[Bug c++/96524] New: missing warning constructing a larger object in a smaller multidimensional array

[Bug c++/96524] New: missing warning constructing a larger object in a smaller multidimensional array

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96524

            Bug ID: 96524
           Summary: missing warning constructing a larger object in a
                    smaller multidimensional array
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Only the first of the two functions is diagnosed for attempting to construct an
int object in an array of size 1.  The equivalent attempt in g() involving a
multidimensional array with insufficient size is not diagnosed, either by
-Wplacement-new or later by -Warray-bounds (or -Wstringop-overflow, although
this warning is only meant to trigger for char accesses).

$ cat t.C && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout t.C
void* operator new (__SIZE_TYPE__, void *p) { return p; }

char a[1];
char b[1][1];

void f ()
{
  new (&a[0]) int (123);     // warning (good)
}

void g ()
{
  new (&b[0][0]) int (456);  // no warning
}
t.C: In function ‘void f()’:
t.C:8:8: warning: placement new constructing an object of type ‘int’ and size
‘4’ in a region of type ‘char [1]’ and size ‘1’ [-Wplacement-new=]
    8 |   new (&a[0]) int (123);     // warning (good)
      |        ^~~~~

;; Function operator new (_ZnwmPv, funcdef_no=0, decl_uid=2337, cgraph_uid=1,
symbol_order=0)

operator new (long unsigned int D.2335, void * p)
{
  <bb 2> [local count: 1073741824]:
  return p_1(D);

}


t.C:8:23: warning: writing 4 bytes into a region of size 1
[-Wstringop-overflow=]
    8 |   new (&a[0]) int (123);     // warning (good)
      |                       ^
t.C:3:6: note: at offset 0 to object ‘a’ with size 1 declared here
    3 | char a[1];
      |      ^

;; Function f (_Z1fv, funcdef_no=1, decl_uid=2341, cgraph_uid=2,
symbol_order=3)

f ()
{
  <bb 2> [local count: 1073741824]:
  MEM[(int *)&a] = 123;
  return;

}



;; Function g (_Z1gv, funcdef_no=2, decl_uid=2346, cgraph_uid=3,
symbol_order=4)

g ()
{
  <bb 2> [local count: 1073741824]:
  MEM[(int *)&b] = 456;
  return;

}

[Bug c++/96524] missing warning constructing a larger object in a smaller multidimensional array

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96524

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=96511,
                   |                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=94195

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
It looks as though warn_placement_new_too_small() simply doesn't take into
consideration milti-dimensional arrays.

The missing -Warray-bounds warning is for the same reason as pr94195.