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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107467] Miscompilation involing -Os , -flto
 and -fno-strict-aliasing
Date: Sun, 30 Oct 2022 23:36:28 +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: 12.2.0
X-Bugzilla-Keywords: lto, wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107467

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is a single file example which just needs -Os -flto -fno-strict-aliasi=
ng:

template <class T>
struct pair
{
    int first;
    T second;
};

template <typename C>
[[gnu::optimize("strict-aliasing")]]
bool __attribute__((noinline))
compare_pairs(const pair<C> &lhs, const pair<C> &rhs) {
  return lhs.first =3D=3D rhs.first && lhs.second =3D=3D rhs.second;
}

template <typename B> struct Combined {
  pair<B *>=20
__attribute__((noinline)) get_const() {
    return pair<B *>{123, nullptr};
  }
[[gnu::optimize("strict-aliasing")]]
  bool=20
__attribute__((noinline)) clashy() {
    return compare_pairs(get_const(), get_const());
  }
};

class SomeClass {};
class OtherClass {};

[[gnu::optimize("strict-aliasing")]]
[[gnu::used]]
void some_func() {
  Combined<SomeClass> myvar;
  __builtin_printf("%i\n", myvar.clashy());
}

[[gnu::optimize("strict-aliasing")]]
void other_func() {
  Combined<OtherClass> myvar;
  __builtin_printf("%i\n", myvar.clashy());
}

[[gnu::optimize("O0")]]
int main()
{
  other_func();
}=