Issue #12805: Make bytes.join and bytearray.join faster when the separator is empty.

Patch by Serhiy Storchaka.

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.4.0 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #12805: Make bytes.join and bytearray.join faster when the separator
+  is empty.  Patch by Serhiy Storchaka.
+
 - Issue #6074: Ensure cached bytecode files can always be updated by the
   user that created them, even when the source file is read-only.
 

Objects/stringlib/join.h

@@ -94,6 +94,16 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
 
     /* Catenate everything. */
     p = STRINGLIB_STR(res);
+    if (!seplen) {
+        /* fast path */
+        for (i = 0; i < nbufs; i++) {
+            Py_ssize_t n = buffers[i].len;
+            char *q = buffers[i].buf;
+            Py_MEMCPY(p, q, n);
+            p += n;
+        }
+        goto done;
+    }
     for (i = 0; i < nbufs; i++) {
         Py_ssize_t n;
         char *q;