https://github.com/python/cpython/commit/16108074b1496b1dd60dfdafc762d9e26d6f149c
commit: 16108074b1496b1dd60dfdafc762d9e26d6f149c
branch: main
author: Victor Stinner <[email protected]>
committer: vstinner <[email protected]>
date: 2026-10-07T13:18:10Z
summary:
gh-158897: Optimize io.BufferedReader.readline() by calling memchr() (#158944)
Optimize io.BufferedReader.readline() when lines are close to the
buffer size (128 kB by default) or longer than the buffer size.
Replace the C loop searching for the newline byte in the buffer with
a memchr() call which is more efficient. In the good case,
it makes readline() up to 4x faster!
files:
A Misc/NEWS.d/next/Library/2026-10-07-02-29-45.gh-issue-158897.jH3FwQ.rst
M Modules/_io/bufferedio.c
diff --git
a/Misc/NEWS.d/next/Library/2026-10-07-02-29-45.gh-issue-158897.jH3FwQ.rst
b/Misc/NEWS.d/next/Library/2026-10-07-02-29-45.gh-issue-158897.jH3FwQ.rst
new file mode 100644
index 00000000000000..4524077daefe9a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-07-02-29-45.gh-issue-158897.jH3FwQ.rst
@@ -0,0 +1,5 @@
+Optimize :meth:`!io.BufferedReader.readline` when lines are close to the
+buffer size (128 kB by default) or longer than the buffer size. Replace the
+C loop searching for the newline byte in the buffer with a :c:func:`!memchr`
+call which is more efficient. In the good case, it makes ``readline()`` up to
+4x faster! Patch by Victor Stinner.
diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index d6318af1263d9c..44c3f56caee18b 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -1280,16 +1280,14 @@ _buffered_readline(buffered *self, Py_ssize_t limit)
n = limit;
}
start = self->buffer;
- const char *end = start + n;
- s = start;
- while (s < end) {
- if (*s++ == '\n') {
- if (PyBytesWriter_WriteBytes(writer, start, s - start) < 0) {
- goto error;
- }
- self->pos = s - start;
- goto found;
+ const char *newline = memchr(start, '\n', n);
+ if (newline != NULL) {
+ n = newline - start + 1;
+ if (PyBytesWriter_WriteBytes(writer, start, n) < 0) {
+ goto error;
}
+ self->pos = n;
+ goto found;
}
if (PyBytesWriter_WriteBytes(writer, start, n) < 0) {
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