[libstdc++] Improve M_check_len

[Jonathan Wakely]

P.S. please CC libstdc++@gcc.gnu.org for all libstdc++ patches.

On Mon, 19 Jun 2023 at 16:13, Jonathan Wakely <jwakely@redhat.com> wrote:

> On Mon, 19 Jun 2023 at 12:20, Jakub Jelinek wrote:
>
>> On Mon, Jun 19, 2023 at 01:05:36PM +0200, Jan Hubicka via Gcc-patches
>> wrote:
>> > -     if (max_size() - size() < __n)
>> > -       __throw_length_error(__N(__s));
>> > +     const size_type __max_size = max_size();
>> > +     // On 64bit systems vectors can not reach overflow by growing
>> > +     // by small sizes; before this happens, we will run out of memory.
>> > +     if (__builtin_constant_p(__n)
>> > +         && __builtin_constant_p(__max_size)
>> > +         && sizeof(ptrdiff_t) >= 8
>> > +         && __max_size * sizeof(_Tp) >= ((ptrdiff_t)1 << 60)
>>
>> Isn't there a risk of overlow in the __max_size * sizeof(_Tp) computation?
>>
>
> For std::allocator, no, because max_size() is size_t(-1) / sizeof(_Tp).
> But for a user-defined allocator that has a silly max_size(), yes, that's
> possible.
>
> I still don't really understand why any change is needed here. The PR says
> that the current _M_check_len brings in the EH code, but how/why does that
> happen? The __throw_length_error function is not inline, it's defined in
> libstdc++.so, so why isn't it just an extern call? Is the problem that it
> makes _M_check_len potentially-throwing? Because that's basically the
> entire point of _M_check_len: to throw the exception that is required by
> the C++ standard. We need to be very careful about removing that required
> throw! And after we call _M_check_len we call allocate unconditionally, so
> _M_realloc_insert can always throw (we only call _M_realloc_insert in the
> case where we've already decided a reallocation is definitely needed).
>
> Would this version of _M_check_len help?
>
>       size_type
>       _M_check_len(size_type __n, const char* __s) const
>       {
>         const size_type __size = size();
>         const size_type __max_size = max_size();
>
>         if (__is_same(allocator_type, allocator<_Tp>)
>               && __size > __max_size / 2)
>           __builtin_unreachable(); // Assume std::allocator can't fill
> memory.
>         else if (__size > __max_size)
>           __builtin_unreachable();
>
>         if (__max_size - __size < __n)
>           __throw_length_error(__N(__s));
>
>         const size_type __len = __size + (std::max)(__size, __n);
>         return (__len < __size || __len > __max_size) ? __max_size : __len;
>       }
>
> This only applies to std::allocator, not user-defined allocators (because
> we don't know their semantics). It also seems like less of a big hack!
>
>
>