[Bug ipa/106389] New: IPA modref breaks Safe Bitfields in C++

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106389

            Bug ID: 106389
           Summary: IPA modref breaks Safe Bitfields in C++
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ipa
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

The following test is miscompiled since r11-3308-gd119f34c952f87 (so the
problem goes away with -fno-ipa-modref).

void __assert_fail(const char *, const char *, int, const char *) {
__builtin_abort(); }
typedef int uint32_t;
template <int Offset> struct BitFieldMember {
  int value;
  operator int() { return value & 1; }
  BitFieldMember operator=(int v) {
    value = value | v << Offset;
    return *this;
  }
  BitFieldMember operator-=(int v) {
    *this ? void() : __assert_fail("", "", 9, __PRETTY_FUNCTION__);
    value -= v << Offset;
    return *this;
  }
};
union Status {
  struct {
    uint32_t value;
  } wrapper;
  Status(uint32_t v = 0) { wrapper.value = v; }
  operator uint32_t() { return wrapper.value; }
  BitFieldMember<0> readers;
  BitFieldMember<10> waitToRead;
  BitFieldMember<20> writers;
};
uint32_t u;
struct N {
  uint32_t m_status;
  void foo () {
    Status oldStatus = m_status, newStatus = oldStatus;
    u = oldStatus.waitToRead;
    newStatus.writers -= u;
    newStatus.readers = u;
    m_status = newStatus;
  }
};
int main() {
  Status status;
  status.writers = status = 1;
  N lock;
  lock.m_status = status;
  lock.foo ();
  if (lock.m_status != 1)
    __builtin_abort();
}