Don't allow multiple entries for same PC in BTB

Necessary for RVC forward-progress guarantee.

src/main/scala/rocket/btb.scala

@@ -246,6 +246,11 @@ class BTB(implicit p: Parameters) extends BtbModule {
   io.resp.bits.bridx := (if (fetchWidth > 1) Mux1H(hitsVec, brIdx) else UInt(0))
   io.resp.bits.mask := Cat((UInt(1) << ~Mux(io.resp.bits.taken, ~io.resp.bits.bridx, UInt(0)))-1, UInt(1))
 
+  // if multiple entries for same PC land in BTB, zap them
+  when (PopCountAtLeast(hits, 2)) {
+    isValid := isValid & ~hits
+  }
+
   if (nBHT > 0) {
     val bht = new BHT(nBHT)
     val isBranch = !(hits & isJump).orR

src/main/scala/util/Package.scala

@@ -41,4 +41,21 @@ package object util {
     // this one's snagged from scalaz
     def option[T](z: => T): Option[T] = if (x) Some(z) else None
   }
+
+  object PopCountAtLeast {
+    private def two(x: UInt): (Bool, Bool) = x.getWidth match {
+      case 1 => (Bool(true), Bool(false))
+      case n =>
+        val half = x.getWidth / 2
+        val (leftOne, leftTwo) = two(x(half - 1, 0))
+        val (rightOne, rightTwo) = two(x(x.getWidth - 1, half))
+        (leftOne || rightOne, leftTwo || rightTwo || (leftOne && rightOne))
+    }
+    def apply(x: UInt, n: Int): Bool = n match {
+      case 0 => Bool(true)
+      case 1 => x.orR
+      case 2 => two(x)._2
+      case 3 => PopCount(x) >= UInt(n)
+    }
+  }
 }