path: root/src/synth/netlists.adb

--  Netlist.
--  Copyright (C) 2017 Tristan Gingold
--
--  This file is part of GHDL.
--
--  This program is free software; you can redistribute it and/or modify
--  it under the terms of the GNU General Public License as published by
--  the Free Software Foundation; either version 2 of the License, or
--  (at your option) any later version.
--
--  This program is distributed in the hope that it will be useful,
--  but WITHOUT ANY WARRANTY; without even the implied warranty of
--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
--  GNU General Public License for more details.
--
--  You should have received a copy of the GNU General Public License
--  along with this program; if not, write to the Free Software
--  Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
--  MA 02110-1301, USA.

with Netlists.Utils; use Netlists.Utils;
with Tables;

package body Netlists is

   --  Names

   package Snames_Table is new Tables
     (Table_Component_Type => Sname_Record,
      Table_Index_Type => Sname,
      Table_Low_Bound => 0,
      Table_Initial => 1024);

   function New_Sname_User (Id : Name_Id) return Sname is
   begin
      Snames_Table.Append ((Kind => Sname_User,
                            Prefix => No_Sname,
                            Suffix => Uns32 (Id)));
      return Snames_Table.Last;
   end New_Sname_User;

   function New_Sname_Artificial (Id : Name_Id) return Sname is
   begin
      Snames_Table.Append ((Kind => Sname_Artificial,
                            Prefix => No_Sname,
                            Suffix => Uns32 (Id)));
      return Snames_Table.Last;
   end New_Sname_Artificial;

   function New_Sname (Prefix : Sname; Suffix : Name_Id) return Sname is
   begin
      Snames_Table.Append ((Kind => Sname_User,
                            Prefix => Prefix,
                            Suffix => Uns32 (Suffix)));
      return Snames_Table.Last;
   end New_Sname;

   function New_Sname_Version (Prefix : Sname; Ver : Uns32) return Sname is
   begin
      Snames_Table.Append ((Kind => Sname_Version,
                            Prefix => Prefix,
                            Suffix => Ver));
      return Snames_Table.Last;
   end New_Sname_Version;

   function Is_Valid (Name : Sname) return Boolean is
   begin
      return Name > No_Sname and Name <= Snames_Table.Last;
   end Is_Valid;

   function Get_Sname_Kind (Name : Sname) return Sname_Kind is
   begin
      pragma Assert (Is_Valid (Name));
      return Snames_Table.Table (Name).Kind;
   end Get_Sname_Kind;

   function Get_Sname_Prefix (Name : Sname) return Sname is
   begin
      pragma Assert (Is_Valid (Name));
      return Snames_Table.Table (Name).Prefix;
   end Get_Sname_Prefix;

   function Get_Sname_Suffix (Name : Sname) return Name_Id
   is
      subtype Snames_Suffix is Sname_Kind range Sname_User .. Sname_Artificial;
   begin
      pragma Assert (Is_Valid (Name));
      pragma Assert (Get_Sname_Kind (Name) in Snames_Suffix);
      return Name_Id (Snames_Table.Table (Name).Suffix);
   end Get_Sname_Suffix;

   function Get_Sname_Version (Name : Sname) return Uns32 is
   begin
      pragma Assert (Is_Valid (Name));
      pragma Assert (Get_Sname_Kind (Name) = Sname_Version);
      return Snames_Table.Table (Name).Suffix;
   end Get_Sname_Version;

   function Get_Sname_Num (Name : Sname) return Uns32 is
   begin
      pragma Assert (Is_Valid (Name));
      pragma Assert (Get_Sname_Kind (Name) = Sname_Artificial);
      return Snames_Table.Table (Name).Suffix;
   end Get_Sname_Num;


   --  Modules

   package Modules_Table is new Tables
     (Table_Component_Type => Module_Record,
      Table_Index_Type => Module,
      Table_Low_Bound => No_Module,
      Table_Initial => 1024);

   function New_Design (Name : Sname) return Module
   is
      Res : Module;
      Self : Instance;
   begin
      Modules_Table.Append ((Parent => No_Module,
                             Name => Name,
                             Id => Id_Design,
                             First_Port_Desc => No_Port_Desc_Idx,
                             Nbr_Inputs => 0,
                             Nbr_Outputs => 0,
                             First_Param_Desc => No_Param_Desc_Idx,
                             Nbr_Params => 0,
                             First_Sub_Module => No_Module,
                             Last_Sub_Module => No_Module,
                             Next_Sub_Module => No_Module,
                             First_Instance => No_Instance,
                             Last_Instance => No_Instance));
      Res := Modules_Table.Last;
      Self := Create_Self_Instance (Res);
      pragma Unreferenced (Self);

      return Res;
   end New_Design;

   function Is_Valid (M : Module) return Boolean is
   begin
      return M > No_Module and then M <= Modules_Table.Last;
   end Is_Valid;

   function New_User_Module (Parent : Module;
                             Name : Sname;
                             Id : Module_Id;
                             Nbr_Inputs : Port_Nbr;
                             Nbr_Outputs : Port_Nbr;
                             Nbr_Params : Param_Nbr := 0)
                            return Module
   is
      pragma Assert (Is_Valid (Parent));
      pragma Assert (Nbr_Inputs + Nbr_Outputs > 0);
      Parent_Rec : Module_Record renames Modules_Table.Table (Parent);
      Res : Module;
   begin
      Modules_Table.Append
        ((Parent => Parent,
          Name => Name,
          Id => Id,
          First_Port_Desc => No_Port_Desc_Idx,
          Nbr_Inputs => Nbr_Inputs,
          Nbr_Outputs => Nbr_Outputs,
          First_Param_Desc => No_Param_Desc_Idx,
          Nbr_Params => Nbr_Params,
          First_Sub_Module => No_Module,
          Last_Sub_Module => No_Module,
          Next_Sub_Module => No_Module,
          First_Instance => No_Instance,
          Last_Instance => No_Instance));
      Res := Modules_Table.Last;

      --  Append
      if Parent_Rec.First_Sub_Module = No_Module then
         Parent_Rec.First_Sub_Module := Res;
      else
         Modules_Table.Table (Parent_Rec.Last_Sub_Module).Next_Sub_Module :=
           Res;
      end if;
      Parent_Rec.Last_Sub_Module := Res;

      return Res;
   end New_User_Module;

   function Get_Module_Name (M : Module) return Sname is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).Name;
   end Get_Module_Name;

   function Get_Id (M : Module) return Module_Id is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).Id;
   end Get_Id;

   function Get_Nbr_Inputs (M : Module) return Port_Nbr is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).Nbr_Inputs;
   end Get_Nbr_Inputs;

   function Get_Nbr_Outputs (M : Module) return Port_Nbr is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).Nbr_Outputs;
   end Get_Nbr_Outputs;

   function Get_Nbr_Params (M : Module) return Param_Nbr is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).Nbr_Params;
   end Get_Nbr_Params;

   function Get_First_Port_Desc (M : Module) return Port_Desc_Idx is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).First_Port_Desc;
   end Get_First_Port_Desc;

   function Get_Input_First_Desc (M : Module) return Port_Desc_Idx
   is
      pragma Assert (Is_Valid (M));
   begin
      return Modules_Table.Table (M).First_Port_Desc;
   end Get_Input_First_Desc;

   function Get_Output_First_Desc (M : Module) return Port_Desc_Idx
   is
      pragma Assert (Is_Valid (M));
   begin
      return Modules_Table.Table (M).First_Port_Desc
        + Port_Desc_Idx (Modules_Table.Table (M).Nbr_Inputs);
   end Get_Output_First_Desc;

   function Get_Self_Instance (M : Module) return Instance is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).First_Instance;
   end Get_Self_Instance;

   function Get_First_Instance (M : Module) return Instance
   is
      First : constant Instance := Get_Self_Instance (M);
   begin
      if First = No_Instance then
         --  Empty module.
         return No_Instance;
      else
         return Get_Next_Instance (First);
      end if;
   end Get_First_Instance;

   function Get_First_Sub_Module (M : Module) return Module is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).First_Sub_Module;
   end Get_First_Sub_Module;

   function Get_Next_Sub_Module (M : Module) return Module is
   begin
      pragma Assert (Is_Valid (M));
      return Modules_Table.Table (M).Next_Sub_Module;
   end Get_Next_Sub_Module;

   --  Instances

   package Instances_Table is new Tables
     (Table_Component_Type => Instance_Record,
      Table_Index_Type => Instance,
      Table_Low_Bound => No_Instance,
      Table_Initial => 1024);

   package Nets_Table is new Tables
     (Table_Component_Type => Net_Record,
      Table_Index_Type => Net,
      Table_Low_Bound => No_Net,
      Table_Initial => 1024);

   package Inputs_Table is new Tables
     (Table_Component_Type => Input_Record,
      Table_Index_Type => Input,
      Table_Low_Bound => No_Input,
      Table_Initial => 1024);

   package Params_Table is new Tables
     (Table_Component_Type => Uns32,
      Table_Index_Type => Param_Idx,
      Table_Low_Bound => No_Param_Idx,
      Table_Initial => 256);

   procedure Extract_All_Instances (M : Module; First_Instance : out Instance)
   is
      pragma Assert (Is_Valid (M));
      M_Ent : Module_Record renames Modules_Table.Table (M);
   begin
      First_Instance := M_Ent.First_Instance;

      --  Clear the instance list.
      M_Ent.First_Instance := No_Instance;
      M_Ent.Last_Instance := No_Instance;
   end Extract_All_Instances;

   procedure Append_Instance (M : Module; Inst : Instance)
   is
      M_Ent : Module_Record renames Modules_Table.Table (M);
   begin
      if M_Ent.First_Instance = No_Instance then
         M_Ent.First_Instance := Inst;
      else
         Instances_Table.Table (M_Ent.Last_Instance).Next_Instance := Inst;
      end if;
      M_Ent.Last_Instance := Inst;
   end Append_Instance;

   function New_Instance_Internal (Parent : Module;
                                   M : Module;
                                   Name : Sname;
                                   Nbr_Inputs : Port_Nbr;
                                   Nbr_Outputs : Port_Nbr;
                                   Nbr_Params : Param_Nbr)
                                  return Instance
   is
      pragma Assert (Is_Valid (Parent));
      pragma Assert (Is_Valid (M));
      Res : Instance;
      Inputs : constant Input := Inputs_Table.Allocate (Natural (Nbr_Inputs));
      Outputs : constant Net := Nets_Table.Allocate (Natural (Nbr_Outputs));
      Params : constant Param_Idx :=
        Params_Table.Allocate (Natural (Nbr_Params));
   begin
      Instances_Table.Append ((Parent => Parent,
                               Next_Instance => No_Instance,
                               Klass => M,
                               Name => Name,
                               First_Param => Params,
                               First_Input => Inputs,
                               First_Output => Outputs));
      Res := Instances_Table.Last;

      --  Setup inputs.
      if Nbr_Inputs > 0 then
         for I in 0 .. Nbr_Inputs - 1 loop
            Inputs_Table.Table (Inputs + Input (I)) :=
              (Parent => Res,
               Driver => No_Net,
               Next_Sink => No_Input);
         end loop;
      end if;

      --  Setup nets.
      if Nbr_Outputs > 0 then
         for I in 0 .. Nbr_Outputs - 1 loop
            Nets_Table.Table (Outputs + Net (I)) := (Parent => Res,
                                                     First_Sink => No_Input,
                                                     W => 0);
         end loop;
      end if;

      --  Init params (to 0).
      if Nbr_Params > 0 then
         for I in 0 .. Nbr_Params - 1 loop
            Params_Table.Table (Params + I) := 0;
         end loop;
      end if;

      return Res;
   end New_Instance_Internal;

   procedure Set_Outputs_Width_From_Desc (Inst : Instance;
                                          Nbr_Outputs : Port_Nbr;
                                          Outputs_Desc : Port_Desc_Idx) is
   begin
      if Nbr_Outputs > 0 then
         for I in 0 .. Nbr_Outputs - 1 loop
            Set_Width
              (Get_Output (Inst, I),
               Get_Port_Desc (Outputs_Desc + Port_Desc_Idx (I)).W);
         end loop;
      end if;
   end Set_Outputs_Width_From_Desc;

   function New_Instance (Parent : Module; M : Module; Name : Sname)
                         return Instance
   is
      Nbr_Inputs : constant Port_Nbr := Get_Nbr_Inputs (M);
      Nbr_Outputs : constant Port_Nbr := Get_Nbr_Outputs (M);
      Nbr_Params : constant Param_Nbr := Get_Nbr_Params (M);
      Res : Instance;
   begin
      Res := New_Instance_Internal
        (Parent, M, Name, Nbr_Inputs, Nbr_Outputs, Nbr_Params);
      Set_Outputs_Width_From_Desc
        (Res, Nbr_Outputs, Get_Output_First_Desc (M));

      --  Link instance
      Append_Instance (Parent, Res);

      return Res;
   end New_Instance;

   function New_Var_Instance (Parent : Module;
                              M : Module;
                              Name : Sname;
                              Nbr_Inputs : Port_Nbr;
                              Nbr_Outputs : Port_Nbr;
                              Nbr_Params : Param_Nbr)
                             return Instance
   is
      Res : Instance;
   begin
      Res := New_Instance_Internal
        (Parent, M, Name, Nbr_Inputs, Nbr_Outputs, Nbr_Params);

      --  Link instance
      Append_Instance (Parent, Res);

      return Res;
   end New_Var_Instance;

   function Create_Self_Instance (M : Module) return Instance
   is
      --  Can be done only once.
      pragma Assert (Get_Self_Instance (M) = No_Instance);
      Nbr_Inputs : constant Port_Nbr := Get_Nbr_Inputs (M);
      Nbr_Outputs : constant Port_Nbr := Get_Nbr_Outputs (M);
      Res : Instance;
   begin
      --  Swap inputs and outputs; no parameters.
      Res := New_Instance_Internal
        (M, M, Get_Name (M), Nbr_Outputs, Nbr_Inputs, 0);
      Set_Outputs_Width_From_Desc
        (Res, Nbr_Inputs, Get_Input_First_Desc (M));

      Append_Instance (M, Res);

      return Res;
   end Create_Self_Instance;

   function Is_Valid (I : Instance) return Boolean is
   begin
      return I > No_Instance and then I <= Instances_Table.Last;
   end Is_Valid;

   function Is_Self_Instance (I : Instance) return Boolean is
      Irec : Instance_Record renames Instances_Table.Table (I);
   begin
      return Irec.Parent = Irec.Klass;
   end Is_Self_Instance;

   procedure Free_Instance (Inst : Instance)
   is
      pragma Assert (Is_Valid (Inst));
   begin
      Instances_Table.Table (Inst).Klass := Free_Module;
   end Free_Instance;

   function Get_Module (Inst : Instance) return Module is
   begin
      pragma Assert (Is_Valid (Inst));
      return Instances_Table.Table (Inst).Klass;
   end Get_Module;

   function Get_Instance_Name (Inst : Instance) return Sname is
   begin
      pragma Assert (Is_Valid (Inst));
      return Instances_Table.Table (Inst).Name;
   end Get_Instance_Name;

   function Get_Instance_Parent (Inst : Instance) return Module is
   begin
      pragma Assert (Is_Valid (Inst));
      return Instances_Table.Table (Inst).Parent;
   end Get_Instance_Parent;

   function Get_Next_Instance (Inst : Instance) return Instance is
   begin
      pragma Assert (Is_Valid (Inst));
      return Instances_Table.Table (Inst).Next_Instance;
   end Get_Next_Instance;

   function Get_First_Output (Inst : Instance) return Net is
   begin
      pragma Assert (Is_Valid (Inst));
      return Instances_Table.Table (Inst).First_Output;
   end Get_First_Output;

   function Get_Output (Inst : Instance; Idx : Port_Idx) return Net is
   begin
      pragma Assert (Is_Valid (Inst));
      pragma Assert (Idx < Get_Nbr_Outputs (Inst));
      return Instances_Table.Table (Inst).First_Output + Net (Idx);
   end Get_Output;

   function Get_Input (Inst : Instance; Idx : Port_Idx) return Input is
   begin
      pragma Assert (Is_Valid (Inst));
      pragma Assert (Idx < Get_Nbr_Inputs (Inst));
      return Instances_Table.Table (Inst).First_Input + Input (Idx);
   end Get_Input;

   --  Nets

   function Is_Valid (N : Net) return Boolean is
   begin
      return N > No_Net and then N <= Nets_Table.Last;
   end Is_Valid;

   function Get_Net_Parent (O : Net) return Instance is
   begin
      pragma Assert (Is_Valid (O));
      return Nets_Table.Table (O).Parent;
   end Get_Net_Parent;

   function Get_Port_Idx (O : Net) return Port_Idx
   is
      pragma Assert (Is_Valid (O));
      Parent : constant Instance := Get_Parent (O);
   begin
      return Port_Idx (O - Instances_Table.Table (Parent).First_Output);
   end Get_Port_Idx;

   function Get_First_Sink (O : Net) return Input is
   begin
      pragma Assert (Is_Valid (O));
      return Nets_Table.Table (O).First_Sink;
   end Get_First_Sink;

   function Get_Width (N : Net) return Width
   is
      pragma Assert (Is_Valid (N));
   begin
      return Nets_Table.Table (N).W;
   end Get_Width;

   procedure Set_Width (N : Net; W : Width)
   is
      pragma Assert (Is_Valid (N));
   begin
      if Nets_Table.Table (N).W /= No_Width then
         raise Internal_Error;
      end if;
      Nets_Table.Table (N).W := W;
   end Set_Width;


   --  Inputs

   function Is_Valid (N : Input) return Boolean is
   begin
      return N > No_Input and then N <= Inputs_Table.Last;
   end Is_Valid;

   function Get_Input_Parent (I : Input) return Instance is
   begin
      pragma Assert (Is_Valid (I));
      return Inputs_Table.Table (I).Parent;
   end Get_Input_Parent;

   function Get_Port_Idx (I : Input) return Port_Idx
   is
      pragma Assert (Is_Valid (I));
      Parent : constant Instance := Get_Parent (I);
   begin
      return Port_Idx (I - Instances_Table.Table (Parent).First_Input);
   end Get_Port_Idx;

   function Get_Driver (I : Input) return Net is
   begin
      pragma Assert (Is_Valid (I));
      return Inputs_Table.Table (I).Driver;
   end Get_Driver;

   function Get_Next_Sink (I : Input) return Input is
   begin
      pragma Assert (Is_Valid (I));
      return Inputs_Table.Table (I).Next_Sink;
   end Get_Next_Sink;


   --  Port_Desc

   package Port_Desc_Table is new Tables
     (Table_Component_Type => Port_Desc,
      Table_Index_Type => Port_Desc_Idx,
      Table_Low_Bound => No_Port_Desc_Idx,
      Table_Initial => 1024);

   function Get_Port_Desc (Idx : Port_Desc_Idx) return Port_Desc is
   begin
      return Port_Desc_Table.Table (Idx);
   end Get_Port_Desc;

   function Get_Input_Desc (M : Module; I : Port_Idx) return Port_Desc
   is
      F : constant Port_Desc_Idx := Get_Input_First_Desc (M);
      pragma Assert (I < Get_Nbr_Inputs (M));
   begin
      return Port_Desc_Table.Table (F + Port_Desc_Idx (I));
   end Get_Input_Desc;

   function Get_Output_Desc (M : Module; O : Port_Idx) return Port_Desc
   is
      F : constant Port_Desc_Idx := Get_Output_First_Desc (M);
      pragma Assert (O < Get_Nbr_Outputs (M));
   begin
      return Port_Desc_Table.Table (F + Port_Desc_Idx (O));
   end Get_Output_Desc;

   procedure Set_Port_Desc (M : Module;
                            Input_Descs : Port_Desc_Array;
                            Output_Descs : Port_Desc_Array)
   is
      pragma Assert (Is_Valid (M));
      pragma Assert (Input_Descs'Length = Get_Nbr_Inputs (M));
      pragma Assert (Output_Descs'Length = Get_Nbr_Outputs (M));
   begin
      pragma Assert
        (Modules_Table.Table (M).First_Port_Desc = No_Port_Desc_Idx);

      Modules_Table.Table (M).First_Port_Desc := Port_Desc_Table.Last + 1;

      for I of Input_Descs loop
         pragma Assert (I.Dir = Port_In);
         Port_Desc_Table.Append (I);
      end loop;

      for O of Output_Descs loop
         pragma Assert (O.Dir in Port_Outs);
         Port_Desc_Table.Append (O);
      end loop;
   end Set_Port_Desc;

   --  Param_Desc

   package Param_Desc_Table is new Tables
     (Table_Component_Type => Param_Desc,
      Table_Index_Type => Param_Desc_Idx,
      Table_Low_Bound => No_Param_Desc_Idx,
      Table_Initial => 256);

   procedure Set_Param_Desc (M : Module;
                             Params : Param_Desc_Array)
   is
      pragma Assert (Is_Valid (M));
      pragma Assert (Params'Length = Get_Nbr_Params (M));
   begin
      pragma Assert
        (Modules_Table.Table (M).First_Param_Desc = No_Param_Desc_Idx);

      Modules_Table.Table (M).First_Param_Desc := Param_Desc_Table.Last + 1;

      for P of Params loop
         Param_Desc_Table.Append (P);
      end loop;
   end Set_Param_Desc;

   function Get_Param_Desc (M : Module; Param : Param_Idx) return Param_Desc
   is
      pragma Assert (Is_Valid (M));
      pragma Assert (Param < Get_Nbr_Params (M));
   begin
      return Param_Desc_Table.Table
        (Modules_Table.Table (M).First_Param_Desc + Param_Desc_Idx (Param));
   end Get_Param_Desc;

   function Get_Param_Idx (Inst : Instance; Param : Param_Idx) return Param_Idx
   is
      pragma Assert (Is_Valid (Inst));
      pragma Assert (Param < Get_Nbr_Params (Inst));
   begin
      return Instances_Table.Table (Inst).First_Param + Param;
   end Get_Param_Idx;

   function Get_Param_Uns32 (Inst : Instance; Param : Param_Idx) return Uns32
   is
      pragma Assert (Is_Valid (Inst));
      M : constant Module := Get_Module (Inst);
      pragma Assert (Param < Get_Nbr_Params (M));
      pragma Assert (Get_Param_Desc (M, Param).Typ = Param_Uns32);
   begin
      return Params_Table.Table (Get_Param_Idx (Inst, Param));
   end Get_Param_Uns32;

   procedure Set_Param_Uns32 (Inst : Instance; Param : Param_Idx; Val : Uns32)
   is
      pragma Assert (Is_Valid (Inst));
      M : constant Module := Get_Module (Inst);
      pragma Assert (Param < Get_Nbr_Params (M));
      pragma Assert (Get_Param_Desc (M, Param).Typ = Param_Uns32);
   begin
      Params_Table.Table (Get_Param_Idx (Inst, Param)) := Val;
   end Set_Param_Uns32;

   procedure Connect (I : Input; O : Net)
   is
      pragma Assert (Is_Valid (I));
      pragma Assert (Is_Valid (O));
      --  Check Width compatibility
      --  pragma assert (get_width (i) = get_width (o));
      pragma Assert (Get_Driver (I) = No_Net);
      I_Ent : Input_Record renames Inputs_Table.Table (I);
      O_Ent : Net_Record renames Nets_Table.Table (O);
   begin
      I_Ent.Driver := O;
      I_Ent.Next_Sink := O_Ent.First_Sink;
      O_Ent.First_Sink := I;
   end Connect;

   procedure Disconnect (I : Input)
   is
      pragma Assert (Is_Valid (I));
      Drv : constant Net := Get_Driver (I);
      pragma Assert (Drv /= No_Net);
      Next_Sink : constant Input := Get_Next_Sink (I);
      I_Ent : Input_Record renames Inputs_Table.Table (I);
      D_Ent : Net_Record renames Nets_Table.Table (Drv);
      S, N_S : Input;
   begin
      I_Ent.Next_Sink := No_Input;
      I_Ent.Driver := No_Net;

      if D_Ent.First_Sink = I then
         --  Was the first sink.
         D_Ent.First_Sink := Next_Sink;
      else
         --  Walk
         S := D_Ent.First_Sink;
         loop
            pragma Assert (Is_Valid (S));
            N_S := Get_Next_Sink (S);
            if N_S = I then
               Inputs_Table.Table (S).Next_Sink := Next_Sink;
               exit;
            else
               S := N_S;
            end if;
         end loop;
      end if;
   end Disconnect;

   procedure Redirect_Inputs (Old : Net; N : Net)
   is
      First_I, I : Input;
      Prev_I : Input;
   begin
      First_I := Get_First_Sink (Old);
      if First_I = No_Input then
         return;
      end if;

      I := First_I;
      Prev_I := No_Input;
      while I /= No_Input loop
         declare
            I_Rec : Input_Record renames Inputs_Table.Table (I);
         begin
            pragma Assert (I_Rec.Driver = Old);
            I_Rec.Driver := N;

            if Prev_I /= No_Input then
               Inputs_Table.Table (Prev_I).Next_Sink := I;
            end if;
            Prev_I := I;

            I := I_Rec.Next_Sink;
         end;
      end loop;
      if Prev_I /= No_Input then
         Inputs_Table.Table (Prev_I).Next_Sink := Get_First_Sink (N);
         Nets_Table.Table (N).First_Sink := First_I;
      end if;
   end Redirect_Inputs;

begin
   --  Initialize snames_table: create the first entry for No_Sname.
   Snames_Table.Append ((Kind => Sname_Artificial,
                         Prefix => No_Sname,
                         Suffix => 0));
   pragma Assert (Snames_Table.Last = No_Sname);

   Modules_Table.Append ((Parent => No_Module,
                          Name => No_Sname,
                          Id => Id_None,
                          First_Port_Desc => No_Port_Desc_Idx,
                          Nbr_Inputs => 0,
                          Nbr_Outputs => 0,
                          First_Param_Desc => No_Param_Desc_Idx,
                          Nbr_Params => 0,
                          First_Sub_Module => No_Module,
                          Last_Sub_Module => No_Module,
                          Next_Sub_Module => No_Module,
                          First_Instance => No_Instance,
                          Last_Instance => No_Instance));
   pragma Assert (Modules_Table.Last = No_Module);

   Modules_Table.Append ((Parent => No_Module,
                          Name => No_Sname,
                          Id => Id_Free,
                          First_Port_Desc => No_Port_Desc_Idx,
                          Nbr_Inputs => 0,
                          Nbr_Outputs => 0,
                          First_Param_Desc => No_Param_Desc_Idx,
                          Nbr_Params => 0,
                          First_Sub_Module => No_Module,
                          Last_Sub_Module => No_Module,
                          Next_Sub_Module => No_Module,
                          First_Instance => No_Instance,
                          Last_Instance => No_Instance));
   pragma Assert (Modules_Table.Last = Free_Module);

   Instances_Table.Append ((Parent => No_Module,
                            Next_Instance => No_Instance,
                            Klass => No_Module,
                            Name => No_Sname,
                            First_Param => No_Param_Idx,
                            First_Input => No_Input,
                            First_Output => No_Net));
   pragma Assert (Instances_Table.Last = No_Instance);

   Nets_Table.Append ((Parent => No_Instance,
                       First_Sink => No_Input,
                       W => 0));
   pragma Assert (Nets_Table.Last = No_Net);

   Inputs_Table.Append ((Parent => No_Instance,
                         Driver => No_Net,
                         Next_Sink => No_Input));
   pragma Assert (Inputs_Table.Last = No_Input);

   Port_Desc_Table.Append ((Name => No_Sname,
                            W => 0,
                            Dir => Port_In,
                            Left => 0,
                            Right => 0));
   pragma Assert (Port_Desc_Table.Last = No_Port_Desc_Idx);

   Param_Desc_Table.Append ((Name => No_Sname,
                             Typ => Param_Uns32));
   pragma Assert (Param_Desc_Table.Last = No_Param_Desc_Idx);

   Params_Table.Append (0);
   pragma Assert (Params_Table.Last = No_Param_Idx);
end Netlists;