Why is overflow silently allowed in Ada?

Question

Silent failure for such trivial code. How can they claim high integrity?

with ada.text_io;
use  ada.text_io;
procedure overflow is
    procedure p (i: positive) is
        x: integer := integer'last;
    begin
        x := (x+i)/2;
        put_line (integer'image(x) & " should be positive");
    end;
begin
    p(10);
end;

Not even a compiler warning like "cannot ensure the lack of overflow" or smth.

This seems at least as error-prone as some of the C++ pitfalls the Ada folks like to point out. Correct me if I am wrong. I would like to have a better language than C++, but I keep facing language complexity that is supposed to protect from all pitfalls, and still.

Answer 1

Not all Ada compilers will behave like this. You must be using a version of the GNAT GPL compiler prior to 2015, or FSF GCC prior to 5; at that point, GNAT didn’t check for integer overflow by default - you had to compile with -gnato.

This was stated to be in the pursuit of efficiency, but many users thought that the gain in efficiency wasn’t worth the cost of explaining this behaviour to new (and, sometimes, not-so-new) users. And, of course, using compiler-provided types such as Integer isn’t best practice.

Nowadays, your program compiles OK without -gnato (admittedly without warnings, even with -gnatwa) but runs with

$ ./overflow 

raised CONSTRAINT_ERROR : overflow.adb:7 overflow check failed

SPARK Ada would detect this problem:

$ /opt/gnat-gpl-2016/bin/gnatprove -P overflow.gpr
Phase 1 of 2: generation of Global contracts ...
Phase 2 of 2: flow analysis and proof ...
overflow.adb:4:14: warning: subprogram "p" has no effect
overflow.adb:7:15: medium: overflow check might fail, in call inlined at overflow.adb:11 (e.g. when x = 2147483647)
overflow.adb:8:07: warning: no Global contract available for "Put_Line"
overflow.adb:8:07: warning: assuming "Put_Line" has no effect on global items
Summary logged in /Users/simon/tmp/so/gnatprove/gnatprove.out

Answer 2

If a compiler behaves like this, it isn't an Ada compiler.

My guess is that you use GCC (GNAT), which for some odd reason requires that you pass it a very specific combination of command line arguments, to behave like an Ada compiler:

• -fstack-check -- Stack overflow
• -gnato -- Arithmetic overflow
• -gnat2012 -- Use Ada 2012 (instead of older versions of the language)

And if you want contracts to behave sensibly:

• -gnata -- Contracts and assertions