[libc++] Allow __move_constexpr to work with unrelated pointers

[philnik]

Summary

Allow __move_constexpr to work with unrelated pointers and _LIBCPP_ASSERT that __copy_constexpr, __move_constexpr and __assign_constexpr are only run during constant evaluation

[• Quuxplusone]

FWIW, I'm a little leery of how we're making these codepaths more and more complicated. If it wouldn't merge-conflict horribly with the "constexpr std::string" stuff, I would be very tempted to take a close look at all of these codepaths and see how they could be simplified.
For example, __copy_constexpr is an overload set: there's a __copy_constexpr(T*, T*, size_t) here in <__string> but also a __copy_constexpr(In, In, Out) in <__algorithm/copy.h> (which is tied up with all the __unwrap_iter/__rewrap_iter stuff). We don't need that overload set to exist.
For example, __assign_constexpr and this overload of __copy_constexpr are one-liners; they could probably just be inlined into their three callers.

Orthogonally, consider renaming __s1 and __s2 to __src and __dest respectively (or vice versa).

Orthogonally, I wonder if we're 100% sure that constexpr new and delete (as used here) are fully supported in Clang 13. Libc++ 14 will still have to work with Clang 13.

[philnik]

Feel free to change the __{copy, move, assign}_constexpr functions, as long as you make char_traits::move work with unrelated pointers. They aren't used directly in <string>, so I should be able to just drop the changes in <__string>.

[ldionne]

What do you mean "with unrelated pointers"? Can you explain what breaks when you use std::copy to implement __move_constexpr?

[philnik]

In D115986#3202891, @ldionne wrote:

What do you mean "with unrelated pointers"? Can you explain what breaks when you use std::copy to implement __move_constexpr?

The problem isn't std::copy. The problem is __s1 < __s2, which only works if you have two pointers that were derived by __s2 = __s1 + c. Otherwise the constant evaluator will fail, because there is no rule to determine which pointer is larger.

[ldionne]

As @Quuxplusone mentions, I would really support an effort to clean up our various __foo_constexpr functions.

LGTM with the renaming.

libcxx/include/__string

293

__s1 and __s2 is super confusing, especially since the order is reversed from std::copy_n. Please apply this throughout as a fly-by.

295–297

This is pretty annoying -- if we ever end up calling this function outside of constexpr (by mistake or otherwise), we'll end up with an assertion that fails at runtime. I don't see a way to make this fail at compile-time, since if you use __libcpp_is_constant_evaluated() in a place where a constant expression is required, it'll always return true. There's nothing actionable to this comment, but I thought I'd point out that this is a pretty bad defect of is_constant_evaluated().

[philnik]

• renamed

libcxx/include/__string

for (; __n; --__n, ++__s)
         assign(*__s, __a);
     return __r;
 }
 
 // constexpr versions of move/copy/assign.
 
 template <class _CharT>
 static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
-_CharT* __move_constexpr(_CharT* __s1, const _CharT* __s2, size_t __n) _NOEXCEPT
+_CharT* __copy_constexpr(_CharT* __s1, const _CharT* __s2, size_t __n) _NOEXCEPT
 {
-    if (__n == 0) return __s1;
-    if (__s1 < __s2) {
-      _VSTD::copy(__s2, __s2 + __n, __s1);
-    } else if (__s2 < __s1) {
-      _VSTD::copy_backward(__s2, __s2 + __n, __s1 + __n);
-    }
-    return __s1;
+  _LIBCPP_ASSERT(__libcpp_is_constant_evaluated(), "__copy_constexpr() should always be constant evaluated");
+  _VSTD::copy_n(__s2, __n, __s1);
 }
 
 template <class _CharT>
 static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
-_CharT* __copy_constexpr(_CharT* __s1, const _CharT* __s2, size_t __n) _NOEXCEPT
+_CharT* __move_constexpr(_CharT* __dest, const _CharT* __source, size_t __n) _NOEXCEPT
 {
-    _VSTD::copy_n(__s2, __n, __s1);
-    return __s1;
+  _LIBCPP_ASSERT(__libcpp_is_constant_evaluated(), "__move_constexpr() should always be constant evaluated");
+  if (__n == 0)
+    return __dest;
+  _CharT* __allocation = new _CharT[__n];
+  _VSTD::__copy_constexpr(__allocation, __source, __n);
+  _VSTD::__copy_constexpr(__dest, static_cast<const _CharT*>(__allocation), __n);
+  delete[] __allocation;
+  return __dest;
 }
 
 template <class _CharT>
 static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 _CharT* __assign_constexpr(_CharT* __s, size_t __n, _CharT __a) _NOEXCEPT
 {
-     _VSTD::fill_n(__s, __n, __a);
-     return __s;
+  _LIBCPP_ASSERT(__libcpp_is_constant_evaluated(), "__assign_constexpr() should always be constant evaluated");
 }
 
 // char_traits<char>
 
 template <>
 struct _LIBCPP_TEMPLATE_VIS char_traits<char>
 {
     typedef char      char_type;