From 4d14c714a744a2892543ec063f31c17fed208501 Mon Sep 17 00:00:00 2001
From: Ross Thompson <stephen.thompson.37@us.af.mil>
Date: Thu, 4 Mar 2021 13:01:41 -0600
Subject: [PATCH] Fixed forwarding around the 2 bit predictor.

---
 wally-pipelined/src/ifu/BTBPredictor.sv    |  3 +--
 wally-pipelined/src/ifu/twoBitPredictor.sv | 20 ++++++++++++++++++--
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/wally-pipelined/src/ifu/BTBPredictor.sv b/wally-pipelined/src/ifu/BTBPredictor.sv
index 4a78353f..85cb586c 100644
--- a/wally-pipelined/src/ifu/BTBPredictor.sv
+++ b/wally-pipelined/src/ifu/BTBPredictor.sv
@@ -80,9 +80,8 @@ module BTBPredictor
   assign Valid = ValidBits[LookUpPCIndexQ];
 
   // the BTB contains the target address.
-  // *** future version may contain the instruction class, a tag or partial tag,
-  // and other indirection branch data.
   // Another optimization may be using a PC relative address.
+  // *** need to add forwarding.
 
   SRAM2P1R1W #(Depth, `XLEN+4) memory(.clk(clk),
 				      .reset(reset),
diff --git a/wally-pipelined/src/ifu/twoBitPredictor.sv b/wally-pipelined/src/ifu/twoBitPredictor.sv
index 704d7fb0..f01f48dc 100644
--- a/wally-pipelined/src/ifu/twoBitPredictor.sv
+++ b/wally-pipelined/src/ifu/twoBitPredictor.sv
@@ -31,7 +31,7 @@ module twoBitPredictor
   #(parameter int Depth = 10
     )
   (input logic clk,
-   input logic reset,
+   input logic 		   reset,
    input logic [`XLEN-1:0] LookUpPC,
    output logic [1:0] 	   Prediction,
    // update
@@ -42,6 +42,8 @@ module twoBitPredictor
 
   logic [Depth-1:0] 	   LookUpPCIndex, UpdatePCIndex;
   logic [1:0] 		   PredictionMemory;
+  logic 		   DoForwarding, DoForwardingF;
+  logic [1:0] 		   UpdatePredictionF;
   
 
   // hashing function for indexing the PC
@@ -63,6 +65,20 @@ module twoBitPredictor
 				.BitWEN1(2'b11));
 
   // need to forward when updating to the same address as reading.
-  assign Prediction = (UpdatePC == LookUpPC) ? UpdatePrediction : PredictionMemory;
+  // first we compare to see if the update and lookup addreses are the same
+  assign DoForwarding = UpdatePCIndex == LookUpPCIndex;
+
+  // register the update value and the forwarding signal into the Fetch stage
+  flopr #(1) DoForwardingReg(.clk(clk),
+			     .reset(reset),
+			     .d(DoForwarding),
+			     .q(DoForwardingF));
+  
+  flopr #(2) UpdatePredictionReg(.clk(clk),
+				 .reset(reset),
+				 .d(UpdatePrediction),
+				 .q(UpdatePredictionF));
+
+  assign Prediction = DoForwardingF ? UpdatePredictionF : PredictionMemory;
   
 endmodule