From 3895b75a560f6a337b4822e0b283115b9aed53b1 Mon Sep 17 00:00:00 2001
From: Andrew Waterman <waterman@cs.berkeley.edu>
Date: Sat, 24 Aug 2013 17:33:11 -0700
Subject: [PATCH] Support non-power-of-2 BTBs; prefer invalid entries

---
 rocket/src/main/scala/dpath_util.scala | 16 +++++++++-------
 rocket/src/main/scala/util.scala       | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/rocket/src/main/scala/dpath_util.scala b/rocket/src/main/scala/dpath_util.scala
index 6e3d00a9..127ac851 100644
--- a/rocket/src/main/scala/dpath_util.scala
+++ b/rocket/src/main/scala/dpath_util.scala
@@ -23,27 +23,29 @@ class rocketDpathBTB(entries: Int) extends Module
 {
   val io = new DpathBTBIO
 
-  val repl_way = LFSR16(io.wen)(log2Up(entries)-1,0) // TODO: pseudo-LRU
-
   var hit_reduction = Bool(false)
   val hit = Bool()
   val update = Bool()
   var update_reduction = Bool(false)
+  val valid = Vec.fill(entries){Reg(init=Bool(false))}
   val hits = Vec.fill(entries){Bool()}
   val updates = Vec.fill(entries){Bool()}
   val targets = Vec.fill(entries){Reg(UInt())}
   val anyUpdate = updates.toBits.orR
 
+  val random_way = Random(entries, io.wen)
+  val invalid_way = valid.indexWhere((x: Bool) => !x)
+  val repl_way = Mux(valid.contains(Bool(false)), invalid_way, random_way)
+
   for (i <- 0 until entries) {
     val tag = Reg(UInt())
-    val valid = Reg(init=Bool(false))
-    hits(i) := valid && tag === io.current_pc
-    updates(i) := valid && tag === io.correct_pc
+    hits(i) := valid(i) && tag === io.current_pc
+    updates(i) := valid(i) && tag === io.correct_pc
 
     when (io.wen && (updates(i) || !anyUpdate && UInt(i) === repl_way)) {
-      valid := Bool(false)
+      valid(i) := Bool(false)
       when (!io.clr) {
-        valid := Bool(true)
+        valid(i) := Bool(true)
         tag := io.correct_pc
         targets(i) := io.correct_target
       }
diff --git a/rocket/src/main/scala/util.scala b/rocket/src/main/scala/util.scala
index 9b735f1d..b99d3d43 100644
--- a/rocket/src/main/scala/util.scala
+++ b/rocket/src/main/scala/util.scala
@@ -130,3 +130,21 @@ case class WideCounter(width: Int, inc: Bool = Bool(true))
     if (isWide) large := (if (w < smallWidth) UInt(0) else x(w.min(width)-1,smallWidth))
   }
 }
+
+object Random
+{
+  def apply(mod: Int, inc: Bool = Bool(true)): UInt = {
+    if (isPow2(mod)) {
+      require(mod <= 65536)
+      LFSR16(inc)(log2Up(mod)-1,0).toUInt
+    } else {
+      val max = 1 << log2Up(mod*8)
+      val rand_pow2 = apply(max, inc)
+
+      var res = UInt(mod-1)
+      for (i <- mod-1 to 1 by -1)
+        res = Mux(rand_pow2 < UInt(i*max/mod), UInt(i-1), res)
+      res
+    }
+  }
+}