[Bug c/99797] New: accessing uninitialized automatic variables

["muecker at gwdg dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99797

            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 = 0;
void bar(int c);
void foo(void)
{
    char c;
    //&c;
    //char *p = &c;

    if (!c)
        bar(0);

    if (d)
        c = 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 would
prefer a more consistent behavior. 

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 the
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 unspecified
yet consistent value is read. Producing code that assumes c == 0 and c != 0 at
different points in time (without intervening write) is really dangerous and
should be avoided.