[Bug middle-end/78914] New: missing -Wnonnull for a trivial null pointer dereference

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78914

            Bug ID: 78914
           Summary: missing -Wnonnull for a trivial null pointer
                    dereference
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Now that the -Wnonnull warning has been made smarter (bug 17308) it would be
nice if, in addition to calls to functions declared with attribute nonnnull, it
could also diagnose other null pointer dereferences such as the one in the test
case below.  As is evident from __builtin_trap() in the dump, GCC already
"knows" that the pointer being dereferenced is null, but issuing a warning
would help catch that case earlier than at runtime.

$ cat z.c && gcc -O2 -S -Wall -Wextra -Wpedantic
-fdump-tree-optimized=/dev/stdout z.c
struct S { int i; };

int foo (struct S *s)
{
  if (s)
    return 6;

  return s->i;
}


;; Function foo (foo, funcdef_no=0, decl_uid=1797, cgraph_uid=0,
symbol_order=0)

foo (struct S * s)
{
  int _4;

  <bb 2> [100.00%]:
  if (s_2(D) != 0B)
    goto <bb 4>; [51.18%]
  else
    goto <bb 3>; [48.82%]

  <bb 3> [48.82%]:
  _4 ={v} MEM[(struct S *)0B].i;
  __builtin_trap ();

  <bb 4> [100.00%]:
  return 6;

}