Added popcount util

rocket/src/main/scala/util.scala

@@ -32,6 +32,41 @@ object FillInterleaved
   }
 }
 
+// http://aggregate.ee.engr.uky.edu/MAGIC/#Population%20Count%20%28Ones%20Count%29
+// http://bits.stephan-brumme.com/countBits.html
+object PopCount
+{
+  def apply(in: Bits) =
+  {
+    require(in.width <= 32)
+    val w = log2up(in.width+1)
+    var x = in
+    if(in.width == 2) { 
+      x = x - ((x >> UFix(1)) & Bits("h_5555_5555"))
+    } else if(in.width <= 4) {
+      x = x - ((x >> UFix(1)) & Bits("h_5555_5555"))
+      x = (((x >> UFix(2)) & Bits("h_3333_3333")) + (x & Bits("h_3333_3333")))
+    } else if(in.width <= 8) {
+      x = x - ((x >> UFix(1)) & Bits("h_5555_5555"))
+      x = (((x >> UFix(2)) & Bits("h_3333_3333")) + (x & Bits("h_3333_3333")))
+      x = ((x >> UFix(4)) + x) 
+    } else {
+      // count bits of each 2-bit chunk
+      x = x - ((x >> UFix(1)) & Bits("h_5555_5555"))
+      // count bits of each 4-bit chunk
+      x = (((x >> UFix(2)) & Bits("h_3333_3333")) + (x & Bits("h_3333_3333")))
+      // count bits of each 8-bit chunk
+      x = ((x >> UFix(4)) + x) 
+      // mask junk in upper bits
+      x = x & Bits("h_0f0f_0f0f")
+      // add all four 8-bit chunks
+      x = x + (x >> UFix(8))
+      x = x + (x >> UFix(16))
+    }
+    x(w-1,0)
+  }
+}
+
 object Reverse
 {
   def apply(in: Bits) =