From 308e8d84a4f1d41e3557757c3bb904fd377a0864 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Sun, 4 Oct 2026 00:44:29 +0200
Subject: [PATCH] gh-158585: Optimize bytes.join() for non-empty separator

---
 Objects/stringlib/join.h | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/Objects/stringlib/join.h b/Objects/stringlib/join.h
index fe460d41f6700f..5fd2ca70f98dd3 100644
--- a/Objects/stringlib/join.h
+++ b/Objects/stringlib/join.h
@@ -134,13 +134,15 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
         }
     }
     else {
-        for (i = 0; i < nbufs; i++) {
-            Py_ssize_t n;
-            char *q;
-            if (i) {
-                memcpy(p, sepstr, seplen);
-                p += seplen;
-            }
+        Py_ssize_t n = buffers[0].len;
+        char *q = buffers[0].buf;
+        memcpy(p, q, n);
+        p += n;
+
+        for (i = 1; i < nbufs; i++) {
+            memcpy(p, sepstr, seplen);
+            p += seplen;
+
             n = buffers[i].len;
             q = buffers[i].buf;
             memcpy(p, q, n);