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[/] [openrisc/] [trunk/] [gnu-src/] [gcc-4.5.1/] [gcc/] [ada/] [s-expmod.adb] - Blame information for rev 410

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1 281 jeremybenn
------------------------------------------------------------------------------
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--                                                                          --
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--                         GNAT RUN-TIME COMPONENTS                         --
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--                                                                          --
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--                       S Y S T E M . E X P _ M O D                        --
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--                                                                          --
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--                                 B o d y                                  --
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--                                                                          --
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--          Copyright (C) 1992-2009 Free Software Foundation, Inc.          --
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--                                                                          --
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-- GNAT is free software;  you can  redistribute it  and/or modify it under --
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-- terms of the  GNU General Public License as published  by the Free Soft- --
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-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
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-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
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-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
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-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
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--                                                                          --
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-- As a special exception under Section 7 of GPL version 3, you are granted --
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-- additional permissions described in the GCC Runtime Library Exception,   --
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-- version 3.1, as published by the Free Software Foundation.               --
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--                                                                          --
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-- You should have received a copy of the GNU General Public License and    --
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-- a copy of the GCC Runtime Library Exception along with this program;     --
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-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
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-- <http://www.gnu.org/licenses/>.                                          --
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--                                                                          --
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-- GNAT was originally developed  by the GNAT team at  New York University. --
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-- Extensive contributions were provided by Ada Core Technologies Inc.      --
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--                                                                          --
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------------------------------------------------------------------------------
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package body System.Exp_Mod is
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   -----------------
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   -- Exp_Modular --
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   -----------------
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   function Exp_Modular
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     (Left    : Integer;
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      Modulus : Integer;
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      Right   : Natural)
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      return    Integer
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   is
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      Result : Integer := 1;
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      Factor : Integer := Left;
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      Exp    : Natural := Right;
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      function Mult (X, Y : Integer) return Integer;
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      pragma Inline (Mult);
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      --  Modular multiplication. Note that we can't take advantage of the
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      --  compiler's circuit, because the modulus is not known statically.
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      function Mult (X, Y : Integer) return Integer is
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      begin
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         return Integer
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           (Long_Long_Integer (X) * Long_Long_Integer (Y)
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             mod Long_Long_Integer (Modulus));
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      end Mult;
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   --  Start of processing for Exp_Modular
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   begin
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      --  We use the standard logarithmic approach, Exp gets shifted right
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      --  testing successive low order bits and Factor is the value of the
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      --  base raised to the next power of 2.
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      --  Note: it is not worth special casing the cases of base values -1,0,+1
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      --  since the expander does this when the base is a literal, and other
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      --  cases will be extremely rare.
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      if Exp /= 0 then
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         loop
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            if Exp rem 2 /= 0 then
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               Result := Mult (Result, Factor);
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            end if;
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            Exp := Exp / 2;
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            exit when Exp = 0;
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            Factor := Mult (Factor, Factor);
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         end loop;
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      end if;
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      return Result;
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   end Exp_Modular;
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end System.Exp_Mod;

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