cvw/pipelined/src/fpu/fcmp.sv

///////////////////////////////////////////
// fcmp.sv
//
// Written: me@KatherineParry.com
// Modified: 7/5/2022
//
// Purpose: Comparison unit
// 
// A component of the 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"

// OpCtrl values
//    110   min
//    101   max
//    010   equal
//    001   less than
//    011   less than or equal

module fcmp (   
   input logic  [`FMTBITS-1:0]   Fmt,      // format of fp number
   input logic  [2:0]            OpCtrl,   // see above table
   input logic                   Xs, Ys,   // input signs
   input logic  [`NE-1:0]        Xe, Ye,   // input exponents
   input logic  [`NF:0]          Xm, Ym,   // input mantissa
   input logic                   XZero, YZero, // is zero
   input logic                   XNaN, YNaN,   // is NaN
   input logic                   XSNaN, YSNaN, // is signaling NaN
   input logic  [`FLEN-1:0]      X, Y,       // original inputs (before unpacker)
   output logic                  CmpNV,      // invalid flag
   output logic [`FLEN-1:0]      CmpFpRes,   // compare floating-point result
   output logic [`XLEN-1:0]      CmpIntRes   // compare integer result
   );

   logic LTabs, LT, EQ;         // is X < or > or = Y
   logic [`FLEN-1:0] NaNRes;    // NaN result
   logic BothZero;              // are both inputs zero
   logic EitherNaN, EitherSNaN; // are either input a (signaling) NaN
   
   assign LTabs= {1'b0, Xe, Xm} < {1'b0, Ye, Ym}; // unsigned comparison, treating FP as integers
   assign LT = (Xs & ~Ys) | (Xs & Ys & ~LTabs & ~EQ) | (~Xs & ~Ys & LTabs); // signed comparison
   assign EQ = (X == Y);

   assign BothZero = XZero&YZero;
   assign EitherNaN = XNaN|YNaN;
   assign EitherSNaN = XSNaN|YSNaN;


   // flags
   //    Min/Max - if an input is a signaling NaN set invalid flag
   //    LT/LE - signaling - sets invalid if NaN input
   //    EQ - quiet - sets invalid if signaling NaN input
   always_comb begin
      case (OpCtrl[2:0])
         3'b110: CmpNV = EitherSNaN;//min 
         3'b101: CmpNV = EitherSNaN;//max
         3'b010: CmpNV = EitherSNaN;//equal
         3'b001: CmpNV = EitherNaN;//less than
         3'b011: CmpNV = EitherNaN;//less than or equal
         default: CmpNV = 1'bx;
      endcase
   end 

   // fmin/fmax of two NaNs returns a quiet NaN of the appropriate size
   // for IEEE, return the payload of X
   // for RISC-V, return the canonical NaN

   // select the NaN result
   if (`FPSIZES == 1)
      if(`IEEE754) assign NaNRes = {Xs, {`NE{1'b1}}, 1'b1, Xm[`NF-2:0]};
      else         assign NaNRes = {1'b0, {`NE{1'b1}}, 1'b1, {`NF-1{1'b0}}};

   else if (`FPSIZES == 2) 
      if(`IEEE754) assign NaNRes = Fmt ? {Xs, {`NE{1'b1}}, 1'b1, Xm[`NF-2:0]} : {{`FLEN-`LEN1{1'b1}}, Xs, {`NE1{1'b1}}, 1'b1, Xm[`NF-2:`NF-`NF1]};
      else         assign NaNRes = Fmt ? {1'b0, {`NE{1'b1}}, 1'b1, {`NF-1{1'b0}}} : {{`FLEN-`LEN1{1'b1}}, 1'b0, {`NE1{1'b1}}, 1'b1, (`NF1-1)'(0)};
   
   else if (`FPSIZES == 3)
      always_comb
            case (Fmt)
               `FMT:  
                  if(`IEEE754) NaNRes = {Xs, {`NE{1'b1}}, 1'b1, Xm[`NF-2:0]};
                  else         NaNRes = {1'b0, {`NE{1'b1}}, 1'b1, {`NF-1{1'b0}}};
               `FMT1:
                  if(`IEEE754) NaNRes = {{`FLEN-`LEN1{1'b1}}, Xs, {`NE1{1'b1}}, 1'b1, Xm[`NF-2:`NF-`NF1]};
                  else         NaNRes = {{`FLEN-`LEN1{1'b1}}, 1'b0, {`NE1{1'b1}}, 1'b1, (`NF1-1)'(0)};
               `FMT2:
                  if(`IEEE754) NaNRes = {{`FLEN-`LEN2{1'b1}}, Xs, {`NE2{1'b1}}, 1'b1, Xm[`NF-2:`NF-`NF2]};
                  else         NaNRes = {{`FLEN-`LEN2{1'b1}}, 1'b0, {`NE2{1'b1}}, 1'b1, (`NF2-1)'(0)};
               default:        NaNRes = {`FLEN{1'bx}};
            endcase

   else if (`FPSIZES == 4)
      always_comb
            case (Fmt)
               2'h3:  
                  if(`IEEE754) NaNRes = {Xs, {`NE{1'b1}}, 1'b1, Xm[`NF-2:0]};
                  else         NaNRes = {1'b0, {`NE{1'b1}}, 1'b1, {`NF-1{1'b0}}};
               2'h1:  
                  if(`IEEE754) NaNRes = {{`FLEN-`D_LEN{1'b1}}, Xs, {`D_NE{1'b1}}, 1'b1, Xm[`NF-2:`NF-`D_NF]};
                  else         NaNRes = {{`FLEN-`D_LEN{1'b1}}, 1'b0, {`D_NE{1'b1}}, 1'b1, (`D_NF-1)'(0)};
               2'h0: 
                  if(`IEEE754) NaNRes = {{`FLEN-`S_LEN{1'b1}}, Xs, {`S_NE{1'b1}}, 1'b1, Xm[`NF-2:`NF-`S_NF]};
                  else         NaNRes = {{`FLEN-`S_LEN{1'b1}}, 1'b0, {`S_NE{1'b1}}, 1'b1, (`S_NF-1)'(0)};
               2'h2:
                  if(`IEEE754) NaNRes = {{`FLEN-`H_LEN{1'b1}}, Xs, {`H_NE{1'b1}}, 1'b1, Xm[`NF-2:`NF-`H_NF]};
                  else         NaNRes = {{`FLEN-`H_LEN{1'b1}}, 1'b0, {`H_NE{1'b1}}, 1'b1, (`H_NF-1)'(0)};
            endcase


   // Min/Max
   //    - outputs the min/max of X and Y
   //    - -0 < 0
   //    - if both are NaN return quiet X
   //    - if one is a NaN output the non-NaN
   always_comb
      if(OpCtrl[0]) // MAX
         if(XNaN)
            if(YNaN)    CmpFpRes = NaNRes;   // X = NaN Y = NaN
            else        CmpFpRes = Y;        // X = NaN Y != NaN
         else
            if(YNaN)    CmpFpRes = X; // X != NaN Y = NaN
            else // X,Y != NaN
               if(LT)   CmpFpRes = Y; // X < Y
               else     CmpFpRes = X; // X > Y
      else  // MIN
         if(XNaN)
            if(YNaN)    CmpFpRes = NaNRes;   // X = NaN Y = NaN
            else        CmpFpRes = Y;        // X = NaN Y != NaN
         else
            if(YNaN)    CmpFpRes = X; // X != NaN Y = NaN
            else // X,Y != NaN
               if(LT)   CmpFpRes = X; // X < Y
               else     CmpFpRes = Y; // X > Y
                                    
   // LT/LE/EQ
   //    - -0 = 0
   //    - inf = inf and -inf = -inf
   //    - return 0 if comparison with NaN (unordered)
   assign CmpIntRes = {(`XLEN-1)'(0), (((EQ|BothZero)&OpCtrl[1])|(LT&OpCtrl[0]&~BothZero))&~EitherNaN};
   
endmodule