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From: "vultkayn at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug analyzer/110830] New: -Wanalyzer-use-of-uninitialized-value
 false negative due to use-after-free::supercedes_p.
Date: Thu, 27 Jul 2023 12:41:41 +0000
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X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110830

            Bug ID: 110830
           Summary: -Wanalyzer-use-of-uninitialized-value false negative
                    due to use-after-free::supercedes_p.
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: vultkayn at gcc dot gnu.org
  Target Milestone: ---

state_machine::supercedes_p is called when two diagnostics are emitted for =
the
same statement, without regarding the path that led to this statement.
See reproducer on trunk https://godbolt.org/z/GqebW5s5h=20

#include <stdlib.h>

extern int ext();

int* foo()
{
  int *p =3D 0;
  if (ext() > 5)
  {
    p =3D malloc (sizeof(int));
    *p =3D 0;
    free (p);
    return p;
  }
  else
    return malloc(sizeof(int));
}

void test()
{
  int *y =3D foo(); // (*)
  int x =3D 4 + *y;
  free (y);
}

At statement (*) both -Wanalyzer-use-of-uninitialized-value and
-Wanalyzer-use-after-free should be emitted as solving the latter won't imp=
act
the former, since they result from two independent branches. But
use_after_free::supercedes_p hides the other.

In the case of a false positive -Wanalyzer-use-after-free, or simply one
ignored by the user, the adjacent -Wanalyzer-use-of-uninitialized-value wou=
ld
therefore never be emitted.=