[Bug c++/80265] __builtin_{memcmp,memchr,strlen} are not usable in constexpr functions

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80265

--- Comment #24 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, but then we'd have *three* versions of std::equal: the default
implementation using a for-loop, the optimized one using memcmp for PODs, and
the constexpr form of the latter which also uses a for-loop.

It would be more sensible to just use the default (not memcmp-optimized) one in
constexpr contexts, but that requires using __builtin_constant_p in every
function that is affected, instead of just replacing __builtin_memcmp with
__constexpr_memcmp in the optimized version.

Also, the more work that the algorithm does, the more concerned I'd be that
although the inputs are constant (and so we use the naive for-loop) the actual
work done in the loop might be too much to inline, so we make a run-time call
to the slow version when we could use memcmp.