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From: "malat at debian dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109175] error: 'void* __builtin_memset(void*,
 int, long unsigned int)' writing 4 or more bytes into a region of size 0
 overflows the destination [-Werror=stringop-overflow=]
Date: Fri, 24 Mar 2023 18:20:39 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: diagnostic, needs-reduction
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109175

--- Comment #5 from Mathieu Malaterre <malat at debian dot org> ---
c-reduce(d) ugly version:

% cat demo3.cc
struct d {
  using b =3D float &;
};
template <typename> using c =3D d::b;
struct e {
  using b =3D c<float>;
};
template <long, typename> struct j;
template <long f, typename g> using h =3D typename j<f, g>::b;
template <long, typename...> struct p;
template <typename k, typename... l> struct p<0, k, l...> { using b =3D k; =
};
template <typename...> class m;
template <typename n> struct aa {
  aa(n t) : o(t) {}
  static n ab(aa t) { return t.o; }
  n o;
};
template <long, typename...> struct q;
template <long r, typename n, typename... s>
struct q<r, n, s...> : q<1, s...>, aa<n> {
  q(n t, s... ac) : q<1, s...>(ac...), aa<n>(t) {}
};
template <long r, typename n> struct q<r, n> : aa<n> {
  q(n t) : aa<n>(t) {}
};
template <typename ad, typename ae> class m<ad, ae> : public q<0, ad, ae> {
public:
  m(ad t, ae ac) : q<0, ad, ae>(t, ac) {}
};
template <long f, typename... af> struct j<f, m<af...>> {
  using b =3D typename p<f, af...>::b;
};
template <long f, typename n, typename... s> n ag(q<f, n, s...> t) {
  return q<f, n>::ab(t);
}
template <int f, typename... ah> h<f, m<ah...>> ai(m<ah...> t) { return ag(=
t);
}
class u {
  struct aj {
    using b =3D float *;
  };

public:
  using ak =3D aj::b;
  u(ak t, int) : al(t, long()) {}
  ak w() { return ai<0>(al); }
  m<ak, int> al;
};
struct x : u {
  using u::u;
};
class y {
  x al;

public:
  using ak =3D u::ak;
  using am =3D int;
  template <typename an> y(an t, am ac) : al(t, ac) {}
  e::b operator[](long t) { return z()[t]; }
  ak z() { return al.w(); }
};
void *ao();
template <typename ap> ap *aq(long t) {
  if (t)
    return nullptr;
  return static_cast<ap *>(ao());
}
template <typename> using ar =3D y;
template <typename ap> ar<ap> as(long t, void *) {
  return ar<ap>(aq<ap>(t), int());
}
template <typename ap> ar<ap> as(long t) { return as<ap>(t, nullptr); }
float v;
void at(long t) {
  long a =3D sizeof(0), i =3D 0;
  auto b =3D as<float>(t);
  for (; i < a; ++i)
    b[i] =3D v;
  for (; i < t; ++i)
    b[i] =3D 0.0f;
}

 % /usr/lib/gcc-snapshot/bin/g++ -Wall -O2 -o t.o -c demo3.cc
demo3.cc: In function 'void at(long int)':
demo3.cc:79:10: warning: 'void* __builtin_memset(void*, int, long unsigned
int)' writing 4 or more bytes into a region of size 0 overflows the destina=
tion
[-Wstringop-overflow=3D]
   79 |     b[i] =3D 0.0f;
cc1plus: note: destination object is likely at address zero=