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From: "stefansf at linux dot ibm.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/101066] New: Wrong code after fixup_cfg3
Date: Mon, 14 Jun 2021 13:45:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101066

            Bug ID: 101066
           Summary: Wrong code after fixup_cfg3
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stefansf at linux dot ibm.com
  Target Milestone: ---
            Target: s390*-*-*, x86_64-*-*

int a =3D 1, c, d, e;
int *b =3D &a;
static int g(int *h) {
  c =3D *h;
  return d;
}
static void f(int *h) {
  e =3D *h;
  *b =3D 0;
  g(h);
}
int main() {
  f(b);
  printf("%d\n", c);
}

Running `gcc t.c -Os && ./a.out` results in printed 1 whereas 0 is expected.
This does not happen for -O[0,1,2,3] i.e. there 0 is printed.

Prior fixup_cfg3 the code looks good to me and afterwards the assignment to=
 c
uses a cached/initial value of variable a which is wrong:

int main ()
{
  int * b.0_1;
  int c.1_2;
  int _6;
  int _7;
  int * b.2_8;
  int _10;
  int _11;

  <bb 2> [local count: 1073741824]:
  b.0_1 =3D b;
  _6 =3D *b.0_1;
  _7 =3D _6;
  e =3D _7;
  b.2_8 =3D b;
  *b.2_8 =3D 0;
  _10 =3D _6;
  c =3D _10;
  _11 =3D d;
  c.1_2 =3D c;
  printf ("%d\n", c.1_2);
  return 0;

}

Reproducible on IBM Z as well as x86_64 using commit 831589c227c.=