[Bug c++/100370] New: [11.1.0 regression] Incorrect warning for placement new

[Bug c++/100370] New: [11.1.0 regression] Incorrect warning for placement new

[foldy at rmki dot kfki.hu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

            Bug ID: 100370
           Summary: [11.1.0 regression] Incorrect warning for placement
                    new
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: foldy at rmki dot kfki.hu
  Target Milestone: ---

With g++ 11.1.0:

tmp> cat test.cc
#include <new>

int main()
{
  struct s1 { int iv[4]; };
  struct s2 { union { char* cp; int* ip; }; };

  s2 b;
  b.ip=new int[8];
  new (b.ip+4) s1;
}
tmp> g++ -c test.cc
test.cc: In function 'int main()':
test.cc:10:12: warning: placement new constructing an object of type
'main()::s1' and size '16' in a region of type 'main()::s2' and size '0'
[-Wplacement-new=]
   10 |   new (b.ip+4) s1;
      |        ~~~~^~
test.cc:8:6: note: at offset 16 from 'b' declared here
    8 |   s2 b;
      |      ^

g++ 10.2.1 is OK.

[Bug c++/100370] [11/12 Regression] Incorrect warning for placement new

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[11.1.0 regression]         |[11/12 Regression]
                   |Incorrect warning for       |Incorrect warning for
                   |placement new               |placement new
   Target Milestone|---                         |11.2
            Version|unknown                     |11.1.0

[Bug c++/100370] [11/12 Regression] Incorrect warning for placement new

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |100399
   Last reconfirmed|                            |2021-05-03
      Known to fail|                            |11.1.0
             Status|UNCONFIRMED                 |NEW
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.  The false positive here is the result of trying to avoid other
false positives for unions and not considering that the pointer might point to
space that's larger than that.  Like the false negative in pr100307 comment 2
(AKA pr100325), this would also be prevented by running -Wplacement-new later,
when the program is in SSA form, rather than from the front end.

  if (code == COMPONENT_REF)
    {
      tree ref = TREE_OPERAND (ptr, 0);
      if (TREE_CODE (TREE_TYPE (ref)) == UNION_TYPE)
        /* In accesses through union types consider the entire unions
           rather than just their members.  */
        ostype = 0;


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100399
[Bug 100399] bogus/missing -Wplacement-new

[Bug c++/100370] [11/12 Regression] Incorrect -Wplacement-new with union

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.2                        |11.3

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 11.2 is being released, retargeting bugs to GCC 11.3

[Bug c++/100370] [11/12 Regression] Incorrect -Wplacement-new with union

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c++/100370] [11/12 Regression] Incorrect -Wplacement-new with union

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=70834

[Bug c++/100370] [11/12 Regression] Incorrect -Wplacement-new with union

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Patch posted: https://gcc.gnu.org/pipermail/gcc-patches/2022-April/592856.html

[Bug c++/100370] [11/12 Regression] Incorrect -Wplacement-new with union

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:790b02af6a1fcfa07dba6129909b3578a55a51fa

commit r12-8079-g790b02af6a1fcfa07dba6129909b3578a55a51fa
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Apr 5 22:29:06 2022 -0400

    c++: -Wplacement-new and anon union member [PR100370]

    This bug was an object/value confusion; we are interested in the size
    of *b.ip, but instead the code was calculating the size of b.ip itself.

    This seems to be because compute_objsize will compute the size of whatever
    object it can find in the argument: if you pass it a VAR_DECL, it gives you
    the size of that variable.  If you pass it an ADDR_EXPR of a VAR_DECL, it
    again gives you the size of the variable.  The way you can tell the
    difference is by looking at the deref member of access_ref: if it's -1, the
    argument is a pointer to the object.  Since that's what we're interested
in,
    we should check for that, like check_dangling_stores does.

    This regressed some tests because compute_objsize_r was wrongly zeroing
    deref in the POINTER_PLUS_EXPR handling; adding an offset to a pointer
    doesn't change whether the pointer is itself a variable or a pointer to
    one.  In fact, handling POINTER_PLUS_EXPR only really makes sense for deref
    == -1, where we're adjusting a pointer to the variable.

            PR c++/100370

    gcc/cp/ChangeLog:

            * init.cc (warn_placement_new_too_small): Check deref.

    gcc/ChangeLog:

            * pointer-query.cc (compute_objsize_r) [POINTER_PLUS_EXPR]: Require
            deref == -1.

    gcc/testsuite/ChangeLog:

            * g++.dg/warn/Wplacement-new-size-11.C: New test.

[Bug c++/100370] [11 Regression] Incorrect -Wplacement-new with union

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.3                        |11.4

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 11.3 is being released, retargeting bugs to GCC 11.4.

[Bug c++/100370] [11 Regression] Incorrect -Wplacement-new with union

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100370

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.4                        |11.5

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 11.4 is being released, retargeting bugs to GCC 11.5.