auto merge of #16999 : brson/rust/fannkuch, r=alexcrichton

From the discussion on reddit:
http://www.reddit.com/r/rust/comments/2fenlg/benchmark_improvement_fannkuchredux/

This adds two variants: the primary, that uses an unsafe block, and a secondary
that is completely safe.

The one with the unsafe block matches clang's performance and beats gcc's.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs
index c3c18e36617..cc2b01e3bb5 100644
--- a/src/libcore/slice.rs
+++ b/src/libcore/slice.rs
@@ -806,7 +806,12 @@ impl<'a,T> MutableSlice<'a, T> for &'a mut [T] {
         let mut i: uint = 0;
         let ln = self.len();
         while i < ln / 2 {
-            self.swap(i, ln - i - 1);
+            // Unsafe swap to avoid the bounds check in safe swap.
+            unsafe {
+                let pa: *mut T = self.unsafe_mut_ref(i);
+                let pb: *mut T = self.unsafe_mut_ref(ln - i - 1);
+                ptr::swap(pa, pb);
+            }
             i += 1;
         }
     }