diff --git a/src/libstd/io/extensions.rs b/src/libstd/io/extensions.rs
index 09876f15a8b..7c7d7350055 100644
--- a/src/libstd/io/extensions.rs
+++ b/src/libstd/io/extensions.rs
@@ -465,4 +465,86 @@ mod test {
         assert!(reader.read_le_f32() == 8.1250);
     }
 
+    #[test]
+    fn test_u64_from_be_bytes() {
+        use super::u64_from_be_bytes;
+
+        let buf = [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09];
+
+        // Aligned access
+        assert_eq!(u64_from_be_bytes(buf, 0, 0), 0);
+        assert_eq!(u64_from_be_bytes(buf, 0, 1), 0x01);
+        assert_eq!(u64_from_be_bytes(buf, 0, 2), 0x0102);
+        assert_eq!(u64_from_be_bytes(buf, 0, 3), 0x010203);
+        assert_eq!(u64_from_be_bytes(buf, 0, 4), 0x01020304);
+        assert_eq!(u64_from_be_bytes(buf, 0, 5), 0x0102030405);
+        assert_eq!(u64_from_be_bytes(buf, 0, 6), 0x010203040506);
+        assert_eq!(u64_from_be_bytes(buf, 0, 7), 0x01020304050607);
+        assert_eq!(u64_from_be_bytes(buf, 0, 8), 0x0102030405060708);
+
+        // Unaligned access
+        assert_eq!(u64_from_be_bytes(buf, 1, 0), 0);
+        assert_eq!(u64_from_be_bytes(buf, 1, 1), 0x02);
+        assert_eq!(u64_from_be_bytes(buf, 1, 2), 0x0203);
+        assert_eq!(u64_from_be_bytes(buf, 1, 3), 0x020304);
+        assert_eq!(u64_from_be_bytes(buf, 1, 4), 0x02030405);
+        assert_eq!(u64_from_be_bytes(buf, 1, 5), 0x0203040506);
+        assert_eq!(u64_from_be_bytes(buf, 1, 6), 0x020304050607);
+        assert_eq!(u64_from_be_bytes(buf, 1, 7), 0x02030405060708);
+        assert_eq!(u64_from_be_bytes(buf, 1, 8), 0x0203040506070809);
+    }
+}
+
+#[cfg(test)]
+mod bench {
+    use extra::test::BenchHarness;
+    use container::Container;
+
+    macro_rules! u64_from_be_bytes_bench_impl(
+        ($size:expr, $stride:expr, $start_index:expr) =>
+        ({
+            use vec;
+            use super::u64_from_be_bytes;
+
+            let data = vec::from_fn($stride*100+$start_index, |i| i as u8);
+            let mut sum = 0u64;
+            bh.iter(|| {
+                let mut i = $start_index;
+                while (i < data.len()) {
+                    sum += u64_from_be_bytes(data, i, $size);
+                    i += $stride;
+                }
+            });
+        })
+    )
+
+    #[bench]
+    fn u64_from_be_bytes_4_aligned(bh: &mut BenchHarness) {
+        u64_from_be_bytes_bench_impl!(4, 4, 0);
+    }
+
+    #[bench]
+    fn u64_from_be_bytes_4_unaligned(bh: &mut BenchHarness) {
+        u64_from_be_bytes_bench_impl!(4, 4, 1);
+    }
+
+    #[bench]
+    fn u64_from_be_bytes_7_aligned(bh: &mut BenchHarness) {
+        u64_from_be_bytes_bench_impl!(7, 8, 0);
+    }
+
+    #[bench]
+    fn u64_from_be_bytes_7_unaligned(bh: &mut BenchHarness) {
+        u64_from_be_bytes_bench_impl!(7, 8, 1);
+    }
+
+    #[bench]
+    fn u64_from_be_bytes_8_aligned(bh: &mut BenchHarness) {
+        u64_from_be_bytes_bench_impl!(8, 8, 0);
+    }
+
+    #[bench]
+    fn u64_from_be_bytes_8_unaligned(bh: &mut BenchHarness) {
+        u64_from_be_bytes_bench_impl!(8, 8, 1);
+    }
 }