diff --git a/src/libcollections/slice.rs b/src/libcollections/slice.rs
index 85aefbfffb1..25ff1f1a82a 100644
--- a/src/libcollections/slice.rs
+++ b/src/libcollections/slice.rs
@@ -1068,6 +1068,18 @@ fn merge_sort<T, F>(v: &mut [T], mut compare: F) where F: FnMut(&T, &T) -> Order
                 let mut out = buf_tmp.offset(start as isize);
                 let out_end = buf_tmp.offset(right_end_idx as isize);
 
+                // If left[last] <= right[0], they are already in order:
+                // fast-forward the left side (the right side is handled
+                // in the loop).
+                // If `right` is not empty then left is not empty, and
+                // the offsets are in bounds.
+                if right != right_end && compare(&*right.offset(-1), &*right) != Greater {
+                    let elems = (right_start as usize - left as usize) / mem::size_of::<T>();
+                    ptr::copy_nonoverlapping(&*left, out, elems);
+                    out = out.offset(elems as isize);
+                    left = right_start;
+                }
+
                 while out < out_end {
                     // Either the left or the right run are exhausted,
                     // so just copy the remainder from the other run