// SPDX-FileCopyrightText: 2022 RIOSLab
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// 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.
//
// SPDX-License-Identifier: Apache-2.0
module gshare 
#(
  parameter GSHARE_WIDTH = 4,
  parameter PHT_LEN = 16
)
(
  input clk,
  input reset,

  // from fetch: current instr
  /* verilator lint_off UNUSED */
  input [31:0] pc,

  // from execute: last instr
  input [31:0] prev_pc,  // prev pc
  input prev_branch_in, // whether prev instr is branch
  input prev_taken, // whether prev instr taken
  // input prev_pred, // prev instr pred result
  // output prev_mispred, // whether prev instr mispred

  // to out: taken or not taken
  output cur_pred// cur instr pred
);

integer i =0;

// pattern history table 
reg [1:0] PHT [0:PHT_LEN-1];

// global history register
reg [GSHARE_WIDTH-1:0] GHR = 0;

// reg [GSHARE_WIDTH-1:0] prev_idx = 0;

// previous instr's index into PHT; aliasing issue exists
wire [GSHARE_WIDTH-1:0] prev_idx = prev_pc[GSHARE_WIDTH+1:2] ^ GHR;

wire [GSHARE_WIDTH-1:0] cur_idx = pc[GSHARE_WIDTH+1:2] ^ GHR;

// cur pred result
assign cur_pred = PHT[cur_idx][1];

// whether last instr is mispred
// assign prev_mispred = (prev_taken != prev_pred);

always @(posedge clk) begin
  if (reset) begin
    // prev_idx <= 0;
    GHR <= 0;
    for (i = 0; i < PHT_LEN; i = i + 1) begin
      PHT[i] <= 2'b10; // initial: weak taken
    end
  end

  // update GHR using last pc from execute
  if (prev_branch_in) begin
    GHR <= {GHR[GSHARE_WIDTH-2:0], prev_taken};
  end

  // update PHT using previous taken or not taken result
  if (prev_branch_in && prev_taken) begin
    case (PHT[prev_idx])
      2'b00:
        PHT[prev_idx] <= 2'b01;
      2'b01:
        PHT[prev_idx] <= 2'b10;
      2'b10:
        PHT[prev_idx] <= 2'b11;
      2'b11:
        PHT[prev_idx] <= 2'b11;
    endcase
  end else if (prev_branch_in && !prev_taken) begin
    case (PHT[prev_idx])
      2'b00:
        PHT[prev_idx] <= 2'b00;
      2'b01:
        PHT[prev_idx] <= 2'b00;
      2'b10:
        PHT[prev_idx] <= 2'b01;
      2'b11:
        PHT[prev_idx] <= 2'b10;
    endcase
  end

  /* verilator lint_on UNUSED */
end

endmodule