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From: "bernd.edlinger at hotmail dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/103830] New: volatile optimized away
Date: Sun, 26 Dec 2021 10:00:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103830

            Bug ID: 103830
           Summary: volatile optimized away
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bernd.edlinger at hotmail dot de
  Target Milestone: ---

the following test case is intentionally writing at address 0,
it is IMHO invalid to optimize it away (at -Og):

$ cat empty-inline.cc
/* This testcase is part of GDB, the GNU debugger.

   Copyright 2021 Free Software Foundation, Inc.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

/* PR 25987 */
struct MyClass;
struct ptr {
    MyClass* get() { return t; }     /* line 21 */
    MyClass* t;
};
struct MyClass { void call(); };
void MyClass::call() {
    *(volatile char*)(nullptr) =3D 1;  /* line 26 */
}
static void intermediate(ptr p) {
    p.get()->call();                 /* line 29 */
}
int main() {
    intermediate(ptr{new MyClass});
}
/* EOF */

previously this used to SIGSEGV in line 26, but no longer with
git master from December 18th at least.
I think it is rather common to have something at address zero,
for instance interrupt tables or device registers,
and therefore it's not OK to optimize those volatile accesses away.=