add benchmark for slice vectorization... expected it to be little or
zero benefit... turns out to be 20x speedup. Something is wrong.

bench/benchmarkSlice.cpp

diff --git a/bench/benchmarkSlice.cpp b/bench/benchmarkSlice.cpp
new file mode 100644
index 0000000..197fe24
--- /dev/null
+++ b/bench/benchmarkSlice.cpp
@@ -0,0 +1,35 @@
+// g++ -O3 -DNDEBUG benchmarkX.cpp -o benchmarkX && time ./benchmarkX
+
+#include <Eigen/Array>
+
+using namespace std;
+USING_PART_OF_NAMESPACE_EIGEN
+
+#ifndef REPEAT
+#define REPEAT 10000
+#endif
+
+#ifndef SCALAR
+#define SCALAR float
+#endif
+
+int main(int argc, char *argv[])
+{
+  typedef Matrix<SCALAR, Eigen::Dynamic, Eigen::Dynamic> Mat;
+  Mat m(100, 100);
+  m.setRandom();
+
+  for(int a = 0; a < REPEAT; a++)
+  {
+    int r, c, nr, nc;
+    r = Eigen::ei_random<int>(0,10);
+    c = Eigen::ei_random<int>(0,10);
+    nr = Eigen::ei_random<int>(50,80);
+    nc = Eigen::ei_random<int>(50,80);
+    m.block(r,c,nr,nc) += Mat::ones(nr,nc);
+    m.block(r,c,nr,nc) *= SCALAR(10);
+    m.block(r,c,nr,nc) -= Mat::constant(nr,nc,10);
+  }
+  cout << m[0] << endl;
+  return 0;
+}