gcc/ada/s-exngen.adb

   1 ------------------------------------------------------------------------------
   2 --                                                                          --
   3 --                         GNAT RUNTIME COMPONENTS                          --
   4 --                                                                          --
   5 --                       S Y S T E M . E X N _ G E N                        --
   6 --                                                                          --
   7 --                                 B o d y                                  --
   8 --                                                                          --
   9 --                                                                          --
  10 --          Copyright (C) 1992-2001, Free Software Foundation, Inc.         --
  11 --                                                                          --
  12 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
  13 -- terms of the  GNU General Public License as published  by the Free Soft- --
  14 -- ware  Foundation;  either version 2,  or (at your option) any later ver- --
  15 -- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
  16 -- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
  17 -- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License --
  18 -- for  more details.  You should have  received  a copy of the GNU General --
  19 -- Public License  distributed with GNAT;  see file COPYING.  If not, write --
  20 -- to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston, --
  21 -- MA 02111-1307, USA.                                                      --
  22 --                                                                          --
  23 -- As a special exception,  if other files  instantiate  generics from this --
  24 -- unit, or you link  this unit with other files  to produce an executable, --
  25 -- this  unit  does not  by itself cause  the resulting  executable  to  be --
  26 -- covered  by the  GNU  General  Public  License.  This exception does not --
  27 -- however invalidate  any other reasons why  the executable file  might be --
  28 -- covered by the  GNU Public License.                                      --
  29 --                                                                          --
  30 -- GNAT was originally developed  by the GNAT team at  New York University. --
  31 -- Extensive contributions were provided by Ada Core Technologies Inc.      --
  32 --                                                                          --
  33 ------------------------------------------------------------------------------
  34
  35 package body System.Exn_Gen is
  36
  37    --------------------
  38    -- Exn_Float_Type --
  39    --------------------
  40
  41    function Exn_Float_Type
  42      (Left  : Type_Of_Base;
  43       Right : Integer)
  44       return  Type_Of_Base
  45    is
  46       pragma Suppress (Division_Check);
  47       pragma Suppress (Overflow_Check);
  48       pragma Suppress (Range_Check);
  49
  50       Result : Type_Of_Base := 1.0;
  51       Factor : Type_Of_Base := Left;
  52       Exp    : Integer := Right;
  53
  54    begin
  55       --  We use the standard logarithmic approach, Exp gets shifted right
  56       --  testing successive low order bits and Factor is the value of the
  57       --  base raised to the next power of 2. For positive exponents we
  58       --  multiply the result by this factor, for negative exponents, we
  59       --  Division by this factor.
  60
  61       if Exp >= 0 then
  62          loop
  63             if Exp rem 2 /= 0 then
  64                Result := Result * Factor;
  65             end if;
  66
  67             Exp := Exp / 2;
  68             exit when Exp = 0;
  69             Factor := Factor * Factor;
  70          end loop;
  71
  72          return Result;
  73
  74       --  Negative exponent. For a zero base, we should arguably return an
  75       --  infinity of the right sign, but it is not clear that there is
  76       --  proper authorization to do so, so for now raise Constraint_Error???
  77
  78       elsif Factor = 0.0 then
  79          raise Constraint_Error;
  80
  81       --  Here we have a non-zero base and a negative exponent
  82
  83       else
  84          --  For the negative exponent case, a constraint error during this
  85          --  calculation happens if Factor gets too large, and the proper
  86          --  response is to return 0.0, since what we essentially have is
  87          --  1.0 / infinity, and the closest model number will be zero.
  88
  89          begin
  90             loop
  91                if Exp rem 2 /= 0 then
  92                   Result := Result * Factor;
  93                end if;
  94
  95                Exp := Exp / 2;
  96                exit when Exp = 0;
  97                Factor := Factor * Factor;
  98             end loop;
  99
 100             return 1.0 / Result;
 101
 102          exception
 103
 104             when Constraint_Error =>
 105                return 0.0;
 106          end;
 107       end if;
 108    end Exn_Float_Type;
 109
 110    ----------------------
 111    -- Exn_Integer_Type --
 112    ----------------------
 113
 114    --  Note that negative exponents get a constraint error because the
 115    --  subtype of the Right argument (the exponent) is Natural.
 116
 117    function Exn_Integer_Type
 118      (Left  : Type_Of_Base;
 119       Right : Natural)
 120       return  Type_Of_Base
 121    is
 122       pragma Suppress (Division_Check);
 123       pragma Suppress (Overflow_Check);
 124
 125       Result : Type_Of_Base := 1;
 126       Factor : Type_Of_Base := Left;
 127       Exp    : Natural := Right;
 128
 129    begin
 130       --  We use the standard logarithmic approach, Exp gets shifted right
 131       --  testing successive low order bits and Factor is the value of the
 132       --  base raised to the next power of 2.
 133
 134       --  Note: it is not worth special casing the cases of base values -1,0,+1
 135       --  since the expander does this when the base is a literal, and other
 136       --  cases will be extremely rare.
 137
 138       if Exp /= 0 then
 139          loop
 140             if Exp rem 2 /= 0 then
 141                Result := Result * Factor;
 142             end if;
 143
 144             Exp := Exp / 2;
 145             exit when Exp = 0;
 146             Factor := Factor * Factor;
 147          end loop;
 148       end if;
 149
 150       return Result;
 151    end Exn_Integer_Type;
 152
 153 end System.Exn_Gen;