Use PseudoLRU in BTB when possible (for powers of two)

src/main/scala/rocket/btb.scala

@@ -7,6 +7,7 @@ import junctions._
 import cde.{Parameters, Field}
 import Util._
 import uncore.util._
+import uncore.agents.PseudoLRU
 
 case object BtbKey extends Field[BtbParameters]
 
@@ -180,7 +181,16 @@ class BTB(implicit p: Parameters) extends BtbModule {
   val updateHits = tagMatch(r_btb_update.bits.pc, updatePageHit)
   val updateHit = if (updatesOutOfOrder) updateHits.orR else r_btb_update.bits.prediction.valid
   val updateHitAddr = if (updatesOutOfOrder) OHToUInt(updateHits) else r_btb_update.bits.prediction.bits.entry
-  val nextRepl = Counter(r_btb_update.valid && !updateHit, entries)._1
+
+  // we'd prefer PseudoLRU replacement, but it only works for powers of 2
+  val nextRepl =
+    if (!isPow2(entries)) {
+      Counter(r_btb_update.valid && !updateHit, entries)._1
+    } else {
+      val plru = new PseudoLRU(entries)
+      when (hits.orR) { plru.access(OHToUInt(hits)) }
+      plru.replace
+    }
 
   val useUpdatePageHit = updatePageHit.orR
   val usePageHit = pageHit.orR