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gh-158451: Optimize PyUnicode_Join() by vstinner · Pull Request #158693 · python/cpython · GitHub

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gh-158451: Optimize PyUnicode_Join() #158693

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gh-158451: Optimize PyUnicode_Join() #158693

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d43b1b5
gh-158451: Optimize PyUnicode_Join()
vstinner Oct 3, 2026
7f65635
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vstinner Oct 5, 2026
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gh-158451: Optimize PyUnicode_Join() 
Move tests on the separator outside the loop. Add a loop version for
empty separator.
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vstinner committed Oct 3, 2026
commit d43b1b5d2ab1b5cd5fd1ceecae2fb42d930ab0d3
90 changes: 57 additions & 33 deletions Objects/unicodeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1835,8 +1835,7 @@ PyUnicode_Resize(PyObject **p_unicode, Py_ssize_t length)
static PyObject*
get_latin1_char(Py_UCS1 ch)
{
PyObject *o = LATIN1(ch);
return o;
return LATIN1(ch);
}

static PyObject*
Expand Down Expand Up @@ -10489,9 +10488,7 @@ _PyUnicode_JoinArray(PyObject *separator, PyObject *const *items, Py_ssize_t seq
/* Set up sep and seplen */
if (separator == NULL) {
/* fall back to a blank space separator */
sep = PyUnicode_FromOrdinal(' ');
if (!sep)
goto onError;
sep = get_latin1_char(' ');
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seplen = 1;
maxchar = 32;
}
Expand Down Expand Up @@ -10562,51 +10559,78 @@ _PyUnicode_JoinArray(PyObject *separator, PyObject *const *items, Py_ssize_t seq
use_memcpy = 0;
#else
if (use_memcpy) {
res_data = PyUnicode_1BYTE_DATA(res);
res_data = PyUnicode_DATA(res);
kind = PyUnicode_KIND(res);
if (seplen != 0)
sep_data = PyUnicode_1BYTE_DATA(sep);
sep_data = PyUnicode_DATA(sep);
}
#endif
if (use_memcpy) {
for (i = 0; i < seqlen; ++i) {
Py_ssize_t itemlen;
item = items[i];

/* Copy item, and maybe the separator. */
if (i && seplen != 0) {
memcpy(res_data,
sep_data,
kind * seplen);
res_data += kind * seplen;
}

itemlen = PyUnicode_GET_LENGTH(item);
if (seplen != 0) {
item = items[0];
Py_ssize_t itemlen = PyUnicode_GET_LENGTH(item);
if (itemlen != 0) {
memcpy(res_data,
PyUnicode_DATA(item),
kind * itemlen);
memcpy(res_data, PyUnicode_DATA(item), kind * itemlen);
res_data += kind * itemlen;
}

for (i = 1; i < seqlen; ++i) {
/* Copy item, and maybe the separator. */
memcpy(res_data, sep_data, kind * seplen);
res_data += kind * seplen;

item = items[i];
itemlen = PyUnicode_GET_LENGTH(item);
if (itemlen != 0) {
memcpy(res_data, PyUnicode_DATA(item), kind * itemlen);
res_data += kind * itemlen;
}
}
}
else {
for (i = 0; i < seqlen; ++i) {
item = items[i];
Py_ssize_t itemlen = PyUnicode_GET_LENGTH(item);
if (itemlen != 0) {
memcpy(res_data, PyUnicode_DATA(item), kind * itemlen);
res_data += kind * itemlen;
}
}
}
assert(res_data == PyUnicode_1BYTE_DATA(res)
+ kind * PyUnicode_GET_LENGTH(res));
}
else {
for (i = 0, res_offset = 0; i < seqlen; ++i) {
Py_ssize_t itemlen;
item = items[i];
if (seplen != 0) {
res_offset = 0;
item = items[0];
Py_ssize_t itemlen = PyUnicode_GET_LENGTH(item);
if (itemlen != 0) {
_PyUnicode_FastCopyCharacters(res, res_offset, item, 0, itemlen);
res_offset += itemlen;
}

/* Copy item, and maybe the separator. */
if (i && seplen != 0) {
for (i = 1; i < seqlen; ++i) {
/* Copy item, and maybe the separator. */
_PyUnicode_FastCopyCharacters(res, res_offset, sep, 0, seplen);
res_offset += seplen;
}

itemlen = PyUnicode_GET_LENGTH(item);
if (itemlen != 0) {
_PyUnicode_FastCopyCharacters(res, res_offset, item, 0, itemlen);
res_offset += itemlen;
item = items[i];
itemlen = PyUnicode_GET_LENGTH(item);
if (itemlen != 0) {
_PyUnicode_FastCopyCharacters(res, res_offset, item, 0, itemlen);
res_offset += itemlen;
}
}
}
else {
for (i = 0, res_offset = 0; i < seqlen; ++i) {
item = items[i];
Py_ssize_t itemlen = PyUnicode_GET_LENGTH(item);
if (itemlen != 0) {
_PyUnicode_FastCopyCharacters(res, res_offset, item, 0, itemlen);
res_offset += itemlen;
}
}
}
assert(res_offset == PyUnicode_GET_LENGTH(res));
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