[Bug libstdc++/110287] _M_check_len is expensive

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110287

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jan Hubicka from comment #5)
> > Do you mean something like this?
> I sent my own version, but yours looks nicer.
> > 
> > diff --git a/libstdc++-v3/include/bits/stl_vector.h
> > b/libstdc++-v3/include/bits/stl_vector.h
> > index 70ced3d101f..a4dbfeb8b5b 100644
> > --- a/libstdc++-v3/include/bits/stl_vector.h
> > +++ b/libstdc++-v3/include/bits/stl_vector.h
> > @@ -1902,6 +1902,21 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> >         return (__len < size() || __len > max_size()) ? max_size() : __len;
> >        }
> > 
> > +      // Called by _M_insert_aux etc.
> > +      _GLIBCXX20_CONSTEXPR
> > +      size_type
> > +      _M_check_len_1(const char* __s) const
> > +      {
> > +       if (__builtin_constant_p(size()))
> Perhaps ruling this out for 32bit ports? Or can we assume that half of
> address space can never be allocated in single block?

No, I don't think we can assume that.

> > +         {
> > +           if (size() == 0)
> > +             return 1;
> > +           else if (size() < max_size() / 2)
> I think even this conditional is safe to be assumed to be true,
> since we can not allocate half of 64bit address space.  

There is no guarantee that std::vector<T, A>::max_size() is PTRDIFF_MAX. It
depends on the Allocator type, A. A user-defined allocator could have
max_size() == 100.


> In general it is importnat to not fall through to _M_check_len.
> 
> As I noticed, we may want to use assume attribute to make clear that
> retval <= max_size ()
> to avoid other unnecesary throws downstream.
> 
> Honza