[Bug tree-optimization/104017] unexpeted -Warray-bounds popping a fixed number of std::deque elements

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104017

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning triggers for the clobber statement in bb 43 below.  _236 is assumed
to point to the beginning of the block of 512 bytes allocated by new, so
subtracting a positive integer from it or adding one in excess of 512 is
invalid, as is dereferencing the result:

  <bb 2> [local count: 118111600]:
  ...
  _229 = operator new (512);                               >>> _229
  ...
  <bb 42> [local count: 50546886]:
  _176 = p.D.20902._M_impl.D.20257._M_finish._M_first;
  if (_176 != _229)
    goto <bb 43>; [82.57%]
  else
    goto <bb 44>; [17.43%]

  <bb 43> [local count: 41736564]:
  _236 = ASSERT_EXPR <_229, _229 != _176>;                 <<< _229
  _177 = _236 + 18446744073709551608;
  p.D.20951._M_impl.D.20306._M_finish._M_cur = _177;
  MEM[(const struct Node * *)_236 + -8B] ={v} {CLOBBER};   <<< -Warray-bounds
  goto <bb 45>; [100.00%]

I view the warning as helpful here (and so not a false positive even) because
the test function assumes the loop inserts at least three elements into the
container.  If it doesn't, one of the pop() calls will crash.

A more compelling test case would guard each if the pop() calls to prevent the
crash, like below:

#include <deque>

struct Node { Node const * parent = nullptr; };

void func(Node const * n)
{
    std::deque<Node const *> p;

    Node const * e = n;

    while (e != nullptr) {
        p.push_front(e);
        e = e->parent;
    }

    if (p.size ())
      p.pop_front();
    if (p.size ())
      p.pop_front();
    if (p.size ())
      p.pop_back();
}


This test case also triggers a warning, for the same reason: GCC can't
determine the relationship between a deque's internal node pointers and the
result of std::deque::size() (which is a function of the node pointers).