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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/24542] potential unwanted truncation of operation overflow
 should be warned on assignment to wider variable
Date: Fri, 31 Mar 2023 09:18:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D24542

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-03-31
             Status|RESOLVED                    |NEW
         Resolution|WONTFIX                     |---
            Summary|potential integer overflow  |potential unwanted
                   |should be warned on         |truncation of operation
                   |assignment to wider         |overflow should be warned
                   |variable                    |on assignment to wider
                   |                            |variable
     Ever confirmed|0                           |1

--- Comment #13 from Richard Biener <rguenth at gcc dot gnu.org> ---
Let me re-open this.  I agree that it sounds useful to have a diagnostic th=
at
would catch these cases but I also think it might have many false positives.
But that's similar to diagnosing if (a || b && c).

That said, the burden is on whoever is going to prototype patch with
extensive enough test coverage.

The question is whether to diagnose

 int x1, x2;
 long y1;
 y1 =3D x1 * x2;

since when x1 * x2 overflows that even invokes undefined behavior (so it's
even worse than the unsigned case).

The description is misleading, there's no "overflow on assignment" but the
operation itself might overflow and the truncated value is then widened
on assignment.  The assignment is a mere hint that a wider result might
have been intended (and a good enough hint IMHO).=