[Bug libstdc++/122255] New: Pointer tagging in filesystem::path::_List::_Impl assumes at least 4-byte alignment

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122255

            Bug ID: 122255
           Summary: Pointer tagging in filesystem::path::_List::_Impl
                    assumes at least 4-byte alignment
           Product: gcc
           Version: 15.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---
            Target: cris-elf

// Clear the lowest two bits from the pointer (i.e. remove the _Type value)
  static _Impl* notype(_Impl* p)
  {
    constexpr uintptr_t mask = ~(uintptr_t)0x3;
    return reinterpret_cast<_Impl*>(reinterpret_cast<uintptr_t>(p) & mask);
  }


This pointer tagging assumes we have unused low bits in every pointer obtained
from operator new:

    void* p = ::operator new(sizeof(_Impl) + n * sizeof(value_type));

The _Impl type is aligned to alignof(path::_Cmpt):

struct path::_List::_Impl
{
  using value_type = _Cmpt;

  _Impl(int cap) : _M_size(0), _M_capacity(cap) { }

  alignas(value_type) int _M_size;
  int _M_capacity;

But on cris-elf that has alignment of one.

So we need to increase the alignment of the _Impl objects. These are entirely
internal to the library, so there's no ABI change for any code users deal with.