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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Unconstrained base subtype questions
Date: Thu, 31 Mar 2011 22:18:29 +0100
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Date: 2011-03-31T22:18:29+01:00
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"Alex Mentis" <foo@invalid.invalid> writes:

> The following does not cause a constraint error in my version of GNAT
> on my system:
>
> ...
>
> Integer_Result := (Integer'Last + Integer'Last) / 2;
>
> ...
>
>
> If I understand correctly, this is because the Integer operators are
> defined for operands of type Integer'Base, which is an unconstrained
> subtype and allows the operands to be stored in extended-length
> registers so that intermediate values in calculations do not overflow.
>
> My questions are:
>
> 1) Do I understand correctly what's going on?
>
> 2) Does the language make any guarantees about preventing spurious
> overflow, or am I just getting lucky with my compiler/architecture? If
> guarantees are made by the language, what are they?

That's a compile-time calculation, and any Ada compiler should work it
out using infinite-precision arithmetic.

   with Ada.Text_IO; use Ada.Text_IO;
   procedure Very_Large is
      Integer_Result : Integer;
   begin
      Integer_Result := 10**128 / 10**127;
      Put_Line (Integer'Image (Integer_Result));
   end Very_Large;

   $ gnatmake very_large.adb
   gcc -c very_large.adb
   gnatbind -x very_large.ali
   gnatlink very_large.ali
   $ ./very_large
    10

As against

   with Ada.Text_IO; use Ada.Text_IO;
   procedure Very_Large is
      Integer_Result : Integer;
   begin
      Integer_Result := Integer'Last;
      Integer_Result := Integer_Result + Integer'Last;
      Integer_Result := Integer_Result / 2;
      Put_Line (Integer'Image (Integer_Result));
   end Very_Large;

   $ gnatmake very_large.adb
   gcc -c very_large.adb
   very_large.adb:6:37: warning: value not in range of type "Standard.Integer"
   very_large.adb:6:37: warning: "Constraint_Error" will be raised at run time
   gnatbind -x very_large.ali
   gnatlink very_large.ali
   $ ./very_large
   raised CONSTRAINT_ERROR : very_large.adb:6 overflow check failed

Note that the compiler knew that was going to happen. If the overflow
wasn't visible at compile time, you'd have to tell GNAT to perform
run-time integer overflow checks using -gnato. Other compiler writers
may have different views about whether run-time integer overflow checks
should be off by default :-)