speed up mem::swap

src/libcore/mem.rs

@@ -109,7 +109,7 @@ pub use intrinsics::transmute;
 /// [`Clone`][clone]. You need the value's destructor to run only once,
 /// because a double `free` is undefined behavior.
 ///
-/// An example is the definition of [`mem::swap`][swap] in this module:
+/// An example is the (old) definition of [`mem::swap`][swap] in this module:
 ///
 /// ```
 /// use std::mem;
@@ -448,17 +448,29 @@ pub unsafe fn uninitialized<T>() -> T {
 pub fn swap<T>(x: &mut T, y: &mut T) {
     unsafe {
         // Give ourselves some scratch space to work with
-        let mut t: T = uninitialized();
+        let mut t: [u8; 16] = uninitialized();
 
-        // Perform the swap, `&mut` pointers never alias
-        ptr::copy_nonoverlapping(&*x, &mut t, 1);
-        ptr::copy_nonoverlapping(&*y, x, 1);
-        ptr::copy_nonoverlapping(&t, y, 1);
+        let x = x as *mut T as *mut u8;
+        let y = y as *mut T as *mut u8;
+        let t = &mut t as *mut _ as *mut u8;
 
-        // y and t now point to the same thing, but we need to completely
-        // forget `t` because we do not want to run the destructor for `T`
-        // on its value, which is still owned somewhere outside this function.
-        forget(t);
+        // can't use a for loop as the `range` impl calls `mem::swap` recursively
+        let len = size_of::<T>() as isize;
+        let mut i = 0;
+        while i + 16 <= len {
+            // Perform the swap 16 bytes at a time, `&mut` pointers never alias
+            ptr::copy_nonoverlapping(x.offset(i), t, 16);
+            ptr::copy_nonoverlapping(y.offset(i), x.offset(i), 16);
+            ptr::copy_nonoverlapping(t, y.offset(i), 16);
+            i += 16;
+        }
+        if i < len {
+            // Swap any remaining bytes
+            let rem = (len - i) as usize;
+            ptr::copy_nonoverlapping(x.offset(i), t, rem);
+            ptr::copy_nonoverlapping(y.offset(i), x.offset(i), rem);
+            ptr::copy_nonoverlapping(t, y.offset(i), rem);
+        }
     }
 }