[Bug c++/95473] New: GCC misses -Wnull-dereference warning in a simple code

[Bug c++/95473] New: GCC misses -Wnull-dereference warning in a simple code

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95473

            Bug ID: 95473
           Summary: GCC misses -Wnull-dereference warning in a simple code
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

$cat null_ref.cc
int main(){
    int &x1 = *(int *)nullptr; // null reference
    return 0;
}

GCC can not catch any warning when I use -Wnull-dereference option. 
I also try to add -O1~Os, but GCC still produces nothing.

While in clang

$clang++-trunk -Wnull-dereference null_ref.cc 
null_ref.cc:2:15: warning: binding dereferenced null pointer to reference has
undefined behavior [-Wnull-dereference]
    int &x1 = *(int *)nullptr; // null reference
              ^~~~~~~~~~~~~~~
1 warning generated.

$g++ --version
g++ (GCC) 11.0.0 20200526 (experimental)
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[Bug c++/95473] GCC misses -Wnull-dereference warning in a simple code

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95473

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |law at gcc dot gnu.org,
                   |                            |manu at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org
   Last reconfirmed|                            |2020-06-02
     Ever confirmed|0                           |1

[Bug middle-end/95473] GCC misses -Wnull-dereference warning in a simple code

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95473

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The reason for this is the same as in pr95461 comment #1.  Like -Warray-bounds,
for optimal results, -Wnull-dereference relies on optimization.  That includes
dead code elimination.  In the test case, since x1 is unused, it's eliminated
early on, well before the warning sees the code.

All these "late" warnings could also run earlier to detect more instances of
the problems, at the cost of issuing what most users view as false positives. 
Below is another example of such a trade-off.  Clang makes an effort to avoid
issuing these but a front end can only do that to a point.  To do much better
control and data flow analysis is required, which is what GCC does.

$ cat pr95473.c && clang -S -Wall -Wno-unused-variable -xc++ pr95473.c
void f ()
{ 
  if (0)
    int &x1 = *(int *)nullptr; // unreachable null reference (no warning)

}

void g ()
{
  const int i = 0;
  if (i)
    int &x1 = *(int *)nullptr; // unreachable null reference (no warning)

}

void h ()
{
  int i = 0;
  if (i)
    int &x1 = *(int *)nullptr; // unreachable null reference (warning)
}
pr95473.c:20:15: warning: binding dereferenced null pointer to reference has
      undefined behavior [-Wnull-dereference]
    int &x1 = *(int *)nullptr; // unreachable null reference (warning)
              ^~~~~~~~~~~~~~~
1 warning generated.

*** This bug has been marked as a duplicate of bug 95461 ***