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bpo-37348: optimize PyUnicode_FromString #14273

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bpo-37348: optimize PyUnicode_FromString #14273

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bpo-37348: optimize PyUnicode_FromString for ASCII
methane Jun 20, 2019
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add news fragment
methane Jun 20, 2019
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2019-06-21-00-58-57.bpo-37348.PfJkWc.rst
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@@ -0,0 +1 @@
Optimized ``PyUnicode_GetString`` for short ASCII strings.
28 changes: 22 additions & 6 deletions Objects/unicodeobject.c
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Expand Up @@ -2163,12 +2163,27 @@ PyUnicode_FromStringAndSize(const char *u, Py_ssize_t size)
PyObject *
PyUnicode_FromString(const char *u)
{
size_t size = strlen(u);
if (size > PY_SSIZE_T_MAX) {
PyErr_SetString(PyExc_OverflowError, "input too long");
return NULL;
/* Most calls of PyUnicode_FromString is for short ASCII string.
* So we optimize this function for them.
*/
size_t len = 0;
int is_ascii = 1;

while (u[len] != '\0') {
if (u[len] > 127) {
is_ascii = 0;

@vstinner vstinner Jun 20, 2019

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Can't you reuse fast _Py_bytes_isascii() somehow here? It works on unsigned long words (64 bits) rather than working on bytes (8 bits): it should be 8x faster.

}
len++;
if (len > PY_SSIZE_T_MAX) {
PyErr_SetString(PyExc_OverflowError, "input too long");
return NULL;
}
}
return PyUnicode_DecodeUTF8Stateful(u, (Py_ssize_t)size, NULL, NULL);

if (is_ascii) {
return _PyUnicode_FromASCII(u, (Py_ssize_t)len);
}
return PyUnicode_DecodeUTF8Stateful(u, (Py_ssize_t)len, NULL, NULL);

@vstinner vstinner Jun 20, 2019

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Why not modifying PyUnicode_DecodeUTF8Stateful() to detect if the input string is ASCII, or your heuristic is faster? Is it because it make non-ASCII string decoding slower?

}

PyObject *
Expand Down Expand Up @@ -15303,8 +15318,9 @@ PyObject *
PyUnicode_InternFromString(const char *cp)
{
PyObject *s = PyUnicode_FromString(cp);
if (s == NULL)
if (s == NULL) {
return NULL;
}
PyUnicode_InternInPlace(&s);
return s;
}
Expand Down