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From: "dmalcolm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug analyzer/106204] New: False positive from
 -Wanalyzer-use-of-uninitialized-value with -ftrivial-auto-var-init=zero
Date: Tue, 05 Jul 2022 18:04:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106204

            Bug ID: 106204
           Summary: False positive from
                    -Wanalyzer-use-of-uninitialized-value with
                    -ftrivial-auto-var-init=3Dzero
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

I'm seeing a lot of false positives from -Wanalyzer-use-of-uninitialized-va=
lue
when building the upstream Linux kernel.

Seems to be a bad interaction with -ftrivial-auto-var-init=3Dzero; minimal
reproducer:

$ cat ../../src/hooks.c
int foo(unsigned *len);
int test()
{
 unsigned len;
 int rc;

 rc =3D foo(&len);
 if (!rc)
  rc =3D len;
 return rc;
}

$ ./xgcc -B. ../../src/hooks.c -ftrivial-auto-var-init=3Dzero -fanalyzer -S=
=20
../../src/hooks.c: In function =E2=80=98test=E2=80=99:
../../src/hooks.c:4:11: warning: use of uninitialized value =E2=80=98<unkno=
wn>=E2=80=99
[CWE-457] [-Wanalyzer-use-of-uninitialized-value]
    4 |  unsigned len;
      |           ^~~
  =E2=80=98test=E2=80=99: event 1
    |
    |    4 |  unsigned len;
    |      |           ^~~
    |      |           |
    |      |           (1) use of uninitialized value =E2=80=98<unknown>=E2=
=80=99 here
    |

where the uninit is being reported on the assignment to "len =3D _1;" here:

  _1 =3D .DEFERRED_INIT (4, 2, &"len"[0]);
  len =3D _1;=