https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105751

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-05-30
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  We end up expanding

  <bb 2> [local count: 1073741824]:
  _5 = a_2(D) + 8;
  _6 = &MEM[(const struct array *)a_2(D)]._M_elems;
  _8 = _5 - _6;
  __len_9 = (const size_t) _8;
  if (__len_9 != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  _4 = &MEM[(const struct array *)b_3(D)]._M_elems;
  _10 = __builtin_memcmp_eq (_6, _4, __len_9);
  _11 = _10 == 0;

  <bb 4> [local count: 1073741824]:
  # _12 = PHI <1(2), _11(3)>
  return _12;

So somehow the size is not known to be 8 and thus the proposed optimized code
would compare uninitialized memory?

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