auto merge of #15471 : erickt/rust/push_all, r=acrichto

llvm is currently not able to conver `Vec::extend` into a memcpy for `Copy` types, which results in methods like `Vec::push_all` to run twice as slow as it should be running. This patch takes the unsafe `Vec::clone` optimization to speed up all the operations that are cloning a slice into a `Vec`.

before:

```
test vec::tests::bench_clone_from_0000_0000                ... bench:        12 ns/iter (+/- 2)
test vec::tests::bench_clone_from_0000_0010                ... bench:       125 ns/iter (+/- 4) = 80 MB/s
test vec::tests::bench_clone_from_0000_0100                ... bench:       360 ns/iter (+/- 33) = 277 MB/s
test vec::tests::bench_clone_from_0000_1000                ... bench:      2601 ns/iter (+/- 175) = 384 MB/s
test vec::tests::bench_clone_from_0010_0000                ... bench:        12 ns/iter (+/- 2)
test vec::tests::bench_clone_from_0010_0010                ... bench:       125 ns/iter (+/- 10) = 80 MB/s
test vec::tests::bench_clone_from_0010_0100                ... bench:       361 ns/iter (+/- 28) = 277 MB/s
test vec::tests::bench_clone_from_0100_0010                ... bench:       131 ns/iter (+/- 13) = 76 MB/s
test vec::tests::bench_clone_from_0100_0100                ... bench:       360 ns/iter (+/- 9) = 277 MB/s
test vec::tests::bench_clone_from_0100_1000                ... bench:      2575 ns/iter (+/- 168) = 388 MB/s
test vec::tests::bench_clone_from_1000_0100                ... bench:       356 ns/iter (+/- 20) = 280 MB/s
test vec::tests::bench_clone_from_1000_1000                ... bench:      2605 ns/iter (+/- 167) = 383 MB/s
test vec::tests::bench_from_slice_0000                     ... bench:        11 ns/iter (+/- 0)
test vec::tests::bench_from_slice_0010                     ... bench:       115 ns/iter (+/- 5) = 86 MB/s
test vec::tests::bench_from_slice_0100                     ... bench:       309 ns/iter (+/- 170) = 323 MB/s
test vec::tests::bench_from_slice_1000                     ... bench:      2065 ns/iter (+/- 198) = 484 MB/s
test vec::tests::bench_push_all_0000_0000                  ... bench:         7 ns/iter (+/- 0)
test vec::tests::bench_push_all_0000_0010                  ... bench:        79 ns/iter (+/- 7) = 126 MB/s
test vec::tests::bench_push_all_0000_0100                  ... bench:       342 ns/iter (+/- 18) = 292 MB/s
test vec::tests::bench_push_all_0000_1000                  ... bench:      2873 ns/iter (+/- 75) = 348 MB/s
test vec::tests::bench_push_all_0010_0010                  ... bench:       154 ns/iter (+/- 8) = 64 MB/s
test vec::tests::bench_push_all_0100_0100                  ... bench:       518 ns/iter (+/- 18) = 193 MB/s
test vec::tests::bench_push_all_1000_1000                  ... bench:      4490 ns/iter (+/- 223) = 222 MB/s
```

after:

```
test vec::tests::bench_clone_from_0000_0000                ... bench:        12 ns/iter (+/- 1)
test vec::tests::bench_clone_from_0000_0010                ... bench:       123 ns/iter (+/- 5) = 81 MB/s
test vec::tests::bench_clone_from_0000_0100                ... bench:       367 ns/iter (+/- 23) = 272 MB/s
test vec::tests::bench_clone_from_0000_1000                ... bench:      2618 ns/iter (+/- 252) = 381 MB/s
test vec::tests::bench_clone_from_0010_0000                ... bench:        12 ns/iter (+/- 1)
test vec::tests::bench_clone_from_0010_0010                ... bench:       124 ns/iter (+/- 7) = 80 MB/s
test vec::tests::bench_clone_from_0010_0100                ... bench:       369 ns/iter (+/- 34) = 271 MB/s
test vec::tests::bench_clone_from_0100_0010                ... bench:       123 ns/iter (+/- 6) = 81 MB/s
test vec::tests::bench_clone_from_0100_0100                ... bench:       371 ns/iter (+/- 25) = 269 MB/s
test vec::tests::bench_clone_from_0100_1000                ... bench:      2713 ns/iter (+/- 532) = 368 MB/s
test vec::tests::bench_clone_from_1000_0100                ... bench:       369 ns/iter (+/- 14) = 271 MB/s
test vec::tests::bench_clone_from_1000_1000                ... bench:      2611 ns/iter (+/- 194) = 382 MB/s
test vec::tests::bench_from_slice_0000                     ... bench:         7 ns/iter (+/- 0)
test vec::tests::bench_from_slice_0010                     ... bench:       108 ns/iter (+/- 4) = 92 MB/s
test vec::tests::bench_from_slice_0100                     ... bench:       235 ns/iter (+/- 24) = 425 MB/s
test vec::tests::bench_from_slice_1000                     ... bench:      1318 ns/iter (+/- 96) = 758 MB/s
test vec::tests::bench_push_all_0000_0000                  ... bench:         7 ns/iter (+/- 0)
test vec::tests::bench_push_all_0000_0010                  ... bench:        70 ns/iter (+/- 4) = 142 MB/s
test vec::tests::bench_push_all_0000_0100                  ... bench:       176 ns/iter (+/- 16) = 568 MB/s
test vec::tests::bench_push_all_0000_1000                  ... bench:      1125 ns/iter (+/- 94) = 888 MB/s
test vec::tests::bench_push_all_0010_0010                  ... bench:       159 ns/iter (+/- 15) = 62 MB/s
test vec::tests::bench_push_all_0100_0100                  ... bench:       363 ns/iter (+/- 12) = 275 MB/s
test vec::tests::bench_push_all_1000_1000                  ... bench:      2860 ns/iter (+/- 415) = 349 MB/s
```

This also includes extra benchmarks for `Vec` and `MemWriter`.

2 files changed, 497 insertions, 82 deletions

diff --git a/src/libcollections/vec.rs b/src/libcollections/vec.rs
index 6e2dc95cfd6..1e96588dac5 100644
--- a/src/libcollections/vec.rs
+++ b/src/libcollections/vec.rs
@@ -205,7 +205,9 @@ impl<T: Clone> Vec<T> {
     /// ```
     #[inline]
     pub fn from_slice(values: &[T]) -> Vec<T> {
-        values.iter().map(|x| x.clone()).collect()
+        let mut vector = Vec::new();
+        vector.push_all(values);
+        vector
     }
 
     /// Constructs a `Vec` with copies of a value.
@@ -246,7 +248,21 @@ impl<T: Clone> Vec<T> {
     /// ```
     #[inline]
     pub fn push_all(&mut self, other: &[T]) {
-        self.extend(other.iter().map(|e| e.clone()));
+        self.reserve_additional(other.len());
+
+        for i in range(0, other.len()) {
+            let len = self.len();
+
+            // Unsafe code so this can be optimised to a memcpy (or something similarly
+            // fast) when T is Copy. LLVM is easily confused, so any extra operations
+            // during the loop can prevent this optimisation.
+            unsafe {
+                ptr::write(
+                    self.as_mut_slice().unsafe_mut_ref(len),
+                    other.unsafe_ref(i).clone());
+                self.set_len(len + 1);
+            }
+        }
     }
 
     /// Grows the `Vec` in-place.
@@ -326,24 +342,7 @@ impl<T: Clone> Vec<T> {
 #[unstable]
 impl<T:Clone> Clone for Vec<T> {
     fn clone(&self) -> Vec<T> {
-        let len = self.len;
-        let mut vector = Vec::with_capacity(len);
-        // Unsafe code so this can be optimised to a memcpy (or something
-        // similarly fast) when T is Copy. LLVM is easily confused, so any
-        // extra operations during the loop can prevent this optimisation
-        {
-            let this_slice = self.as_slice();
-            while vector.len < len {
-                unsafe {
-                    let len = vector.len;
-                    ptr::write(
-                        vector.as_mut_slice().unsafe_mut_ref(len),
-                        this_slice.unsafe_ref(len).clone());
-                }
-                vector.len += 1;
-            }
-        }
-        vector
+        Vec::from_slice(self.as_slice())
     }
 
     fn clone_from(&mut self, other: &Vec<T>) {
@@ -359,8 +358,8 @@ impl<T:Clone> Clone for Vec<T> {
 
         // self.len <= other.len due to the truncate above, so the
         // slice here is always in-bounds.
-        let len = self.len();
-        self.extend(other.slice_from(len).iter().map(|x| x.clone()));
+        let slice = other.slice_from(self.len());
+        self.push_all(slice);
     }
 }
 
@@ -1550,7 +1549,6 @@ pub mod raw {
     }
 }
 
-
 #[cfg(test)]
 mod tests {
     extern crate test;
@@ -1852,107 +1850,480 @@ mod tests {
     #[bench]
     fn bench_new(b: &mut Bencher) {
         b.iter(|| {
-            let v: Vec<int> = Vec::new();
+            let v: Vec<uint> = Vec::new();
+            assert_eq!(v.len(), 0);
             assert_eq!(v.capacity(), 0);
-            assert!(v.as_slice() == []);
         })
     }
 
-    #[bench]
-    fn bench_with_capacity_0(b: &mut Bencher) {
+    fn do_bench_with_capacity(b: &mut Bencher, src_len: uint) {
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v: Vec<int> = Vec::with_capacity(0);
-            assert_eq!(v.capacity(), 0);
-            assert!(v.as_slice() == []);
+            let v: Vec<uint> = Vec::with_capacity(src_len);
+            assert_eq!(v.len(), 0);
+            assert_eq!(v.capacity(), src_len);
         })
     }
 
+    #[bench]
+    fn bench_with_capacity_0000(b: &mut Bencher) {
+        do_bench_with_capacity(b, 0)
+    }
 
     #[bench]
-    fn bench_with_capacity_5(b: &mut Bencher) {
-        b.iter(|| {
-            let v: Vec<int> = Vec::with_capacity(5);
-            assert_eq!(v.capacity(), 5);
-            assert!(v.as_slice() == []);
-        })
+    fn bench_with_capacity_0010(b: &mut Bencher) {
+        do_bench_with_capacity(b, 10)
     }
 
     #[bench]
-    fn bench_with_capacity_100(b: &mut Bencher) {
+    fn bench_with_capacity_0100(b: &mut Bencher) {
+        do_bench_with_capacity(b, 100)
+    }
+
+    #[bench]
+    fn bench_with_capacity_1000(b: &mut Bencher) {
+        do_bench_with_capacity(b, 1000)
+    }
+
+    fn do_bench_from_fn(b: &mut Bencher, src_len: uint) {
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v: Vec<int> = Vec::with_capacity(100);
-            assert_eq!(v.capacity(), 100);
-            assert!(v.as_slice() == []);
+            let dst = Vec::from_fn(src_len, |i| i);
+            assert_eq!(dst.len(), src_len);
+            assert!(dst.iter().enumerate().all(|(i, x)| i == *x));
         })
     }
 
     #[bench]
-    fn bench_from_fn_0(b: &mut Bencher) {
+    fn bench_from_fn_0000(b: &mut Bencher) {
+        do_bench_from_fn(b, 0)
+    }
+
+    #[bench]
+    fn bench_from_fn_0010(b: &mut Bencher) {
+        do_bench_from_fn(b, 10)
+    }
+
+    #[bench]
+    fn bench_from_fn_0100(b: &mut Bencher) {
+        do_bench_from_fn(b, 100)
+    }
+
+    #[bench]
+    fn bench_from_fn_1000(b: &mut Bencher) {
+        do_bench_from_fn(b, 1000)
+    }
+
+    fn do_bench_from_elem(b: &mut Bencher, src_len: uint) {
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v: Vec<int> = Vec::from_fn(0, |_| 5);
-            assert!(v.as_slice() == []);
+            let dst: Vec<uint> = Vec::from_elem(src_len, 5);
+            assert_eq!(dst.len(), src_len);
+            assert!(dst.iter().all(|x| *x == 5));
         })
     }
 
     #[bench]
-    fn bench_from_fn_5(b: &mut Bencher) {
+    fn bench_from_elem_0000(b: &mut Bencher) {
+        do_bench_from_elem(b, 0)
+    }
+
+    #[bench]
+    fn bench_from_elem_0010(b: &mut Bencher) {
+        do_bench_from_elem(b, 10)
+    }
+
+    #[bench]
+    fn bench_from_elem_0100(b: &mut Bencher) {
+        do_bench_from_elem(b, 100)
+    }
+
+    #[bench]
+    fn bench_from_elem_1000(b: &mut Bencher) {
+        do_bench_from_elem(b, 1000)
+    }
+
+    fn do_bench_from_slice(b: &mut Bencher, src_len: uint) {
+        let src: Vec<uint> = FromIterator::from_iter(range(0, src_len));
+
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v: Vec<int> = Vec::from_fn(5, |_| 5);
-            assert!(v.as_slice() == [5, 5, 5, 5, 5]);
-        })
+            let dst = Vec::from_slice(src.clone().as_slice());
+            assert_eq!(dst.len(), src_len);
+            assert!(dst.iter().enumerate().all(|(i, x)| i == *x));
+        });
+    }
+
+    #[bench]
+    fn bench_from_slice_0000(b: &mut Bencher) {
+        do_bench_from_slice(b, 0)
+    }
+
+    #[bench]
+    fn bench_from_slice_0010(b: &mut Bencher) {
+        do_bench_from_slice(b, 10)
+    }
+
+    #[bench]
+    fn bench_from_slice_0100(b: &mut Bencher) {
+        do_bench_from_slice(b, 100)
     }
 
     #[bench]
-    fn bench_from_slice_0(b: &mut Bencher) {
+    fn bench_from_slice_1000(b: &mut Bencher) {
+        do_bench_from_slice(b, 1000)
+    }
+
+    fn do_bench_from_iter(b: &mut Bencher, src_len: uint) {
+        let src: Vec<uint> = FromIterator::from_iter(range(0, src_len));
+
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v: Vec<int> = Vec::from_slice([]);
-            assert!(v.as_slice() == []);
-        })
+            let dst: Vec<uint> = FromIterator::from_iter(src.clone().move_iter());
+            assert_eq!(dst.len(), src_len);
+            assert!(dst.iter().enumerate().all(|(i, x)| i == *x));
+        });
+    }
+
+    #[bench]
+    fn bench_from_iter_0000(b: &mut Bencher) {
+        do_bench_from_iter(b, 0)
+    }
+
+    #[bench]
+    fn bench_from_iter_0010(b: &mut Bencher) {
+        do_bench_from_iter(b, 10)
+    }
+
+    #[bench]
+    fn bench_from_iter_0100(b: &mut Bencher) {
+        do_bench_from_iter(b, 100)
     }
 
     #[bench]
-    fn bench_from_slice_5(b: &mut Bencher) {
+    fn bench_from_iter_1000(b: &mut Bencher) {
+        do_bench_from_iter(b, 1000)
+    }
+
+    fn do_bench_extend(b: &mut Bencher, dst_len: uint, src_len: uint) {
+        let dst: Vec<uint> = FromIterator::from_iter(range(0, dst_len));
+        let src: Vec<uint> = FromIterator::from_iter(range(dst_len, dst_len + src_len));
+
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v: Vec<int> = Vec::from_slice([1i, 2, 3, 4, 5]);
-            assert!(v.as_slice() == [1, 2, 3, 4, 5]);
-        })
+            let mut dst = dst.clone();
+            dst.extend(src.clone().move_iter());
+            assert_eq!(dst.len(), dst_len + src_len);
+            assert!(dst.iter().enumerate().all(|(i, x)| i == *x));
+        });
+    }
+
+    #[bench]
+    fn bench_extend_0000_0000(b: &mut Bencher) {
+        do_bench_extend(b, 0, 0)
+    }
+
+    #[bench]
+    fn bench_extend_0000_0010(b: &mut Bencher) {
+        do_bench_extend(b, 0, 10)
+    }
+
+    #[bench]
+    fn bench_extend_0000_0100(b: &mut Bencher) {
+        do_bench_extend(b, 0, 100)
     }
 
     #[bench]
-    fn bench_from_iter_0(b: &mut Bencher) {
+    fn bench_extend_0000_1000(b: &mut Bencher) {
+        do_bench_extend(b, 0, 1000)
+    }
+
+    #[bench]
+    fn bench_extend_0010_0010(b: &mut Bencher) {
+        do_bench_extend(b, 10, 10)
+    }
+
+    #[bench]
+    fn bench_extend_0100_0100(b: &mut Bencher) {
+        do_bench_extend(b, 100, 100)
+    }
+
+    #[bench]
+    fn bench_extend_1000_1000(b: &mut Bencher) {
+        do_bench_extend(b, 1000, 1000)
+    }
+
+    fn do_bench_push_all(b: &mut Bencher, dst_len: uint, src_len: uint) {
+        let dst: Vec<uint> = FromIterator::from_iter(range(0, dst_len));
+        let src: Vec<uint> = FromIterator::from_iter(range(dst_len, dst_len + src_len));
+
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v0: Vec<int> = vec!();
-            let v1: Vec<int> = FromIterator::from_iter(v0.move_iter());
-            assert!(v1.as_slice() == []);
-        })
+            let mut dst = dst.clone();
+            dst.push_all(src.as_slice());
+            assert_eq!(dst.len(), dst_len + src_len);
+            assert!(dst.iter().enumerate().all(|(i, x)| i == *x));
+        });
+    }
+
+    #[bench]
+    fn bench_push_all_0000_0000(b: &mut Bencher) {
+        do_bench_push_all(b, 0, 0)
+    }
+
+    #[bench]
+    fn bench_push_all_0000_0010(b: &mut Bencher) {
+        do_bench_push_all(b, 0, 10)
+    }
+
+    #[bench]
+    fn bench_push_all_0000_0100(b: &mut Bencher) {
+        do_bench_push_all(b, 0, 100)
     }
 
     #[bench]
-    fn bench_from_iter_5(b: &mut Bencher) {
+    fn bench_push_all_0000_1000(b: &mut Bencher) {
+        do_bench_push_all(b, 0, 1000)
+    }
+
+    #[bench]
+    fn bench_push_all_0010_0010(b: &mut Bencher) {
+        do_bench_push_all(b, 10, 10)
+    }
+
+    #[bench]
+    fn bench_push_all_0100_0100(b: &mut Bencher) {
+        do_bench_push_all(b, 100, 100)
+    }
+
+    #[bench]
+    fn bench_push_all_1000_1000(b: &mut Bencher) {
+        do_bench_push_all(b, 1000, 1000)
+    }
+
+    fn do_bench_push_all_move(b: &mut Bencher, dst_len: uint, src_len: uint) {
+        let dst: Vec<uint> = FromIterator::from_iter(range(0u, dst_len));
+        let src: Vec<uint> = FromIterator::from_iter(range(dst_len, dst_len + src_len));
+
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v0: Vec<int> = vec!(1, 2, 3, 4, 5);
-            let v1: Vec<int> = FromIterator::from_iter(v0.move_iter());
-            assert!(v1.as_slice() == [1, 2, 3, 4, 5]);
-        })
+            let mut dst = dst.clone();
+            dst.push_all_move(src.clone());
+            assert_eq!(dst.len(), dst_len + src_len);
+            assert!(dst.iter().enumerate().all(|(i, x)| i == *x));
+        });
+    }
+
+    #[bench]
+    fn bench_push_all_move_0000_0000(b: &mut Bencher) {
+        do_bench_push_all_move(b, 0, 0)
+    }
+
+    #[bench]
+    fn bench_push_all_move_0000_0010(b: &mut Bencher) {
+        do_bench_push_all_move(b, 0, 10)
+    }
+
+    #[bench]
+    fn bench_push_all_move_0000_0100(b: &mut Bencher) {
+        do_bench_push_all_move(b, 0, 100)
+    }
+
+    #[bench]
+    fn bench_push_all_move_0000_1000(b: &mut Bencher) {
+        do_bench_push_all_move(b, 0, 1000)
     }
 
     #[bench]
-    fn bench_extend_0(b: &mut Bencher) {
+    fn bench_push_all_move_0010_0010(b: &mut Bencher) {
+        do_bench_push_all_move(b, 10, 10)
+    }
+
+    #[bench]
+    fn bench_push_all_move_0100_0100(b: &mut Bencher) {
+        do_bench_push_all_move(b, 100, 100)
+    }
+
+    #[bench]
+    fn bench_push_all_move_1000_1000(b: &mut Bencher) {
+        do_bench_push_all_move(b, 1000, 1000)
+    }
+
+    fn do_bench_clone(b: &mut Bencher, src_len: uint) {
+        let src: Vec<uint> = FromIterator::from_iter(range(0, src_len));
+
+        b.bytes = src_len as u64;
+
         b.iter(|| {
-            let v0: Vec<int> = vec!();
-            let mut v1: Vec<int> = vec!(1, 2, 3, 4, 5);
-            v1.extend(v0.move_iter());
-            assert!(v1.as_slice() == [1, 2, 3, 4, 5]);
-        })
+            let dst = src.clone();
+            assert_eq!(dst.len(), src_len);
+            assert!(dst.iter().enumerate().all(|(i, x)| i == *x));
+        });
     }
 
     #[bench]
-    fn bench_extend_5(b: &mut Bencher) {
+    fn bench_clone_0000(b: &mut Bencher) {
+        do_bench_clone(b, 0)
+    }
+
+    #[bench]
+    fn bench_clone_0010(b: &mut Bencher) {
+        do_bench_clone(b, 10)
+    }
+
+    #[bench]
+    fn bench_clone_0100(b: &mut Bencher) {
+        do_bench_clone(b, 100)
+    }
+
+    #[bench]
+    fn bench_clone_1000(b: &mut Bencher) {
+        do_bench_clone(b, 1000)
+    }
+
+    fn do_bench_clone_from(b: &mut Bencher, times: uint, dst_len: uint, src_len: uint) {
+        let dst: Vec<uint> = FromIterator::from_iter(range(0, src_len));
+        let src: Vec<uint> = FromIterator::from_iter(range(dst_len, dst_len + src_len));
+
+        b.bytes = (times * src_len) as u64;
+
         b.iter(|| {
-            let v0: Vec<int> = vec!(1, 2, 3, 4, 5);
-            let mut v1: Vec<int> = vec!(1, 2, 3, 4, 5);
-            v1.extend(v0.move_iter());
-            assert!(v1.as_slice() == [1, 2, 3, 4, 5, 1, 2, 3, 4, 5]);
-        })
+            let mut dst = dst.clone();
+
+            for _ in range(0, times) {
+                dst.clone_from(&src);
+
+                assert_eq!(dst.len(), src_len);
+                assert!(dst.iter().enumerate().all(|(i, x)| dst_len + i == *x));
+            }
+        });
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0000_0000(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 0, 0)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0000_0010(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 0, 10)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0000_0100(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 0, 100)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0000_1000(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 0, 1000)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0010_0010(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 10, 10)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0100_0100(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 100, 100)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_1000_1000(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 1000, 1000)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0010_0100(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 10, 100)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0100_1000(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 100, 1000)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0010_0000(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 10, 0)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_0100_0010(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 100, 10)
+    }
+
+    #[bench]
+    fn bench_clone_from_01_1000_0100(b: &mut Bencher) {
+        do_bench_clone_from(b, 1, 1000, 100)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0000_0000(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 0, 0)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0000_0010(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 0, 10)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0000_0100(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 0, 100)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0000_1000(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 0, 1000)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0010_0010(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 10, 10)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0100_0100(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 100, 100)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_1000_1000(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 1000, 1000)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0010_0100(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 10, 100)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0100_1000(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 100, 1000)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0010_0000(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 10, 0)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_0100_0010(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 100, 10)
+    }
+
+    #[bench]
+    fn bench_clone_from_10_1000_0100(b: &mut Bencher) {
+        do_bench_clone_from(b, 10, 1000, 100)
     }
 }
diff --git a/src/libstd/io/mem.rs b/src/libstd/io/mem.rs
index 7d293f363f0..7f9a1180785 100644
--- a/src/libstd/io/mem.rs
+++ b/src/libstd/io/mem.rs
@@ -607,18 +607,62 @@ mod test {
         assert_eq!(buf.as_slice(), &[7, 8, 6]);
     }
 
-    #[bench]
-    fn bench_mem_writer(b: &mut Bencher) {
+    fn do_bench_mem_writer(b: &mut Bencher, times: uint, len: uint) {
+        let src: Vec<u8> = Vec::from_elem(len, 5);
+
         b.iter(|| {
             let mut wr = MemWriter::new();
-            for _i in range(0u, 10) {
-                wr.write([5, .. 10]).unwrap();
+            for _ in range(0, times) {
+                wr.write(src.as_slice()).unwrap();
             }
-            assert_eq!(wr.unwrap().as_slice(), [5, .. 100].as_slice());
+
+            let v = wr.unwrap();
+            assert_eq!(v.len(), times * len);
+            assert!(v.iter().all(|x| *x == 5));
         });
     }
 
     #[bench]
+    fn bench_mem_writer_001_0000(b: &mut Bencher) {
+        do_bench_mem_writer(b, 1, 0)
+    }
+
+    #[bench]
+    fn bench_mem_writer_001_0010(b: &mut Bencher) {
+        do_bench_mem_writer(b, 1, 10)
+    }
+
+    #[bench]
+    fn bench_mem_writer_001_0100(b: &mut Bencher) {
+        do_bench_mem_writer(b, 1, 100)
+    }
+
+    #[bench]
+    fn bench_mem_writer_001_1000(b: &mut Bencher) {
+        do_bench_mem_writer(b, 1, 1000)
+    }
+
+    #[bench]
+    fn bench_mem_writer_100_0000(b: &mut Bencher) {
+        do_bench_mem_writer(b, 100, 0)
+    }
+
+    #[bench]
+    fn bench_mem_writer_100_0010(b: &mut Bencher) {
+        do_bench_mem_writer(b, 100, 10)
+    }
+
+    #[bench]
+    fn bench_mem_writer_100_0100(b: &mut Bencher) {
+        do_bench_mem_writer(b, 100, 100)
+    }
+
+    #[bench]
+    fn bench_mem_writer_100_1000(b: &mut Bencher) {
+        do_bench_mem_writer(b, 100, 1000)
+    }
+
+    #[bench]
     fn bench_mem_reader(b: &mut Bencher) {
         b.iter(|| {
             let buf = Vec::from_slice([5 as u8, ..100]);