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gh-144157: Optimize bytes.translate() by deferring change detection (GH-144158)

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Optimize bytes.translate() by deferring change detection

Move the equality check out of the hot loop to allow better compiler
optimization. Instead of checking each byte during translation, perform
a single memcmp at the end to determine if the input can be returned
unchanged.

This allows compilers to unroll and pipeline the loops, resulting in ~2x
throughput improvement for medium-to-large inputs (tested on an AMD zen2).
No change observed on small inputs.

It will also be faster for bytes subclasses as those do not need change
detection.
This commit is contained in:
Gregory P. Smith 2026-01-22 09:21:07 -08:00 • committed by GitHub
parent 77bf4ba732
commit a966d94e76
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2 changed files with 10 additions and 4 deletions
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Misc/NEWS.d/next/Core_and_Builtins/2026-01-22-16-20-16.gh-issue-144157.dxyp7k.rst Normal file
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@ -0,0 +1,2 @@
:meth:`bytes.translate` now allows the compiler to unroll its loop more
usefully for a 2x speedup in the common no-deletions specified case.

12
Objects/bytesobject.c
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@ -2237,11 +2237,15 @@ bytes_translate_impl(PyBytesObject *self, PyObject *table,
/* If no deletions are required, use faster code */
for (i = inlen; --i >= 0; ) {
c = Py_CHARMASK(*input++);
if (Py_CHARMASK((*output++ = table_chars[c])) != c)
changed = 1;
*output++ = table_chars[c];
}
if (!changed && PyBytes_CheckExact(input_obj)) {
Py_SETREF(result, Py_NewRef(input_obj));
/* Check if anything changed (for returning original object) */
/* We save this check until the end so that the compiler will */
/* unroll the loop above leading to MUCH faster code. */
if (PyBytes_CheckExact(input_obj)) {
if (memcmp(PyBytes_AS_STRING(input_obj), output_start, inlen) == 0) {
Py_SETREF(result, Py_NewRef(input_obj));
}
}
PyBuffer_Release(&del_table_view);
PyBuffer_Release(&table_view);