[Bug c++/96511] New: Incorrect placement-new warning

[Bug c++/96511] New: Incorrect placement-new warning

[mserdarsanli at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96511

            Bug ID: 96511
           Summary: Incorrect placement-new warning
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mserdarsanli at gmail dot com
  Target Milestone: ---

Below code causes the warning in the last line, which seems to be incorrect. It
affects all versions starting from GCC 8
Compiler explorer: https://godbolt.org/z/xhf6cr

#include <new>

struct Foo {
    Foo() = default;
    Foo(int _a) : a(_a) {}
    int a = 0;
};

void f()
{
    Foo arr[2];

    new (arr) Foo(10); // ok
    new (arr + 0) Foo(10); // ok
    new (&arr[0]) Foo(10); // ok

    new (&arr[1]) Foo(15); // ok
    new (arr + 1) Foo(15); // warning: placement new constructing an object of
type 'Foo' and size '4' in a region of type 'Foo [2]' and size '0'
[-Wplacement-new=]
}

[Bug c++/96511] Incorrect placement-new warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96511

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
      Known to fail|                            |10.2.0, 11.0, 8.4.0, 9.3.0
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-08-07
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.  To determine the final offset the warning multiplies the offset in
a POINTER_PLUS expression by the elements size the same way it does the index
in an ARRAY_REF expression.  The latter is correct but the former isn't.

A simplified test case:

$ cat pr96511.C && gcc -S -Wall -Wextra pr96511.C
void* operator new (__SIZE_TYPE__, void *p) { return p; }

void g ()
{
  int a[2];

  new (&a[0] + 1) int;
}

pr96511.C: In function ‘void g()’:
pr96511.C:7:14: warning: placement new constructing an object of type ‘int’ and
size ‘4’ in a region of type ‘int [2]’ and size ‘0’ [-Wplacement-new=]
    7 |   new (&a[0] + 1) int;
      |        ~~~~~~^~~

[Bug c++/96511] Incorrect placement-new warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96511

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |msebor at gcc dot gnu.org
             Status|NEW                         |ASSIGNED
           Keywords|                            |patch

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Patch: https://gcc.gnu.org/pipermail/gcc-patches/2020-August/551783.html

[Bug c++/96511] Incorrect placement-new warning

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96511

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Sebor <msebor@gcc.gnu.org>:

https://gcc.gnu.org/g:de05c19d5fd661ae16dd75a895b49d32d12f5edc

commit r11-3826-gde05c19d5fd661ae16dd75a895b49d32d12f5edc
Author: Martin Sebor <msebor@redhat.com>
Date:   Fri Oct 9 13:56:53 2020 -0600

    Correct handling of indices into arrays with elements larger than 1 (PR
c++/96511)

    Resolves:
    PR c++/96511 - Incorrect -Wplacement-new on POINTER_PLUS into an array with
4-byte elements
    PR middle-end/96384 - bogus -Wstringop-overflow= storing into
multidimensional array with index in range

    gcc/ChangeLog:

            PR c++/96511
            PR middle-end/96384
            * builtins.c (get_range): Return full range of type when neither
            value nor its range is available.  Fail for ranges inverted due
            to the signedness of offsets.
            (compute_objsize): Handle more special array members.  Handle
            POINTER_PLUS_EXPR and VIEW_CONVERT_EXPR that come up in front end
            code.
            (access_ref::offset_bounded): Define new member function.
            * builtins.h (access_ref::eval): New data member.
            (access_ref::offset_bounded): New member function.
            (access_ref::offset_zero): New member function.
            (compute_objsize): Declare a new overload.
            * gimple-array-bounds.cc (array_bounds_checker::check_array_ref):
Use
            enum special_array_member.
            * tree.c (component_ref_size): Use special_array_member.
            * tree.h (special_array_member): Define a new type.
            (component_ref_size): Change signature.

    gcc/cp/ChangeLog:

            PR c++/96511
            PR middle-end/96384
            * init.c (warn_placement_new_too_small): Call builtin_objsize
instead
            of duplicating what it does.

    gcc/testsuite/ChangeLog:

            PR c++/96511
            PR middle-end/96384
            * g++.dg/init/strlen.C: Add expected warning.
            * g++.dg/warn/Wplacement-new-size-1.C: Relax warnings.
            * g++.dg/warn/Wplacement-new-size-2.C: Same.
            * g++.dg/warn/Wplacement-new-size-6.C: Same.
            * gcc.dg/Warray-bounds-58.c: Adjust
            * gcc.dg/Wstringop-overflow-37.c: Same.
            * g++.dg/warn/Wplacement-new-size-7.C: New test.

[Bug c++/96511] Incorrect placement-new warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96511

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|11.0                        |

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
Fixed for GCC 11.

[Bug c++/96511] Incorrect placement-new warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96511

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.0
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
The patch is too intrusive to backport.  Resolving as fixed.