cvw/pipelined/src/cache/cachereplacementpolicy.sv

///////////////////////////////////////////
// dcache (data cache)
//
// Written: ross1728@gmail.com July 20, 2021
//          Implements Pseudo LRU
//
//
// A component of the Wally configurable RISC-V project.
//
// Copyright (C) 2021 Harvey Mudd College & Oklahoma State University
//
// Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy,
// modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software
// is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
// BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT
// OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
///////////////////////////////////////////

`include "wally-config.vh"

module cachereplacementpolicy
  #(parameter NUMWAYS = 4, INDEXLEN = 9, OFFSETLEN = 5, NUMLINES = 128)
  (input logic clk, reset,
   input logic [NUMWAYS-1:0] 			WayHit,
   output logic [NUMWAYS-1:0] 			VictimWay,
   input logic [INDEXLEN+OFFSETLEN-1:OFFSETLEN] LsuPAdrM,
   input logic [INDEXLEN-1:0] 			RAdr,
   input logic 					LRUWriteEn
   );

  //  *** Only implements 2, 4, and 8 way
  // I would like parametersize this in the future.

  logic [NUMWAYS-2:0] 				LRUEn, LRUMask;
  logic [$clog2(NUMWAYS)-1:0] 			EncVicWay;
  logic [NUMWAYS-2:0] 				ReplacementBits [NUMLINES-1:0];
  logic [NUMWAYS-2:0] 				LineReplacementBits;
  logic [NUMWAYS-2:0] 				NewReplacement;
  logic [NUMWAYS-2:0] 				NewReplacementD;  

  logic [INDEXLEN+OFFSETLEN-1:OFFSETLEN] 	LsuPAdrMD;
  logic [INDEXLEN-1:0] 				RAdrD;
  logic 					LRUWriteEnD;
  
  /* verilator lint_off BLKLOOPINIT */
  always_ff @(posedge clk) begin
    if (reset) begin
      RAdrD <= '0;
      LsuPAdrMD <= '0;
      LRUWriteEnD <= 0;
      NewReplacementD <= '0;
      for(int index = 0; index < NUMLINES; index++)
	ReplacementBits[index] <= '0;
    end else begin
      RAdrD <= RAdr;
      LsuPAdrMD <= LsuPAdrM;
      LRUWriteEnD <= LRUWriteEn;
      NewReplacementD <= NewReplacement;
      if (LRUWriteEnD) begin
	ReplacementBits[LsuPAdrMD[INDEXLEN+OFFSETLEN-1:OFFSETLEN]] <= NewReplacementD;
      end
    end
  end
  /* verilator lint_on BLKLOOPINIT */

  assign LineReplacementBits = ReplacementBits[RAdrD];

  genvar 		      index;
  generate
    if(NUMWAYS == 2) begin : TwoWay
      
      assign LRUEn[0] = 1'b0;

      assign NewReplacement[0] = WayHit[1];

      assign VictimWay[1] = ~LineReplacementBits[0];
      assign VictimWay[0] = LineReplacementBits[0];
      
    end else if (NUMWAYS == 4) begin : FourWay


      // VictimWay is a function only of the current value of the LRU.
      // binary encoding
      //assign VictimWay[0] = LineReplacementBits[2] ? LineReplacementBits[1] : LineReplacementBits[0];
      //assign VictimWay[1] = LineReplacementBits[2];

      // 1 hot encoding
      //| WayHit | LRU 2 | LRU 1 | LRU 0 |
      //|--------+-------+-------+-------|
      //|   0000 |     - | -     | -     |
      //|   0001 |     1 | -     | 1     |
      //|   0010 |     1 | -     | 0     |
      //|   0100 |     0 | 1     | -     |
      //|   1000 |     0 | 0     | -     |

      assign VictimWay[0] = ~LineReplacementBits[2] & ~LineReplacementBits[0];
      assign VictimWay[1] = ~LineReplacementBits[2] & LineReplacementBits[0];
      assign VictimWay[2] = LineReplacementBits[2] & ~LineReplacementBits[1];
      assign VictimWay[3] = LineReplacementBits[2] & LineReplacementBits[1];      

      // New LRU bits which are updated is function only of the WayHit.
      // However the not updated bits come from the old LRU.
      assign LRUEn[2] = |WayHit;
      assign LRUEn[1] = WayHit[3] | WayHit[2];
      assign LRUEn[0] = WayHit[1] | WayHit[0];

      assign LRUMask[2] = WayHit[1] | WayHit[0];
      assign LRUMask[1] = WayHit[2];
      assign LRUMask[0] = WayHit[0];
      
/* -----\/----- EXCLUDED -----\/-----

      // selects
      assign LRUEn[2] = 1'b1;
      assign LRUEn[1] = WayHit[3];      
      assign LRUEn[0] = WayHit[3] | WayHit[2];

      // mask
      assign LRUMask[0] = WayHit[1];
      assign LRUMask[1] = WayHit[3];      
      assign LRUMask[2] = WayHit[3] | WayHit[2];
 -----/\----- EXCLUDED -----/\----- */

      for(index = 0; index < NUMWAYS-1; index++)
	assign NewReplacement[index] = LRUEn[index] ? LRUMask[index] : LineReplacementBits[index];

/* -----\/----- EXCLUDED -----\/-----
      assign EncVicWay[1] = LineReplacementBits[2];
      assign EncVicWay[0] = LineReplacementBits[2] ? LineReplacementBits[0] : LineReplacementBits[1];

      onehotdecoder #(2) 
      waydec(.bin(EncVicWay),
	     .decoded({VictimWay[0], VictimWay[1], VictimWay[2], VictimWay[3]}));
 -----/\----- EXCLUDED -----/\----- */

    end else if (NUMWAYS == 8) begin : EightWay

      // selects
      assign LRUEn[6] = 1'b1;
      assign LRUEn[5] = WayHit[7] | WayHit[6] | WayHit[5] | WayHit[4];
      assign LRUEn[4] = WayHit[7] | WayHit[6];
      assign LRUEn[3] = WayHit[5] | WayHit[4];
      assign LRUEn[2] = WayHit[3] | WayHit[2] | WayHit[1] | WayHit[0];
      assign LRUEn[1] = WayHit[3] | WayHit[2];
      assign LRUEn[0] = WayHit[1] | WayHit[0];

      // mask
      assign LRUMask[6] = WayHit[7] | WayHit[6] | WayHit[5] | WayHit[4];
      assign LRUMask[5] = WayHit[7] | WayHit[6];
      assign LRUMask[4] = WayHit[7];
      assign LRUMask[3] = WayHit[5];
      assign LRUMask[2] = WayHit[3] | WayHit[2];
      assign LRUMask[1] = WayHit[2];
      assign LRUMask[0] = WayHit[0];

      for(index = 0; index < NUMWAYS-1; index++)
	assign NewReplacement[index] = LRUEn[index] ? LRUMask[index] : LineReplacementBits[index];

      assign EncVicWay[2] = LineReplacementBits[6];
      assign EncVicWay[1] = LineReplacementBits[6] ? LineReplacementBits[5] : LineReplacementBits[2];
      assign EncVicWay[0] = LineReplacementBits[6] ? LineReplacementBits[5] ? LineReplacementBits[4] : LineReplacementBits[3] :
			    LineReplacementBits[2] ? LineReplacementBits[1] : LineReplacementBits[0];
      

      onehotdecoder #(3) 
      waydec(.bin(EncVicWay),
	     .decoded({VictimWay[0], VictimWay[1], VictimWay[2], VictimWay[3],
		       VictimWay[4], VictimWay[5], VictimWay[6], VictimWay[7]}));
    end
  endgenerate
  
endmodule