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//////////////////////////////////////////////////////////////////////////////////

// Engineer: Agner Fog

// 

// Create Date:    2020-06-13

// Last modified:  2021-08-03

// Module Name:    subfunctions

// Project Name:   ForwardCom soft core

// Target Devices: Artix 7

// Tool Versions:  Vivado v. 2020.1

// License:        CERN-OHL-W v. 2 or later

// Description:    Subfunctions for calculations:

// bitscan:        find highest set bit

// popcount:       count number of 1-bits

// reversebits:    reverse order of bits

// truth_table_lookup: 3-input truth table

//////////////////////////////////////////////////////////////////////////////////

`include "defines.vh"

// 6-input popcount, fits into 6-input LUT.

function [2:0] popcount6;

    input [5:0] inp;

    integer sum;

    sum = 0;

    for (integer k = 0; k < 6; k ++) begin

        sum += {2'b00, inp[k]};

    end

    return sum;

endfunction

// 32 input popcount

function [5:0] popcount32;

    input [31:0] inp;

    logic[5:0] sum;

    sum = 0;

    for (integer j = 0; j < 5; j++) begin

        sum += popcount6(inp[(j*6)+:6]);

    end

    sum += popcount6({4'b0,inp[31:30]});

    return sum;

endfunction

// 64 input popcount

function [6:0] popcount64;

    input [63:0] inp;

    logic[6:0] sum;

    sum = 0;

    for (integer j = 0; j < 10; j++) begin

        sum += popcount6(inp[(j*6)+:6]);

    end

    sum += popcount6({2'b0,inp[63:60]});

    return sum;

endfunction

// 64 input bit scan

// (also known as leading zero counter or priority encoder)

// return value:

// bitscan64[6:1] is an index to the highest 1-bit in the input

// bitscan64[0]   is 1 if all input bits are zero

function [6:0] bitscan64A;

    input [63:0] m0;         // 64 bits input

    logic [5:0]  r;          // index to highest 1-bit

    logic        iszero;     // indicates that input is zero

    logic [15:0] m1;         // subdivision

    logic [3:0]  m2;         // subdivision

    r = 0;

    // divide into four blocks of 16 bits each

    if (|m0[63:48]) begin

        r[5:4] = 3;          // r[5:4] indicates which 16-bit block contains the highest 1-bit

        m1 = m0[63:48];      // m1 is the 16-bit block that contains the highest 1-bit

    end else if (|m0[47:32]) begin

        r[5:4] = 2;

        m1 = m0[47:32];

    end else if (|m0[31:16]) begin

        r[5:4] = 1;

        m1 = m0[31:16];

    end else begin

        r[5:4] = 0;

        m1 = m0[15:0];

    end

    // now subdivide m1 into four blocks of 4 bits each

    if (|m1[15:12]) begin

        r[3:2] = 3;          // r[3:2] indicates which 4-bit block of m1 contains the highest 1-bit

        m2 = m1[15:12];      // m2 is the 4-bit block that contains the highest 1-bit

    end else if (|m1[11:8]) begin

        m2 = m1[11:8];

        r[3:2] = 2;

    end else if (|m1[7:4]) begin

        m2 = m1[7:4];

        r[3:2] = 1;

    end else begin

        m2 = m1[3:0];

        r[3:2] = 0;

    end

    // finally, test each of the four bits in m2

    if (m2[3])      r[1:0] = 3; // r[1:0] indicates which of the 4 bit bits in m2 contains the highest 1-bit

    else if (m2[2]) r[1:0] = 2;

    else if (m2[1]) r[1:0] = 1;

    else            r[1:0] = 0;

    // test if everything is zero

    iszero = ~|m2;

    // return two values

    return {r, iszero};

endfunction

// 64 input bit scan, alternative implementation

// (this one is slightly slower)

// return value:

// bitscan64[6:1] is an index to the highest 1-bit in the input

// bitscan64[0]   is 1 if all input bits are zero

function [6:0] bitscan64B;

    input [63:0] m0;         // 64 bits input

    logic [5:0]  r;          // index to highest 1-bit

    logic        iszero;     // indicates that input is zero

    logic [3:0]  m1;         // subdivision flags

    logic [3:0]  m2;         // subdivision

    r = 0;

    if (|m0[63:48]) begin

        r[5:4] = 3;

        m1[3] = |m0[63:60];

        m1[2] = |m0[59:56];

        m1[1] = |m0[55:52];

        m1[0] = |m0[51:48];

    end else if (|m0[47:32]) begin

        r[5:4] = 2;

        m1[3] = |m0[47:44];

        m1[2] = |m0[43:40];

        m1[1] = |m0[39:36];

        m1[0] = |m0[35:32];

    end else if (|m0[31:16]) begin

        r[5:4] = 1;

        m1[3] = |m0[31:28];

        m1[2] = |m0[27:24];

        m1[1] = |m0[23:20];

        m1[0] = |m0[19:16];

    end else begin

        r[5:4] = 0;

        m1[3] = |m0[15:12];

        m1[2] = |m0[11:8];

        m1[1] = |m0[7:4];

        m1[0] = |m0[3:0];

    end

    if (m1[3]) begin

        r[3:2] = 3;

    end else if (m1[2]) begin

        r[3:2] = 2;

    end else if (m1[1]) begin

        r[3:2] = 1;

    end else begin

        r[3:2] = 0;

    end

    // extract the 4-bit block that contains the highest 1-bit

    m2 = m0[{r[5:2],2'b0}+: 4];

    if      (m2[3]) r[1:0] = 3;

    else if (m2[2]) r[1:0] = 2;

    else if (m2[1]) r[1:0] = 1;

    else            r[1:0] = 0;

    // test if everything is zero

    iszero = ~|m2;

    // return two values

    return {r, iszero};

endfunction

// 64 input bit scan, alternative implementation

// (this one appears to be the fastest)

// return value:

// bitscan64[6:1] is an index to the highest 1-bit in the input

// bitscan64[0]   is 1 if all input bits are zero

function [6:0] bitscan64C;

    input [63:0] m0;         // 64 bits input

    logic [5:0]  r;          // index to highest 1-bit

    logic        iszero;     // indicates that input is zero

    logic [15:0] m1;         // subdivision flags

    logic [3:0]  m2;         // subdivision

    logic [3:0]  m3;         // subdivision

    r = 0;

    m1[15] = |m0[63:60];

    m1[14] = |m0[59:56];

    m1[13] = |m0[55:52];

    m1[12] = |m0[51:48];

    m1[11] = |m0[47:44];

    m1[10] = |m0[43:40];

    m1[9]  = |m0[39:36];

    m1[8]  = |m0[35:32];

    m1[7]  = |m0[31:28];

    m1[6]  = |m0[27:24];

    m1[5]  = |m0[23:20];

    m1[4]  = |m0[19:16];

    m1[3]  = |m0[15:12];

    m1[2]  = |m0[11:8];

    m1[1]  = |m0[7:4];

    m1[0]  = |m0[3:0];

    m2[3]  = |m1[15:12];

    m2[2]  = |m1[11:8];

    m2[1]  = |m1[7:4];

    m2[1]  = |m1[3:0];

    if (m2[3]) begin

        r[5:4] = 3;

        if      (m1[15]) r[3:2] = 3;

        else if (m1[14]) r[3:2] = 2;

        else if (m1[13]) r[3:2] = 1;

        else             r[3:2] = 0;

    end else if (m2[2]) begin

        r[5:4] = 2;

        if      (m1[11]) r[3:2] = 3;

        else if (m1[10]) r[3:2] = 2;

        else if (m1[9])  r[3:2] = 1;

        else             r[3:2] = 0;

    end else if (m2[1]) begin

        r[5:4] = 1;

        if      (m1[7])  r[3:2] = 3;

        else if (m1[6])  r[3:2] = 2;

        else if (m1[5])  r[3:2] = 1;

        else             r[3:2] = 0;

    end else begin

        r[5:4] = 0;

        if      (m1[3])  r[3:2] = 3;

        else if (m1[2])  r[3:2] = 2;

        else if (m1[1])  r[3:2] = 1;

        else             r[3:2] = 0;

    end

    // extract the 4-bit block that contains the highest 1-bit

    m3 = m0[{r[5:2],2'b0}+: 4];

    if      (m3[3]) r[1:0] = 3;

    else if (m3[2]) r[1:0] = 2;

    else if (m3[1]) r[1:0] = 1;

    else            r[1:0] = 0;

    // test if everything is zero

    iszero = ~|m2;

    // return two values

    return {r, iszero};

endfunction

// This function finds the index to a single bit in a 64-bit input

// where only one bit is set. Used when bitscan relies on the output of roundp2

// Use the formula b = a & ~(a-1) to isolate the lowest set bit before

// calling bitindex. Reverse the order of the bits to find the highest set bit.

// The return value is {r, iszero} where r is the position of the single 1-bit,

// iszero is 1 if all input bits are zero.

// Note that this function does not work if more than one input bit is 1.

function [6:0] bitindex;

    input [63:0] m0;         // 64 bits input

    logic [5:0]  r;          // index to highest 1-bit

    logic        iszero;     // indicates that input is zero

    logic [15:0] m2;         // OR combination of groups of four bits

    m2[15] = |m0[63:60];

    m2[14] = |m0[59:56];

    m2[13] = |m0[55:52];

    m2[12] = |m0[51:48];

    m2[11] = |m0[47:44];

    m2[10] = |m0[43:40];

    m2[9]  = |m0[39:36];

    m2[8]  = |m0[35:32];

    m2[7]  = |m0[31:28];

    m2[6]  = |m0[27:24];

    m2[5]  = |m0[23:20];

    m2[4]  = |m0[19:16];

    m2[3]  = |m0[15:12];

    m2[2]  = |m0[11:8];

    m2[1]  = |m0[7:4];

    m2[0]  = 0;//|m0[3:0]; // not used

    r[5] = m2[8]|m2[9]|m2[10]|m2[11]|m2[12]|m2[13]|m2[14]|m2[15];

    r[4] = m2[4]|m2[5]|m2[6]|m2[7]|m2[12]|m2[13]|m2[14]|m2[15];

    r[3] = m2[2]|m2[3]|m2[6]|m2[7]|m2[10]|m2[11]|m2[14]|m2[15];

    r[2] = m2[1]|m2[3]|m2[5]|m2[7]|m2[9]|m2[11]|m2[13]|m2[15];

    r[1] = m0[2]|m0[3]|m0[6]|m0[7]|m0[10]|m0[11]|m0[14]|m0[15]|

           m0[18]|m0[19]|m0[22]|m0[23]|m0[26]|m0[27]|m0[30]|m0[31]|

           m0[34]|m0[35]|m0[38]|m0[39]|m0[42]|m0[43]|m0[46]|m0[47]|

           m0[50]|m0[51]|m0[54]|m0[55]|m0[58]|m0[59]|m0[62]|m0[63];

    r[0] = m0[1]|m0[3]|m0[5]|m0[7]|m0[9]|m0[11]|m0[13]|m0[15]|

           m0[17]|m0[19]|m0[21]|m0[23]|m0[25]|m0[27]|m0[29]|m0[31]|

           m0[33]|m0[35]|m0[37]|m0[39]|m0[41]|m0[43]|m0[45]|m0[47]|

           m0[49]|m0[51]|m0[53]|m0[55]|m0[57]|m0[59]|m0[61]|m0[63];

    iszero = (~|r) && ~(m0[0]);

    // return two values

    return {r, iszero};

endfunction

// reverse order of bits

function [7:0] reversebits8;

    input [7:0] in;          // 8 bits input    

    return {in[0],in[1],in[2],in[3],in[4],in[5],in[6],in[7]};

endfunction

// reverse order of bits

function [15:0] reversebits16;

    input [15:0] in;         // 16 bits input    

    return {reversebits8(in[7:0]),reversebits8(in[15:8])};

endfunction

// reverse order of bits

function [31:0] reversebits32;

    input [31:0] in;         // 32 bits input    

    return {reversebits8(in[7:0]),reversebits8(in[15:8]),reversebits8(in[23:16]),reversebits8(in[31:24])};

endfunction

// reverse order of bits

function [63:0] reversebits64;

    input [63:0] in;         // 32 bits input    

    return {reversebits8(in[7:0]),reversebits8(in[15:8]),reversebits8(in[23:16]),reversebits8(in[31:24]),

    reversebits8(in[39:32]),reversebits8(in[47:40]),reversebits8(in[55:48]),reversebits8(in[63:56])};

endfunction

// Truth table lookup with three inputs for truth_tab3 instruction

function  [`RB1:0] truth_table_lookup;

    input [`RB1:0] in1;      // input 1

    input [`RB1:0] in2;      // input 2

    input [`RB1:0] in3;      // input 3

    input [7:0]    ttable;   // 8 bit truth table

    logic [`RB1:0] res;      // result

    for (integer k = 0; k < `RB; k++) begin       // loop through bits

        res[k] = ttable[{in3[k],in2[k],in1[k]}];  // lookup with 3 bits index

    end

    truth_table_lookup = res;// result

endfunction