[Bug c++/70259] New: [6 Regression] -flifetime-dse=2 bug with empty bases

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70259

            Bug ID: 70259
           Summary: [6 Regression] -flifetime-dse=2 bug with empty bases
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

struct A { A () : a (true) {} bool a; ~A () {} };
struct B { };
struct C : B { C () {} ~C () {} };
struct D { D () : d (true) {} bool d; ~D () {} };
struct E : public A, C, D { E () : e (true) {} bool e; ~E () {} };

int
main ()
{
  E e;
  if (!e.a || !e.d || !e.e)
    __builtin_abort ();
}

is miscompiled with -O2, works with -O2 -flifetime-dse=1 or -O2
-fno-lifetime-dse.
The problem is that the C::C ctor emits a {CLOBBER} for 1 byte, even when C is
empty class and can overlap other fields.  Similarly the {CLOBBER} we emit in
the dtor is 1 byte and can overlap other fields, so in theory one could try to
construct a testcase that would fail already with GCC 5.x or 4.9?

Less reduced testcase for the same issue:
#include <tuple>

struct P1 { } _1;

struct testStruct;

struct _Bind
{
  std::tuple<testStruct*, P1, bool> _M_bound_args;

  explicit _Bind(testStruct* arg)
  : _M_bound_args(arg, _1, true)
  { }

  bool
  operator()()
  {
    return std::get<2>(_M_bound_args);
  }
};

struct testStruct {
  void runme() {
    _Bind c(this);
    if ( c() == false )
      __builtin_abort();
  }
};

int main() {
  testStruct s;
  s.runme();
}