[Bug middle-end/65244] Bogus -Wmaybe-uninitialized warning with posix_memalign() and -Og

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65244

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2015-02-27 00:00:00         |2021-3-31
      Known to fail|                            |10.2.0, 11.0, 4.9.4, 5.5.0,
                   |                            |6.4.0, 7.2.0, 8.3.0, 9.1.0
      Known to work|4.8.2, 5.0                  |
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #15 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning with -Og disappeared with r212559 but still comes back on trunk
with -Og -ftree-bit-ccp.  With that, as the comment in gimple-low.c says, a
call to posix_memalign() is lowered into:

    void *tem;
     res = posix_memalign (&tem, align, size);
     if (res == 0)
       ptr = __builtin_assume_aligned (tem, align);

and at -Og it looks like the two conditionals don't get cleaned up as nicely as
at -O1 and lead to an IL that the warning is designed to trigger on.  This
could be handled by the warning but unless this can come up with other
(especially non-builtin) functions it may not be worth the trouble.

$ cat pr65244.c && gcc -Og -S -Wall -ftree-bit-ccp
-fdump-tree-uninit=/dev/stdout pr65244.c
void exit(int __status) __attribute__ ((__noreturn__));
int posix_memalign(void **__memptr, __SIZE_TYPE__ __alignment, __SIZE_TYPE__
__size);

void *f(void) {
    void *ptr;

    if (posix_memalign(&ptr, 16, 256) != 0)
        exit(1);

    return ptr;
}

;; Function f (f, funcdef_no=0, decl_uid=1949, cgraph_uid=1, symbol_order=0)

pr65244.c: In function ‘f’:
pr65244.c:10:12: warning: ‘ptr’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   10 |     return ptr;
      |            ^~~
pr65244.c:5:11: note: ‘ptr’ was declared here
    5 |     void *ptr;
      |           ^~~
void * f ()
{
  void * D.1957;
  void * ptr;
  int _1;
  void * _6;

  <bb 2> [local count: 1073741824]:
  _1 = posix_memalign (&D.1957, 16, 256);
  if (_1 == 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 7>; [50.00%]

  <bb 7> [local count: 536870912]:
  goto <bb 4>; [100.00%]

  <bb 3> [local count: 536870913]:
  _6 = D.1957;

  <bb 4> [local count: 1073741824]:
  # ptr_2 = PHI <ptr_5(D)(7), _6(3)>
  if (_1 != 0)
    goto <bb 5>; [0.04%]
  else
    goto <bb 6>; [99.96%]

  <bb 5> [local count: 429496]:
  exit (1);

  <bb 6> [local count: 1073312329]:
  return ptr_2;

}