writer, PyObject *obj)
{
/* Return the JSON representation of a string */
PyObject *encoded;

if (s->fast_encode) {
return s->fast_encode(NULL, obj);
return s->fast_encode(writer, obj);
}
encoded = PyObject_CallOneArg(s->encoder, obj);
if (encoded != NULL && !PyUnicode_Check(encoded)) {
if (encoded == NULL) {
return -1;
}
if (!PyUnicode_Check(encoded)) {
PyErr_Format(PyExc_TypeError,
"encoder() must return a string, not %.80s",
Py_TYPE(encoded)->tp_name);
Py_DECREF(encoded);
return NULL;
return -1;
}
return encoded;
return _steal_accumulate(writer, encoded);
}

static int
Expand Down Expand Up @@ -1485,10 +1672,7 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,
return PyUnicodeWriter_WriteASCII(writer, "false", 5);
}
else if (PyUnicode_Check(obj)) {
PyObject *encoded = encoder_encode_string(s, obj);
if (encoded == NULL)
return -1;
return _steal_accumulate(writer, encoded);
return encoder_write_string(s, writer, obj);
}
else if (PyLong_Check(obj)) {
if (PyLong_CheckExact(obj)) {
Expand Down Expand Up @@ -1577,7 +1761,7 @@ encoder_encode_key_value(PyEncoderObject *s, PyUnicodeWriter *writer, bool *firs
PyObject *item_separator)
{
PyObject *keystr = NULL;
PyObject *encoded;
int rv;

if (PyUnicode_Check(key)) {
keystr = Py_NewRef(key);
Expand Down Expand Up @@ -1617,14 +1801,11 @@ encoder_encode_key_value(PyEncoderObject *s, PyUnicodeWriter *writer, bool *firs
}
}

encoded = encoder_encode_string(s, keystr);
rv = encoder_write_string(s, writer, keystr);
Py_DECREF(keystr);
if (encoded == NULL) {
return -1;
}

if (_steal_accumulate(writer, encoded) < 0) {
return -1;
if (rv < 0) {
return rv;
}
if (PyUnicodeWriter_WriteStr(writer, s->key_separator) < 0) {
return -1;
Expand Down
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