From 9f6d1626dd4ce767c18db2f9e986a7a53c77e260 Mon Sep 17 00:00:00 2001
From: Easton Man <manyang.me@outlook.com>
Date: Fri, 1 Sep 2023 20:20:27 +0800
Subject: [PATCH] FTB(timing): use last_stage_entry.carry in fallthrough
 address Mux()

---
 src/main/scala/xiangshan/frontend/FTB.scala   | 19 +++++++++++++++----
 .../xiangshan/frontend/FrontendBundle.scala   | 11 ++++++++---
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/src/main/scala/xiangshan/frontend/FTB.scala b/src/main/scala/xiangshan/frontend/FTB.scala
index ad691ea7a..b8718c420 100644
--- a/src/main/scala/xiangshan/frontend/FTB.scala
+++ b/src/main/scala/xiangshan/frontend/FTB.scala
@@ -176,7 +176,15 @@ class FTBEntry(implicit p: Parameters) extends XSBundle with FTBParams with BPUU
 
   def getOffsetVec = VecInit(brSlots.map(_.offset) :+ tailSlot.offset)
   def isJal = !isJalr
-  def getFallThrough(pc: UInt) = getFallThroughAddr(pc, carry, pftAddr)
+  def getFallThrough(pc: UInt, last_stage_entry: Option[Tuple2[FTBEntry, Bool]] = None) = {
+    if (last_stage_entry.isDefined) {
+      var stashed_carry = RegEnable(last_stage_entry.get._1.carry, last_stage_entry.get._2)
+      getFallThroughAddr(pc, stashed_carry, pftAddr)
+    } else {
+      getFallThroughAddr(pc, carry, pftAddr)
+    }
+  }
+
   def hasBr(offset: UInt) =
     brSlots.map{ s => s.valid && s.offset <= offset}.reduce(_||_) ||
     (tailSlot.valid && tailSlot.offset <= offset && tailSlot.sharing)
@@ -432,13 +440,16 @@ class FTB(implicit p: Parameters) extends BasePredictor with FTBParams with BPUU
   // io.out.bits.resp := RegEnable(io.in.bits.resp_in(0), 0.U.asTypeOf(new BranchPredictionResp), io.s1_fire)
   io.out := io.in.bits.resp_in(0)
 
-  val s1_latch_call_is_rvc   = DontCare // TODO: modify when add RAS
-
   io.out.s2.full_pred.zip(s2_hit_dup).map {case (fp, h) => fp.hit := h}
   io.out.s2.pc                  := s2_pc_dup
   for (full_pred & s2_ftb_entry & s2_pc & s1_pc & s1_fire <-
     io.out.s2.full_pred zip s2_ftb_entry_dup zip s2_pc_dup zip s1_pc_dup zip io.s1_fire) {
-      full_pred.fromFtbEntry(s2_ftb_entry, s2_pc, Some((s1_pc, s1_fire)))
+      full_pred.fromFtbEntry(s2_ftb_entry,
+        s2_pc,
+        // Previous stage meta for better timing
+        Some(s1_pc, s1_fire),
+        Some(ftbBank.io.read_resp, s1_fire)
+      )
   }
 
   io.out.s3.full_pred.zip(s3_hit_dup).map {case (fp, h) => fp.hit := h}
diff --git a/src/main/scala/xiangshan/frontend/FrontendBundle.scala b/src/main/scala/xiangshan/frontend/FrontendBundle.scala
index f59471f25..c25970ff4 100644
--- a/src/main/scala/xiangshan/frontend/FrontendBundle.scala
+++ b/src/main/scala/xiangshan/frontend/FrontendBundle.scala
@@ -511,9 +511,14 @@ class FullBranchPrediction(implicit p: Parameters) extends XSBundle with HasBPUC
 
   def taken = br_taken_mask.reduce(_||_) || slot_valids.last // || (is_jal || is_jalr)
 
-  def fromFtbEntry(entry: FTBEntry, pc: UInt, last_stage: Option[Tuple2[UInt, Bool]] = None) = {
+  def fromFtbEntry(
+                    entry: FTBEntry,
+                    pc: UInt,
+                    last_stage_pc: Option[Tuple2[UInt, Bool]] = None,
+                    last_stage_entry: Option[Tuple2[FTBEntry, Bool]] = None
+                  ) = {
     slot_valids := entry.brSlots.map(_.valid) :+ entry.tailSlot.valid
-    targets := entry.getTargetVec(pc, last_stage) // Use previous stage pc for better timing
+    targets := entry.getTargetVec(pc, last_stage_pc) // Use previous stage pc for better timing
     jalr_target := targets.last
     offsets := entry.getOffsetVec
     is_jal := entry.tailSlot.valid && entry.isJal
@@ -526,7 +531,7 @@ class FullBranchPrediction(implicit p: Parameters) extends XSBundle with HasBPUC
     val startLower        = Cat(0.U(1.W),    pc(instOffsetBits+log2Ceil(PredictWidth)-1, instOffsetBits))
     val endLowerwithCarry = Cat(entry.carry, entry.pftAddr)
     fallThroughErr := startLower >= endLowerwithCarry
-    fallThroughAddr := Mux(fallThroughErr, pc + (FetchWidth * 4).U, entry.getFallThrough(pc))
+    fallThroughAddr := Mux(fallThroughErr, pc + (FetchWidth * 4).U, entry.getFallThrough(pc, last_stage_entry))
   }
 
   def display(cond: Bool): Unit = {
-- 
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