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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/90436] Redundant size checking in vector
Date: Sun, 21 Jun 2020 17:13:57 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D90436

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
      // possibly assumes that ptrdiff_t and size_t have the same size
      size_type
      _M_check_len_one(const char* __str) const
      {
        ptrdiff_t __n =3D sizeof(_Tp);
        ptrdiff_t __ms =3D max_size(); __ms *=3D sizeof(_Tp);
        ptrdiff_t __s =3D size(); __s *=3D sizeof(_Tp);
        if (__s > (__ms - __n))
          __throw_length_error(__N(__str));

        const ptrdiff_t __len =3D __s + (std::max)(__s, __n);
        if (__len <=3D 0) __builtin_unreachable();
        ptrdiff_t __ret =3D (std::min)(__len, __ms);
        return (_Tp*)__ret-(_Tp*)0; // hack to generate divexact, so it
simplifies with * sizeof(_Tp)
      }

generates nicer code. But after those experiments, it seems clear that the
performance of this code is irrelevant (not surprising since it is followed=
 by
a call to operator new), and its effect on global performance is random.
Possibly it causes something to get aligned differently, which can randomly=
 get
this 25% speed-up, but can just as randomly go back to the slow version.
Anyway, I don't think I'll be submitting any patch for this.=