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From: "muecker at gwdg dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/99797] New: accessing uninitialized automatic variables
Date: Sat, 27 Mar 2021 13:47:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99797

            Bug ID: 99797
           Summary: accessing uninitialized automatic variables
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: muecker at gwdg dot de
  Target Milestone: ---

Consider the following example which accesses an uninitialized variable:

static volatile int d =3D 0;
void bar(int c);
void foo(void)
{
    char c;
    //&c;
    //char *p =3D &c;

    if (!c)
        bar(0);

    if (d)
        c =3D 1;

    if (c)
        bar(1);
}


GCC produces code where 'bar' is called twice. According to the C standard,=
 the
code is UB so this is technically OK. Still I think it is dangerous and I w=
ould
prefer a more consistent behavior.=20

When taking the address and assigning it to 'p' GCC produces code which
actually checks the variable 'd'. I am not sure why this happens (it is
unnecessary). Still when taking the address of 'c' the code is not UB and t=
he
generated code is OK.

Finally, when taking the address and not assigning it, the code is the same=
 as
for the first case where 'bar' is called twice. This seems incorrect as the
code is not UB (according to my reading of the C standard).

Ideally, I think GCC should reject code when it is clearly UB (address not
taken). If it does not reject the code, I think it should assume an unspeci=
fied
yet consistent value is read. Producing code that assumes c =3D=3D 0 and c =
!=3D 0 at
different points in time (without intervening write) is really dangerous and
should be avoided.=