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From: "muecker at gwdg dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104800] New: reodering of potentially
 trapping operations and volatile stores
Date: Sat, 05 Mar 2022 18:46:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104800

            Bug ID: 104800
           Summary: reodering of potentially trapping operations and
                    volatile stores
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: muecker at gwdg dot de
  Target Milestone: ---

In the following example, a potentially trapping operation is moved before =
the
store to the volatile variable.  This can change observable behavior.  Beca=
use
division by zero is UB this is a correct optimization in C++ where UB is
allowed to affect previous observable behavior.  For C, I believe that this=
 is
not allowed by the standard. In any case, it annoying (e.g. when debugging =
of
embedded devices) and dangerous (e.g. in device drivers or when controling
machines) and best avoided.


volatile int x;

int foo(int a, int b, _Bool store_to_x)
{
  if (!store_to_x)
    return a / b;
  x =3D b;
  return a / b;
}=