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Partial BTB cleanup.

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This commit is contained in:
Ross Thompson 2023-01-24 16:12:35 -06:00
parent 2157970adf
commit 7d1109fc24
2 changed files with 6 additions and 121 deletions
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114
pipelined/src/ifu/brpred/BTBPredictor.sv
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@ -1,114 +0,0 @@
///////////////////////////////////////////
// ram2p1r1wb
//
// Written: Ross Thomposn
// Email: ross1728@gmail.com
// Created: February 15, 2021
// Modified:
//
// Purpose: BTB model. Outputs type of instruction (currently 1 hot encoded. Probably want
// to encode to reduce storage), valid, target PC.
//
// 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"
module BTBPredictor
#(parameter int Depth = 10
)
(input logic clk,
input logic reset,
input logic StallF, StallE,
input logic [`XLEN-1:0] LookUpPC,
output logic [`XLEN-1:0] TargetPC,
output logic [3:0] InstrClass,
output logic Valid,
// update
input logic UpdateEN,
input logic [`XLEN-1:0] UpdatePC,
input logic [`XLEN-1:0] UpdateTarget,
input logic [3:0] UpdateInstrClass,
input logic UpdateInvalid
);
localparam TotalDepth = 2 ** Depth;
logic [TotalDepth-1:0] ValidBits;
logic [Depth-1:0] LookUpPCIndex, UpdatePCIndex, LookUpPCIndexQ, UpdatePCIndexQ;
logic UpdateENQ;
// hashing function for indexing the PC
// We have Depth bits to index, but XLEN bits as the input.
// bit 0 is always 0, bit 1 is 0 if using 4 byte instructions, but is not always 0 if
// using compressed instructions. XOR bit 1 with the MSB of index.
assign UpdatePCIndex = {UpdatePC[Depth+1] ^ UpdatePC[1], UpdatePC[Depth:2]};
assign LookUpPCIndex = {LookUpPC[Depth+1] ^ LookUpPC[1], LookUpPC[Depth:2]};
flopenr #(Depth) UpdatePCIndexReg(.clk(clk),
.reset(reset),
.en(~StallE),
.d(UpdatePCIndex),
.q(UpdatePCIndexQ));
// The valid bit must be resetable.
always_ff @ (posedge clk) begin
if (reset) begin
ValidBits <= #1 {TotalDepth{1'b0}};
end else
if (UpdateENQ) begin
ValidBits[UpdatePCIndexQ] <= #1 ~ UpdateInvalid;
end
end
assign Valid = ValidBits[LookUpPCIndexQ];
flopenr #(1) UpdateENReg(.clk(clk),
.reset(reset),
.en(~StallF),
.d(UpdateEN),
.q(UpdateENQ));
flopenr #(Depth) LookupPCIndexReg(.clk(clk),
.reset(reset),
.en(~StallF),
.d(LookUpPCIndex),
.q(LookUpPCIndexQ));
// the BTB contains the target address.
// Another optimization may be using a PC relative address.
// *** need to add forwarding.
// *** optimize for byte write enables
// *** switch to ram2p1r1wbefix
ram2p1r1wb #(Depth, `XLEN+4) memory(.clk(clk),
.reset(reset),
.ra1(LookUpPCIndex),
.rd1({{InstrClass, TargetPC}}),
.ren1(~StallF),
.wa2(UpdatePCIndex),
.wd2({UpdateInstrClass, UpdateTarget}),
.wen2(UpdateEN),
.bwe2({4'hF, {`XLEN{1'b1}}})); // *** definitely not right.
endmodule

13
pipelined/src/ifu/brpred/bpred.sv
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@ -135,20 +135,19 @@ module bpred (
// Part 2 Branch target address prediction
// *** For now the BTB will house the direct and indirect targets
// *** getting to many false positivies from the BTB, we need a partial TAG to reduce this.
BTBPredictor TargetPredictor(.clk(clk),
btb TargetPredictor(.clk(clk),
.reset(reset),
.*, // Stalls and flushes
.LookUpPC(PCNextF),
.TargetPC(BTBPredPCF),
.PCNextF,
.BTBPredPCF,
.InstrClass(PredInstrClassF),
.Valid(BTBValidF),
// update
.UpdateEN((|InstrClassE | (PredictionInstrClassWrongE)) & ~StallE),
.UpdatePC(PCE),
.UpdateTarget(IEUAdrE),
.PCE,
.IEUAdrE,
.UpdateInvalid(PredictionInstrClassWrongE),
.UpdateInstrClass(InstrClassE));
.InstrClassE);
// Part 3 RAS
// *** need to add the logic to restore RAS on flushes. We will use incr for this.