diff options
Diffstat (limited to 'gcc/ada/s-expmod.adb')
| -rw-r--r-- | gcc/ada/s-expmod.adb | 91 |
1 files changed, 91 insertions, 0 deletions
diff --git a/gcc/ada/s-expmod.adb b/gcc/ada/s-expmod.adb new file mode 100644 index 00000000000..a87002bcbf1 --- /dev/null +++ b/gcc/ada/s-expmod.adb @@ -0,0 +1,91 @@ +------------------------------------------------------------------------------ +-- -- +-- GNAT RUNTIME COMPONENTS -- +-- -- +-- S Y S T E M . E X P _ M O D -- +-- -- +-- B o d y -- +-- -- +-- $Revision: 1.7 $ -- +-- -- +-- Copyright (C) 1992,1993,1994,1995 Free Software Foundation, Inc. -- +-- -- +-- GNAT is free software; you can redistribute it and/or modify it under -- +-- terms of the GNU General Public License as published by the Free Soft- -- +-- ware Foundation; either version 2, or (at your option) any later ver- -- +-- sion. GNAT is distributed in the hope that it will be useful, but WITH- -- +-- OUT 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 distributed with GNAT; see file COPYING. If not, write -- +-- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, -- +-- MA 02111-1307, USA. -- +-- -- +-- As a special exception, if other files instantiate generics from this -- +-- unit, or you link this unit with other files to produce an executable, -- +-- this unit does not by itself cause the resulting executable to be -- +-- covered by the GNU General Public License. This exception does not -- +-- however invalidate any other reasons why the executable file might be -- +-- covered by the GNU Public License. -- +-- -- +-- GNAT was originally developed by the GNAT team at New York University. -- +-- It is now maintained by Ada Core Technologies Inc (http://www.gnat.com). -- +-- -- +------------------------------------------------------------------------------ + +package body System.Exp_Mod is + + ----------------- + -- Exp_Modular -- + ----------------- + + function Exp_Modular + (Left : Integer; + Modulus : Integer; + Right : Natural) + return Integer + is + Result : Integer := 1; + Factor : Integer := Left; + Exp : Natural := Right; + + function Mult (X, Y : Integer) return Integer; + pragma Inline (Mult); + -- Modular multiplication. Note that we can't take advantage of the + -- compiler's circuit, because the modulus is not known statically. + + function Mult (X, Y : Integer) return Integer is + begin + return Integer + (Long_Long_Integer (X) * Long_Long_Integer (Y) + mod Long_Long_Integer (Modulus)); + end Mult; + + -- Start of processing for Exp_Modular + + begin + -- We use the standard logarithmic approach, Exp gets shifted right + -- testing successive low order bits and Factor is the value of the + -- base raised to the next power of 2. + + -- Note: it is not worth special casing the cases of base values -1,0,+1 + -- since the expander does this when the base is a literal, and other + -- cases will be extremely rare. + + if Exp /= 0 then + loop + if Exp rem 2 /= 0 then + Result := Mult (Result, Factor); + end if; + + Exp := Exp / 2; + exit when Exp = 0; + Factor := Mult (Factor, Factor); + end loop; + end if; + + return Result; + + end Exp_Modular; + +end System.Exp_Mod; |