P.S. please CC libstd...@gcc.gnu.org for all libstdc++ patches. On Mon, 19 Jun 2023 at 16:13, Jonathan Wakely <jwak...@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! > > >