DispatchQueue: wrap around tailPtr index when redirect (#800)

This commit fixes the bug when redirect.valid and the last valid instruction is in the last slot.
Previously the tailPtr becomes size.U when there're no instructions before headPtr. It works fine
when DispatchQueueSize is power2.

src/main/scala/xiangshan/backend/dispatch/DispatchQueue.scala

@@ -138,7 +138,8 @@ class DispatchQueue(size: Int, enqnum: Int, deqnum: Int, name: String)(implicit
   val loValidBitVec = Cat((0 until size).map(i => validBitVec(i) && headPtrMask(i)))
   val hiValidBitVec = Cat((0 until size).map(i => validBitVec(i) && ~headPtrMask(i)))
   val flippedFlag = loValidBitVec.orR || validBitVec(size - 1)
-  val lastOneIndex = size.U - PriorityEncoder(Mux(loValidBitVec.orR, loValidBitVec, hiValidBitVec))
+  val leadingZeros = PriorityEncoder(Mux(loValidBitVec.orR, loValidBitVec, hiValidBitVec))
+  val lastOneIndex = Mux(leadingZeros === 0.U, 0.U, size.U - leadingZeros)
   val walkedTailPtr = Wire(new DispatchQueuePtr)
   walkedTailPtr.flag := flippedFlag ^ headPtr(0).flag
   walkedTailPtr.value := lastOneIndex