Simple one-bit branch-predictor.

src/main/scala/CPU.scala

@@ -90,6 +90,9 @@ class CPU extends MultiIOModule {
   IDBarrier.in.r2Address := ID.io.r2Address
   IDBarrier.in.ALUop := ID.io.ALUOp
   IDBarrier.in.returnAddr := ID.io.returnAddr
+  IDBarrier.in.branchAddr := ID.io.branchAddr
+  IDBarrier.in.nextOpAddr := ID.io.nextOpAddr
+  IDBarrier.in.branchPrediction := ID.io.branchPrediction
   IDBarrier.in.jump := ID.io.jump
   IDBarrier.in.branchType := ID.io.branchType
   IDBarrier.in.writeEnable := ID.io.writeEnableOut
@@ -103,10 +106,12 @@ class CPU extends MultiIOModule {
   EX.io.branchType := IDBarrier.out.branchType
   EX.io.rs1ValueIn := forward(IDBarrier.out.r1Value, IDBarrier.out.r1Address, true.B, mem = MEMBarrier.forwardMem, wb = MEMBarrier.forwardWb, id = MEMBarrier.forwardId).asSInt()
   EX.io.rs2ValueIn := forward(IDBarrier.out.r2Value, IDBarrier.out.r2Address, true.B, mem = MEMBarrier.forwardMem, wb = MEMBarrier.forwardWb, id = MEMBarrier.forwardId).asSInt()
+  EX.io.branchAddr := IDBarrier.out.branchAddr
+  EX.io.nextOpAddr := IDBarrier.out.nextOpAddr
+  EX.io.branchPrediction := IDBarrier.out.branchPrediction
 
   EXBarrier.in.r2Value := EX.io.rs2ValueOut.asUInt()
   EXBarrier.in.ALUResult := EX.io.ALUResult.asUInt()
-  EXBarrier.branchIn := EX.io.branch
   EXBarrier.in.jump := IDBarrier.out.jump
   EXBarrier.in.returnAddr := IDBarrier.out.returnAddr
   EXBarrier.in.writeEnable := IDBarrier.out.writeEnable
@@ -126,19 +131,29 @@ class CPU extends MultiIOModule {
   MEMBarrier.in.writeEnable := EXBarrier.out.writeEnable
   MEMBarrier.in.writeAddr := EXBarrier.out.writeAddr
 
+  // Fast branching forwarding
+  ID.io.forwardEx := EXBarrier.forwardEx
+  ID.io.forwardMem := MEMBarrier.forwardMem
+  ID.io.forwardWb := MEMBarrier.forwardWb
+  ID.io.forwardId := MEMBarrier.forwardId
+
   // Write back
   ID.io.writeData := MEMBarrier.out.data
   ID.io.writeEnableIn := MEMBarrier.out.writeEnable
   ID.io.writeAddrIn := MEMBarrier.out.writeAddr
 
   // Branching
-  IF.io.branch := EXBarrier.branchOut
-  IF.io.branchAddress := EXBarrier.branchAddr
+  IF.io.branch := ID.io.branchPrediction || EX.io.misprediction
+  IF.io.branchAddress := Mux(EX.io.misprediction, EX.io.actualBranchAddr, ID.io.branchAddr)
+
+  // Flush
+  IFBarrier.flush := EX.io.misprediction
+
+  // Update branch prediction
+  ID.io.hasBranchResult := EX.io.branchOperation
+  ID.io.branchResult := EX.io.branchTaken
 
   // Stall
   IF.io.stall := IDBarrier.stall
   IFBarrier.stall := IDBarrier.stall
-
-  // Flush
-  IFBarrier.flush := EXBarrier.flush
 }

src/main/scala/Decoder.scala

@@ -87,7 +87,7 @@ class Decoder() extends Module {
 
     // signal      regWrite, memRead, memWrite, branch,  jump, branchType,      Op1Select,      Op2Select, ImmSelect,        ALUOp
     JAL    -> List(Y,        N,       N,        Y,       Y,    branchType.jump, PC,             imm,       ImmFormat.JTYPE,  ALUOps.ADD   ),
-    JALR   -> List(Y,        N,       N,        Y,       Y,    branchType.jump, rs1,            imm,       ImmFormat.ITYPE,  ALUOps.ADDR  ),
+    JALR   -> List(Y,        N,       N,        Y,       Y,    branchType.jump, rs1,            imm,       ImmFormat.ITYPE,  ALUOps.ADD   ),
   )
 
 

src/main/scala/EX.scala

@@ -14,7 +14,13 @@ class Execute extends MultiIOModule {
       val rs2ValueIn = Input(SInt(32.W))
       val rs2ValueOut = Output(SInt(32.W))
       val branchType = Input(UInt(3.W))
-      val branch = Output(Bool())
+      val branchPrediction = Input(Bool())
+      val branchAddr = Input(UInt(32.W))
+      val nextOpAddr = Input(UInt(32.W))
+      val actualBranchAddr = Output(UInt(32.W))
+      val branchTaken = Output(Bool())
+      val branchOperation = Output(Bool())
+      val misprediction = Output(Bool())
       val ALUOp = Input(UInt(4.W))
       val ALUResult = Output(SInt(32.W))
     }
@@ -31,7 +37,6 @@ class Execute extends MultiIOModule {
     ALUOps.SRA    -> (io.op1 >> io.op2(4, 0)),
     ALUOps.SRL    -> (io.op1.asUInt() >> io.op2(4, 0)).asSInt(),
     ALUOps.SLL    -> (io.op1.asUInt() << io.op2(4, 0)).asSInt(),
-    ALUOps.ADDR   -> ((io.op1 + io.op2) & -2.S),
     ALUOps.COPY_A -> io.op1,
     ALUOps.COPY_B -> io.op2,
   )
@@ -47,6 +52,9 @@ class Execute extends MultiIOModule {
   )
 
   io.rs2ValueOut := io.rs2ValueIn
-  io.branch := MuxLookup(io.branchType, false.B, BranchALUOpsMap)
+  io.branchTaken := MuxLookup(io.branchType, false.B, BranchALUOpsMap)
+  io.misprediction := io.branchOperation && (io.branchTaken =/= io.branchPrediction)
+  io.actualBranchAddr := Mux(io.branchTaken, io.branchAddr, io.nextOpAddr)
+  io.branchOperation := io.branchType =/= branchType.DC
   io.ALUResult := MuxLookup(io.ALUOp, 0.S(32.W), ALUOpsMap)
 }

src/main/scala/EXBarrier.scala

@@ -19,17 +19,14 @@ class EXBarrier extends MultiIOModule {
     new Bundle {
       val in = Input(new EXBarrierIO)
       val out = Output(new EXBarrierIO)
-      val flush = Output(Bool())
-      val branchAddr = Output(UInt(32.W))
-      val branchIn = Input(Bool())
-      val branchOut = Output(Bool())
+      val forwardEx = Output(new Forwarding)
     })
 
   val delay = Reg(new EXBarrierIO)
   delay := io.in
   io.out := delay
 
-  io.flush := io.branchIn
-  io.branchOut := io.branchIn
-  io.branchAddr := io.in.ALUResult
+  io.forwardEx.write := io.in.writeEnable
+  io.forwardEx.writeAddr := io.in.writeAddr
+  io.forwardEx.writeData := Mux(io.in.jump, io.in.returnAddr, io.in.ALUResult)
 }

src/main/scala/ID.scala

@@ -19,6 +19,8 @@ class InstructionDecode extends MultiIOModule {
   val io = IO(
     new Bundle {
       val instruction = Input(new Instruction)
+      val hasBranchResult = Input(Bool())
+      val branchResult = Input(Bool())
       val pc = Input(UInt(32.W))
       val op1 = Output(SInt(32.W))
       val isOp1RValue = Output(Bool())
@@ -37,11 +39,43 @@ class InstructionDecode extends MultiIOModule {
       val memWrite = Output(Bool())
       val memRead = Output(Bool())
       val branchType = Output(UInt(3.W))
+      val branchPrediction = Output(Bool())
       val jump = Output(Bool())
       val returnAddr = Output(UInt(32.W))
+      val branchAddr = Output(UInt(32.W))
+      val nextOpAddr = Output(UInt(32.W))
+
+      val forwardEx = Input(new Forwarding)
+      val forwardMem = Input(new Forwarding)
+      val forwardWb = Input(new Forwarding)
+      val forwardId = Input(new Forwarding)
     }
   )
 
+  def forward(data: UInt, addr: UInt, useForward: Bool, ex: Forwarding, mem: Forwarding, wb: Forwarding, id: Forwarding): UInt = {
+    Mux(
+      !useForward,
+      data,
+      Mux(
+        ex.valid && ex.writeAddr === addr,
+        ex.writeData,
+        Mux(
+          mem.valid && mem.writeAddr === addr,
+          mem.writeData,
+          Mux(
+            wb.valid && wb.writeAddr === addr,
+            wb.writeData,
+            Mux(
+              id.valid && id.writeAddr === addr,
+              id.writeData,
+              data,
+            )
+          )
+        )
+      )
+    )
+  }
+
   val registers = Module(new Registers)
   val decoder   = Module(new Decoder).io
 
@@ -94,4 +128,12 @@ class InstructionDecode extends MultiIOModule {
   io.writeEnableOut := decoder.controlSignals.regWrite
   io.memRead := decoder.controlSignals.memRead
   io.memWrite := decoder.controlSignals.memWrite
+
+  val op1 = forward(io.op1.asUInt(), io.r1Address, io.isOp1RValue, ex = io.forwardEx, mem = io.forwardMem, wb = io.forwardWb,  id = io.forwardId).asSInt()
+  io.branchAddr := ((op1 + io.op2) & -2.S).asUInt()
+  io.nextOpAddr := io.pc + 4.U
+
+  val lastBranchWasTaken = RegInit(Bool(), true.B)
+  lastBranchWasTaken := Mux(io.hasBranchResult, io.branchResult, lastBranchWasTaken)
+  io.branchPrediction := Mux(io.branchType =/= branchType.DC, lastBranchWasTaken, false.B)
 }

src/main/scala/IDBarrier.scala

@@ -13,6 +13,9 @@ class IDBarrierIO extends Bundle {
   val r2Value = UInt(32.W)
   val r2Address = UInt(5.W)
   val returnAddr = UInt(32.W)
+  val branchAddr = UInt(32.W)
+  val nextOpAddr = UInt(32.W)
+  val branchPrediction = Bool()
   val jump = Bool()
   val ALUop = UInt(4.W)
   val branchType = UInt(3.W)

src/main/scala/IF.scala

@@ -46,9 +46,9 @@ class InstructionFetch extends MultiIOModule {
     * 
     * You should expand on or rewrite the code below.
     */
-  io.PC := PC
-  IMEM.io.instructionAddress := PC
-  PC := Mux(io.branch, io.branchAddress, Mux(io.stall, PC, PC + 4.U))
+  io.PC := Mux(io.branch, io.branchAddress, PC)
+  IMEM.io.instructionAddress := io.PC
+  PC := Mux(io.branch, io.branchAddress + 4.U, Mux(io.stall, PC, PC + 4.U))
 
   val instruction = Wire(new Instruction)
   instruction := IMEM.io.instruction.asTypeOf(new Instruction)

src/main/scala/IFBarrier.scala

@@ -20,22 +20,12 @@ class IFBarrier extends MultiIOModule {
 
   val instruction = Reg(new Instruction)
   val replay = RegInit(Bool(), false.B)
-  val flushRemaining = RegInit(UInt(2.W), 0.U)
-  flushRemaining := Mux(
-    io.flush,
-    1.U,
-    Mux(
-      flushRemaining === 0.U,
-      0.U,
-      flushRemaining - 1.U
-    )
-  )
 
   replay := io.stall
   instruction := io.instructionIn
 
   io.instructionOut := Mux(
-    io.stall || io.flush || flushRemaining > 0.U,
+    io.stall || io.flush,
     Instruction.NOP,
     Mux(
       replay,

src/main/scala/ToplevelSignals.scala

@@ -119,7 +119,6 @@ object ALUOps {
   val SRA    = 9.U(4.W)
   val COPY_A = 10.U(4.W)
   val COPY_B = 11.U(4.W)
-  val ADDR   = 12.U(4.W)
 
   val DC     = 15.U(4.W)
 }

src/test/scala/Manifest.scala

@@ -19,7 +19,7 @@ import LogParser._
 
 object Manifest {
 
-  val singleTest = "branch.s"
+  val singleTest = "square.s"
 
   val nopPadded = false