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From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/98029] New: volatile triggers incorrect "warning: right-hand
 operand of comma expression has no effect [-Wunused-value]"
Date: Fri, 27 Nov 2020 12:09:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98029

            Bug ID: 98029
           Summary: volatile triggers incorrect "warning: right-hand
                    operand of comma expression has no effect
                    [-Wunused-value]"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

With gcc (Debian 20201125-2) 11.0.0 20201125 (experimental) [master revision
a4d9837ee4b:97273aee415:b13dacdfb315675803982ad5a3098f7b55e6357a]

double f1 (void)
{
  double d;
  int i;

  for (d =3D 2.0, i =3D 0; i < 5; i++, d *=3D d)
    ;

  return d;
}

double f2 (void)
{
  volatile double d;
  int i;

  for (d =3D 2.0, i =3D 0; i < 5; i++, d *=3D d)
    ;

  return d;
}

yields the following incorrect warning when using "-Wunused-value":

tst.c: In function 'f2':
tst.c:17:10: warning: right-hand operand of comma expression has no effect
[-Wunused-value]
   17 |   for (d =3D 2.0, i =3D 0; i < 5; i++, d *=3D d)
      |        ~~^~~~~

Note that one gets a warning only for f2. The only difference is the
"volatile".

I found this when testing GNU MPFR (a similar code occurs in tests/tset_ld.=
c).=