https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97132
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #2)
> I think there is no compliant way to test alignment assumptions during
> constant evaluations and after all, one should always see the actual objects
> and therefore can check the exact alignment, rather than needing any
> assumptions.
Right, during CE unaligned objects are UB anyway and not allowed. And the
function is only supposed to benefit the optimiser, which isn't relevant during
CE.
> So, I think it doesn't really matter if this is done on the libstdc++ say
> through not using __builtin_assume_aligned if std::is_constant_evaluated()
> or on the C++ FE side (folding __builtin_assume_aligned to the first
> argument during manifestly constant evaluation).
I'll test this:
{
static_assert(std::has_single_bit(_Align));
_GLIBCXX_DEBUG_ASSERT((std::uintptr_t)__ptr % _Align == 0);
- return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
+ if (std::is_constant_evaluated())
+ return __ptr;
+ else
+ return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
}
#endif // C++2a
