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diff --git a/src/synth/synth-ieee-numeric_std.adb b/src/synth/synth-ieee-numeric_std.adb
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+++ b/src/synth/synth-ieee-numeric_std.adb
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+--  numeric_std
+--  Copyright (C) 2019 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.
+
+package body Synth.Ieee.Numeric_Std is
+   Null_Vec : constant Std_Logic_Vector (1 .. 0) := (others => '0');
+
+   subtype Sl_01 is Std_Ulogic range '0' .. '1';
+   subtype Sl_X01 is Std_Ulogic range 'X' .. '1';
+
+   type Carry_Array is array (Sl_01, Sl_01, Sl_01) of Sl_01;
+   Compute_Carry : constant Carry_Array :=
+     ('0' => ('0' => ('0' => '0', '1' => '0'),
+              '1' => ('0' => '0', '1' => '1')),
+      '1' => ('0' => ('0' => '0', '1' => '1'),
+              '1' => ('0' => '1', '1' => '1')));
+   Compute_Sum : constant Carry_Array :=
+     ('0' => ('0' => ('0' => '0', '1' => '1'),
+              '1' => ('0' => '1', '1' => '0')),
+      '1' => ('0' => ('0' => '1', '1' => '0'),
+              '1' => ('0' => '0', '1' => '1')));
+
+   type Sl_To_X01_Array is array (Std_Ulogic) of Sl_X01;
+   Sl_To_X01 : constant Sl_To_X01_Array :=
+     ('0' | 'L' => '0', '1' | 'H' => '1', others => 'X');
+
+   function Add_Uns_Uns (L, R : Std_Logic_Vector) return Std_Logic_Vector
+   is
+      pragma Assert (L'First = 1);
+      pragma Assert (R'First = 1);
+      Len : constant Integer := Integer'Max (L'Last, R'Last);
+      subtype Res_Type is Std_Logic_Vector (1 .. Len);
+      Res : Res_Type;
+      Lb, Rb, Carry : Sl_X01;
+   begin
+      if L'Last < 1 or R'Last < 1 then
+         return Null_Vec;
+      end if;
+      Carry := '0';
+      for I in 1 .. Len loop
+         if I > L'Last then
+            Lb := '0';
+         else
+            Lb := Sl_To_X01 (L (I));
+         end if;
+         if I > R'Last then
+            Rb := '0';
+         else
+            Rb := Sl_To_X01 (R (I));
+         end if;
+         if Lb = 'X' or Rb = 'X' then
+            --assert NO_WARNING
+            --  report "NUMERIC_STD.""+"": non logical value detected"
+            --  severity warning;
+            Res := (others => 'X');
+            exit;
+         end if;
+         Res (I) := Compute_Sum (Carry, Rb, Lb);
+         Carry := Compute_Carry (Carry, Rb, Lb);
+      end loop;
+      return Res;
+   end Add_Uns_Uns;
+end Synth.Ieee.Numeric_Std;