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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99117] [9/10/11 Regression] cannot accumulate
 std::valarray
Date: Tue, 16 Feb 2021 11:47:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99117

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #5)
> int* p =3D sum._M_data;
> int* e1 =3D sum._M_data;
>=20
> If p and e1 aren't __restrict__ too, shouldn't that be fine?

p (called __p below) doesn't use __restrict__:

  template<typename _Tp, class _Dom>
    void
    __valarray_copy(const _Expr<_Dom, _Tp>& __e, size_t __n, _Array<_Tp> __=
a)
    {
      _Tp* __p (__a._M_data);
      for (size_t __i =3D 0; __i < __n; ++__i, ++__p)
        *__p =3D __e[__i];
    }

Here __e is the expression template which has two const valarray<int>& memb=
ers,
so maybe more accurately it's:

struct valarray {
  int* __restrict__ _M_data;
  size_t _M_size;
};
valarray sum{ new int[2]{1,1}, 2 };
valarray rhs{ new int[2]{2,2}, 2 };
int* p =3D sum._M_data;
const valarray& e1 =3D sum;
const valarray& e2 =3D rhs;
for (size_t i =3D 0; i < sum._M_size; ++i, ++p)
  *p =3D e1._M_data[i] + e2._M_data[i];

So e1._M_data is marked with __restrict__.=