[Bug middle-end/98901] missing warning passing a dangling pointer to a function

[msebor at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98901

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-01-31
                 CC|                            |msebor at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=98900
            Summary|Static analyses warning     |missing warning passing a
                   |disappears when the value   |dangling pointer to a
                   |is passed to a function     |function
                   |with variable number of     |
                   |arguments when optimisation |
                   |is on                       |
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
I think the problem here reduces to one of GCC having no support for detecting
the uses of dangling pointers.

In the original test case the variadic function is defined but because GCC
doesn't inline variadic functions it's not relevant.  The smaller test case
below shows that after a variable's life has ended, using its address isn't
diagnosed even though any and all uses of pointers to such objects are
undefined.  Diagnosing the use of such a pointer in this test case is trivial
based on the preceding CLOBBER.  (See also pr98900 for another test case
involving clobbers.)

$ cat pr98901.c && gcc -O2 -S -Wall -fdump-tree-uninit=/dev/stdout pr98901.c
int f (int count, ...);

int main ()
{
  int *p;
  {
    int x = 0;
    p = &x;
  }

  f (1, p);   // missing warning

  __builtin_putchar (*p);
}

;; Function main (main, funcdef_no=0, decl_uid=1944, cgraph_uid=1,
symbol_order=0) (executed once)

int main ()
{
  int x;
  int _1;

  <bb 2> [local count: 1073741824]:
  x ={v} {CLOBBER};
  f (1, &x);
  _1 = x;
  __builtin_putchar (_1);
  return 0;

}