[Bug c++/94867] New: New (since gcc 8) false positive with -Wnull-dereference in very simple code

[Bug c++/94867] New: New (since gcc 8) false positive with -Wnull-dereference in very simple code

[vz-gcc at zeitlins dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

            Bug ID: 94867
           Summary: New (since gcc 8) false positive with
                    -Wnull-dereference in very simple code
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vz-gcc at zeitlins dot org
  Target Milestone: ---

Consider:

% cat -n gcc-null-deref.cpp
     1  #include <vector>
     2
     3  void test()
     4  {
     5      std::vector<int> p;
     6      p.push_back(1);
     7      std::vector<int> q(p.size());
     8      q[0] = p[0];
     9  }
% g++-9 -O2 -c gcc-null-deref.cpp -Wnull-dereference
gcc-null-deref.cpp: In function ‘void test()’:
gcc-null-deref.cpp:8:10: warning: potential null pointer dereference
[-Wnull-dereference]
    8 |     q[0] = p[0];


This is with gcc version 9.3.0 (Debian 9.3.0-11). It doesn't happen with g++-8
but does still happen with the latest version available in Debian which is gcc
version 10.0.1 20200418 (experimental) [master revision
27c171775ab:4c277008be0:c5bac7d127f288fd2f8a1f15c3f30da5903141c6] (Debian
10-20200418-1)

FWIW it looks like -Wnull-dereference really doesn't like push_back() for
whatever reason. Even initializing the vector with fixed size doesn't help to
make the warning go away -- however doing this _and_ removing push_back() does
help.

[Bug c++/94867] [9 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-04-30
      Known to fail|                            |9.1.0, 9.3.0
   Target Milestone|---                         |9.4
            Summary|New (since gcc 8) false     |[9 Regression] New (since
                   |positive with               |gcc 8) false positive with
                   |-Wnull-dereference in very  |-Wnull-dereference in very
                   |simple code                 |simple code
             Status|UNCONFIRMED                 |NEW
      Known to work|                            |10.0, 8.4.0
     Ever confirmed|0                           |1
           Keywords|                            |needs-bisection

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed, only warns with GCC 9 for me.

[Bug c++/94867] [9 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
I see a warning with GCC 10 when passed -O instead of -O2.

[Bug c++/94867] [9 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|needs-bisection             |
                 CC|                            |marxin at gcc dot gnu.org

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
It's gone since r10-3311-gff6686d2e5f797d6.

[Bug c++/94867] [9 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[vz-gcc at zeitlins dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

--- Comment #4 from Vadim Zeitlin <vz-gcc at zeitlins dot org> ---
(In reply to Martin Liška from comment #3)
> It's gone since r10-3311-gff6686d2e5f797d6.

This commit is included in releases/gcc-10.1.0 tag, but I still see the warning
with the provided example when using g++ 10.1 with -O, as mentioned in comment
#2, even though I indeed don't see it with -O2 any longer.

[Bug c++/94867] [9 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org

--- Comment #5 from Martin Liška <marxin at gcc dot gnu.org> ---
You are right, it started with r9-7156-g33579b59aaf02eb7.

[Bug c++/94867] [9 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
I see this warning with GCC 10.2 and with trunk when using the flags -O
-std=c++14 -Wnull-dereference as well.  Should this be marked as a 9/10/11
regression then?

[Bug c++/94867] [9/10 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
            Summary|[9 Regression] New (since   |[9/10 Regression] New
                   |gcc 8) false positive with  |(since gcc 8) false
                   |-Wnull-dereference in very  |positive with
                   |simple code                 |-Wnull-dereference in very
                   |                            |simple code
      Known to fail|                            |10.3.0
      Known to work|10.0                        |11.1.0

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed on the GCC 10 branch.  GCC 11 seems to be unaffected though.

[Bug c++/94867] [9/10 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.4                         |9.5

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9.4 is being released, retargeting bugs to GCC 9.5.

[Bug c++/94867] [10 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.5                         |10.4

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9 branch is being closed

[Bug c++/94867] [10 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug c++/94867] [10 Regression] New (since gcc 8) false positive with -Wnull-dereference in very simple code

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94867

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
      Known to fail|                            |10.5.0
   Target Milestone|10.5                        |11.0
             Status|NEW                         |RESOLVED

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed in GCC 11.