/////////////////////////////////////////// // bpred.sv // // Written: Ross Thomposn ross1728@gmail.com // Created: 12 February 2021 // Modified: 19 January 2023 // // Purpose: Branch direction prediction and jump/branch target prediction. // Prediction made during the fetch stage and corrected in the execution stage. // // A component of the CORE-V-WALLY configurable RISC-V project. // // Copyright (C) 2021-23 Harvey Mudd College & Oklahoma State University // // SPDX-License-Identifier: Apache-2.0 WITH SHL-2.1 // // Licensed under the Solderpad Hardware License v 2.1 (the “License”); you may not use this file // except in compliance with the License, or, at your option, the Apache License version 2.0. You // may obtain a copy of the License at // // https://solderpad.org/licenses/SHL-2.1/ // // Unless required by applicable law or agreed to in writing, any work distributed under the // License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, // either express or implied. See the License for the specific language governing permissions // and limitations under the License. //////////////////////////////////////////////////////////////////////////////////////////////// `include "wally-config.vh" `define INSTR_CLASS_PRED 0 module bpred ( input logic clk, reset, input logic StallF, StallD, StallE, StallM, StallW, input logic FlushD, FlushE, FlushM, FlushW, // Fetch stage // the prediction input logic [31:0] InstrD, // Decompressed decode stage instruction. Used to decode instruction class input logic [`XLEN-1:0] PCNextF, // Next Fetch Address input logic [`XLEN-1:0] PCPlus2or4F, // PCF+2/4 output logic [`XLEN-1:0] PC1NextF, // Branch Predictor predicted or corrected fetch address on miss prediction output logic [`XLEN-1:0] NextValidPCE, // Address of next valid instruction after the instruction in the Memory stage // Update Predictor input logic [`XLEN-1:0] PCF, // Fetch stage instruction address input logic [`XLEN-1:0] PCD, // Decode stage instruction address. Also the address the branch predictor took input logic [`XLEN-1:0] PCE, // Execution stage instruction address input logic [`XLEN-1:0] PCM, // Memory stage instruction address input logic [31:0] PostSpillInstrRawF, // Instruction // Branch and jump outcome input logic InstrValidD, InstrValidE, input logic BranchD, BranchE, input logic JumpD, JumpE, input logic PCSrcE, // Executation stage branch is taken input logic [`XLEN-1:0] IEUAdrE, // The branch/jump target address input logic [`XLEN-1:0] IEUAdrM, // The branch/jump target address input logic [`XLEN-1:0] PCLinkE, // The address following the branch instruction. (AKA Fall through address) output logic [3:0] InstrClassM, // The valid instruction class. 1-hot encoded as call, return, jr (not return), j, br // Report branch prediction status output logic BPWrongE, // Prediction is wrong output logic BPWrongM, // Prediction is wrong output logic BPDirPredWrongM, // Prediction direction is wrong output logic BTAWrongM, // Prediction target wrong output logic RASPredPCWrongM, // RAS prediction is wrong output logic IClassWrongM // Class prediction is wrong ); logic [1:0] BPDirPredF; logic [`XLEN-1:0] BPBTAF, RASPCF; logic BPPCWrongE; logic IClassWrongE; logic BPDirPredWrongE; logic BPPCSrcF; logic [`XLEN-1:0] BPPCF; logic [`XLEN-1:0] PC0NextF; logic [`XLEN-1:0] PCCorrectE; logic [3:0] WrongPredInstrClassD; logic BTBTargetWrongE; logic RASTargetWrongE; logic [`XLEN-1:0] BPBTAD; logic BTBCallF, BTBReturnF, BTBJumpF, BTBBranchF; logic BPBranchF, BPJumpF, BPReturnF, BPCallF; logic BPBranchD, BPJumpD, BPReturnD, BPCallD; logic ReturnD, CallD; logic ReturnE, CallE; logic BranchM, JumpM, ReturnM, CallM; logic BranchW, JumpW, ReturnW, CallW; logic BPReturnWrongD; logic [`XLEN-1:0] BPBTAE; // Part 1 branch direction prediction // look into the 2 port Sram model. something is wrong. if (`BPRED_TYPE == "BP_TWOBIT") begin:Predictor twoBitPredictor #(`BPRED_SIZE) DirPredictor(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .StallW, .FlushD, .FlushE, .FlushM, .FlushW, .PCNextF, .PCM, .BPDirPredF, .BPDirPredWrongE, .BranchE, .BranchM, .PCSrcE); end else if (`BPRED_TYPE == "BP_GSHARE") begin:Predictor gshare #(`BPRED_SIZE) DirPredictor(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .StallW, .FlushD, .FlushE, .FlushM, .FlushW, .PCNextF, .PCF, .PCD, .PCE, .PCM, .BPDirPredF, .BPDirPredWrongE, .BPBranchF, .BranchD, .BranchE, .BranchM, .BranchW, .PCSrcE); end else if (`BPRED_TYPE == "BP_GLOBAL") begin:Predictor gshare #(`BPRED_SIZE, 0) DirPredictor(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .StallW, .FlushD, .FlushE, .FlushM, .FlushW, .PCNextF, .PCF, .PCD, .PCE, .PCM, .BPDirPredF, .BPDirPredWrongE, .BPBranchF, .BranchD, .BranchE, .BranchM, .BranchW, .PCSrcE); end else if (`BPRED_TYPE == "BP_GSHARE_BASIC") begin:Predictor gsharebasic #(`BPRED_SIZE) DirPredictor(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .StallW, .FlushD, .FlushE, .FlushM, .FlushW, .PCNextF, .PCM, .BPDirPredF, .BPDirPredWrongE, .BranchE, .BranchM, .PCSrcE); end else if (`BPRED_TYPE == "BP_GLOBAL_BASIC") begin:Predictor gsharebasic #(`BPRED_SIZE, 0) DirPredictor(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .StallW, .FlushD, .FlushE, .FlushM, .FlushW, .PCNextF, .PCM, .BPDirPredF, .BPDirPredWrongE, .BranchE, .BranchM, .PCSrcE); end else if (`BPRED_TYPE == "BP_LOCAL_BASIC") begin:Predictor localbpbasic #(`BPRED_NUM_LHR, `BPRED_SIZE) DirPredictor(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .StallW, .FlushD, .FlushE, .FlushM, .FlushW, .PCNextF, .PCM, .BPDirPredF, .BPDirPredWrongE, .BranchE, .BranchM, .PCSrcE); end // Part 2 Branch target address prediction // BTB contains target address for all CFI btb #(`BTB_SIZE) TargetPredictor(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .StallW, .FlushD, .FlushE, .FlushM, .FlushW, .PCNextF, .PCF, .PCD, .PCE, .PCM, .BPBTAF, .BPBTAD, .BPBTAE, .BTBIClassF({BTBCallF, BTBReturnF, BTBJumpF, BTBBranchF}), .IClassWrongM, .IClassWrongE, .IEUAdrE, .IEUAdrM, .InstrClassD({CallD, ReturnD, JumpD, BranchD}), .InstrClassE({CallE, ReturnE, JumpE, BranchE}), .InstrClassM({CallM, ReturnM, JumpM, BranchM}), .InstrClassW({CallW, ReturnW, JumpW, BranchW})); icpred #(`INSTR_CLASS_PRED) icpred(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .StallW, .FlushD, .FlushE, .FlushM, .FlushW, .PostSpillInstrRawF, .InstrD, .BranchD, .BranchE, .JumpD, .JumpE, .BranchM, .BranchW, .JumpM, .JumpW, .CallD, .CallE, .CallM, .CallW, .ReturnD, .ReturnE, .ReturnM, .ReturnW, .BTBCallF, .BTBReturnF, .BTBJumpF, .BTBBranchF, .BPCallF, .BPReturnF, .BPJumpF, .BPBranchF, .IClassWrongM, .IClassWrongE, .BPReturnWrongD); // Part 3 RAS RASPredictor RASPredictor(.clk, .reset, .StallF, .StallD, .StallE, .StallM, .FlushD, .FlushE, .FlushM, .BPReturnF, .ReturnD, .ReturnE, .CallE, .BPReturnWrongD, .RASPCF, .PCLinkE); // Check the prediction // if it is a CFI then check if the next instruction address (PCD) matches the branch's target or fallthrough address. // if the class prediction is wrong a regular instruction may have been predicted as a taken branch // this will result in PCD not being equal to the fall through address PCLinkE (PCE+4). // The next instruction is always valid as no other flush would occur at the same time as the branch and not // also flush the branch. This will change in a superscaler cpu. // branch is wrong only if the PC does not match and both the Decode and Fetch stages have valid instructions. assign BPWrongE = (PCCorrectE != PCD) & InstrValidE & InstrValidD; flopenrc #(1) BPWrongMReg(clk, reset, FlushM, ~StallM, BPWrongE, BPWrongM); // Output the predicted PC or corrected PC on miss-predict. assign BPPCSrcF = (BPBranchF & BPDirPredF[1]) | BPJumpF; mux2 #(`XLEN) pcmuxbp(BPBTAF, RASPCF, BPReturnF, BPPCF); // Selects the BP or PC+2/4. mux2 #(`XLEN) pcmux0(PCPlus2or4F, BPPCF, BPPCSrcF, PC0NextF); // If the prediction is wrong select the correct address. mux2 #(`XLEN) pcmux1(PC0NextF, PCCorrectE, BPWrongE, PC1NextF); // Correct branch/jump target. mux2 #(`XLEN) pccorrectemux(PCLinkE, IEUAdrE, PCSrcE, PCCorrectE); // If the fence/csrw was predicted as a taken branch then we select PCF, rather than PCE. // Effectively this is PCM+4 or the non-existant PCLinkM if(`INSTR_CLASS_PRED) mux2 #(`XLEN) pcmuxBPWrongInvalidateFlush(PCE, PCF, BPWrongM, NextValidPCE); else assign NextValidPCE = PCE; if(`ZICOUNTERS_SUPPORTED) begin logic [`XLEN-1:0] RASPCD, RASPCE; logic BTAWrongE, RASPredPCWrongE; // performance counters // 1. class (class wrong / minstret) (IClassWrongM / csr) // Correct now // 2. target btb (btb target wrong / class[0,1,3]) (btb target wrong / (br + j + jal) // 3. target ras (ras target wrong / class[2]) // 4. direction (br dir wrong / class[0]) // Unfortunately we can't use PCD to infer the correctness of the BTB or RAS because the class prediction // could be wrong or the fall through address selected for branch predict not taken. // By pipeline the BTB's PC and RAS address through the pipeline we can measure the accuracy of // both without the above inaccuracies. // **** use BPBTAWrongM from BTB. assign BTAWrongE = (BPBTAE != IEUAdrE) & (BranchE | JumpE & ~ReturnE) & PCSrcE; assign RASPredPCWrongE = (RASPCE != IEUAdrE) & ReturnE & PCSrcE; flopenrc #(`XLEN) RASTargetDReg(clk, reset, FlushD, ~StallD, RASPCF, RASPCD); flopenrc #(`XLEN) RASTargetEReg(clk, reset, FlushE, ~StallE, RASPCD, RASPCE); flopenrc #(3) BPPredWrongRegM(clk, reset, FlushM, ~StallM, {BPDirPredWrongE, BTAWrongE, RASPredPCWrongE}, {BPDirPredWrongM, BTAWrongM, RASPredPCWrongM}); end else begin assign {BTAWrongM, RASPredPCWrongM} = '0; end // **** Fix me assign InstrClassM = {CallM, ReturnM, JumpM, BranchM}; endmodule