[Bug c/105585] New: [12/13 Regression] Spurious stringop-overflow warning with <atomic>

[Bug c/105585] New: [12/13 Regression] Spurious stringop-overflow warning with <atomic>

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585

            Bug ID: 105585
           Summary: [12/13 Regression] Spurious stringop-overflow warning
                    with <atomic>
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

Reduced (from code in abseil-cpp):

#include <atomic>
struct S {
  int i;
  std::atomic<int> a;
};
S* q();
void f();
void g(bool b) {
  auto p = b ? q() : nullptr;
  ++p->a;
  if (p)
    f();
}

In file included from atomic:41,
                 from <source>:1:
In member function 'std::__atomic_base<_IntTp>::__int_type
std::__atomic_base<_IntTp>::operator++() [with _ITp = int]',
    inlined from 'void g(bool)' at <source>:10:3:
bits/atomic_base.h:385:34: warning: 'unsigned int __atomic_add_fetch_4(volatile
void*, unsigned int, int)' writing 4 bytes into a region of size 0 overflows
the destination [-Wstringop-overflow=]
  385 |       { return __atomic_add_fetch(&_M_i, 1, int(memory_order_seq_cst));
}
      |                ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[Bug c/105585] [12/13 Regression] Spurious stringop-overflow warning with <atomic>

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585

--- Comment #1 from Ed Catmur <ed at catmur dot uk> ---
Flags: -O1 -Wstringop-overflow=1
https://godbolt.org/z/8r8roz7Pa

[Bug c/105585] [12/13 Regression] Spurious stringop-overflow warning with <atomic>

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585

--- Comment #2 from Ed Catmur <ed at catmur dot uk> ---
Affected code: https://github.com/abseil/abseil-cpp/issues/1175
The proposed patch to abseil-cpp corresponds to adding an assumption that `b`
is true above.

[Bug tree-optimization/105585] [12/13 Regression] Spurious stringop-overflow warning with <atomic>

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.2
           Keywords|                            |diagnostic
          Component|c                           |tree-optimization

[Bug tree-optimization/105585] [12/13 Regression] Spurious stringop-overflow warning with <atomic> since r12-4725-g88b504b7a8c5affb

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[12/13 Regression] Spurious |[12/13 Regression] Spurious
                   |stringop-overflow warning   |stringop-overflow warning
                   |with <atomic>               |with <atomic> since
                   |                            |r12-4725-g88b504b7a8c5affb
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-05-13
                 CC|                            |marxin at gcc dot gnu.org,
                   |                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Started with r12-4725-g88b504b7a8c5affb.

[Bug tree-optimization/105585] [12/13 Regression] Spurious stringop-overflow warning with <atomic> since r12-4725-g88b504b7a8c5affb

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|NEW                         |RESOLVED

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning is caused by the if statement: if p is null, accessing the p->a
member is undefined.  It triggers because the GCC optimizer splits the code
into two branches: one with a nonnull p and another with a null p.  The second
one triggers the warning.

If p can be null then moving the increment to the body of the if statement
avoids the undefined behavior (and the warning).  If p cannot be null then
making the if statement unconditional also avoids the warning.