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/trunk/subfunctions.vh
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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

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