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GeezerVison is showing github.com — bpo-37348: optimize PyUnicode_FromString by methane · Pull Request #14273 · python/cpython

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@methane

@methane methane commented Jun 20, 2019 •
edited by bedevere-bot
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https://bugs.python.org/issue37348

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methane added 2 commits June 21, 2019 00:57
@methane
bpo-37348: optimize PyUnicode_FromString for ASCII
ba4f73b
@methane
add news fragment
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@the-knights-who-say-ni the-knights-who-say-ni added the CLA signed label Jun 20, 2019
@bedevere-bot bedevere-bot added the awaiting core review label Jun 20, 2019
vstinner
vstinner reviewed Jun 20, 2019
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@vstinner vstinner left a comment

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I don't understand why this change is needed (why it makes the code faster): unicode_decode_utf8() already starts by decoding from ASCII. Is it a performance issue in ascii_decode()? Does _PyUnicodeWriter adds a small overhead which is significant here?

cc @serhiy-storchaka who knows well such hardcores micro-optimizations ;-)

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Comment thread Objects/unicodeobject.c

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.

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Comment thread Objects/unicodeobject.c
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?

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@methane methane closed this Jun 21, 2019
@methane
methane deleted the ascii-fromstring branch June 21, 2019 05:53
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@methane @vstinner @the-knights-who-say-ni @bedevere-bot
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