| ------------------------------------------------------------------------------ |
| -- -- |
| -- GNAT RUN-TIME COMPONENTS -- |
| -- -- |
| -- S Y S T E M . E X P O N N -- |
| -- -- |
| -- B o d y -- |
| -- -- |
| -- Copyright (C) 1992-2025, 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 3, 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. -- |
| -- -- |
| -- As a special exception under Section 7 of GPL version 3, you are granted -- |
| -- additional permissions described in the GCC Runtime Library Exception, -- |
| -- version 3.1, as published by the Free Software Foundation. -- |
| -- -- |
| -- You should have received a copy of the GNU General Public License and -- |
| -- a copy of the GCC Runtime Library Exception along with this program; -- |
| -- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see -- |
| -- <http://www.gnu.org/licenses/>. -- |
| -- -- |
| -- GNAT was originally developed by the GNAT team at New York University. -- |
| -- Extensive contributions were provided by Ada Core Technologies Inc. -- |
| -- -- |
| ------------------------------------------------------------------------------ |
| |
| package body System.Exponn |
| with SPARK_Mode |
| is |
| ----------- |
| -- Expon -- |
| ----------- |
| |
| function Expon (Left : Int; Right : Natural) return Int is |
| |
| -- Note that negative exponents get a constraint error because the |
| -- subtype of the Right argument (the exponent) is Natural. |
| |
| Result : Int := 1; |
| Factor : Int := Left; |
| Exp : Natural := Right; |
| |
| 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: for compilation only, it is not worth special casing base |
| -- values -1, 0, +1 since the expander does this when the base is a |
| -- literal, and other cases will be extremely rare. But for proof, |
| -- special casing zero in both positions makes ghost code and lemmas |
| -- simpler, so we do it. |
| |
| if Right = 0 then |
| Result := 1; |
| elsif Left = 0 then |
| Result := 0; |
| else |
| loop |
| if Exp rem 2 /= 0 then |
| declare |
| pragma Suppress (Overflow_Check); |
| begin |
| Result := Result * Factor; |
| end; |
| end if; |
| |
| Exp := Exp / 2; |
| exit when Exp = 0; |
| |
| declare |
| pragma Suppress (Overflow_Check); |
| begin |
| Factor := Factor * Factor; |
| end; |
| end loop; |
| end if; |
| |
| return Result; |
| end Expon; |
| |
| end System.Exponn; |
