///////////////////////////////////////////
// cnt.sv
//
// Written: Kevin Kim <kekim@hmc.edu>
// Created: 4 February 2023
// Modified: 
//
// Purpose: Count Instruction Submodule
//
// Documentation: RISC-V System on Chip Design Chapter ***
// 
// 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 cnt #(parameter WIDTH = 32) (
  input  logic [WIDTH-1:0] A, B,        // Operands
  input  logic W64,                    // Indicates word operation
  output logic [WIDTH-1:0] czResult,   // count zeros result
  output logic [WIDTH-1:0] cpopResult);// population count result

  //count instructions
  logic [WIDTH-1:0] lzcA, popcntA;
  logic [WIDTH-1:0] revA;

  //in both rv64, rv32
  bitreverse #(WIDTH) brtz(.a(A), .b(revA));
  
  //only in rv64
  if (WIDTH==64) begin
    //NOTE: signal widths can be decreased
    always_comb begin
      //clz input select mux
      case({B,W64})
        5'b00000_0: lzcA = A;                       //clz
        5'b00000_1: lzcA = {A[31:0],{32{1'b1}}};    //clzw
        5'b00001_0: lzcA = revA;                    //ctz
        5'b00001_1: lzcA = {revA[31:0],{32{1'b1}}}; //ctzw
      endcase 

      //cpop select mux
      case ({B,W64})
        5'b00010_0: popcntA = A;
        5'b00010_1: popcntA = {{32{1'b0}}, A[31:0]};
      endcase
    end
  end
  else begin
    assign popcntA = A;
    always_comb begin
      //clz input slect mux
      case(B)
        5'b00000: lzcA = A;
        5'b00001: lzcA = revA;
      endcase
    end
  end

  //NOTE: Signal width mistmatch from log2(WIDTH) to WIDTH but deal with that later.
  lzc #(WIDTH) lzc(.num(lzcA), .ZeroCnt(czResult));
  popcnt #(WIDTH) popcntw(.num(popcntA), .PopCnt(cpopResult));

endmodule