[libcxx-commits] [libcxx] [libc++][ranges] optimize the performance of `ranges::starts_with` (PR #84570)

[Xiaoyang Liu via libcxx-commits]

================
@@ -99,6 +100,10 @@ struct greater_equal {
 // operator are of the same type
 template <class _Tp, class _Up>
 struct __desugars_to<__equal_tag, ranges::equal_to, _Tp, _Up> : true_type {};
+template <class _Tp, class _Up>
+struct __desugars_to<__equal_tag, reference_wrapper<ranges::equal_to>, _Tp, _Up> : true_type {};
+template <class _Tp, class _Up>
+struct __desugars_to<__equal_tag, reference_wrapper<const ranges::equal_to>, _Tp, _Up> : true_type {};
----------------
xiaoyang-sde wrote:

`ranges::equal` will invoke `__equal_impl`. The `__equal_impl` overload that invokes `__builtin_memcmp` requires that the predicate models `__desugars_to<__equal_tag>` and the projection models `__is_identity`.

```cpp
template <class _Tp,
          class _Up,
          class _Pred,
          class _Proj1,
          class _Proj2,
          __enable_if_t<__desugars_to<__equal_tag, _Pred, _Tp, _Up>::value && __is_identity<_Proj1>::value &&
                            __is_identity<_Proj2>::value && !is_volatile<_Tp>::value && !is_volatile<_Up>::value &&
                            __libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
                        int> = 0>
_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
__equal_impl(_Tp* __first1, _Tp* __last1, _Up* __first2, _Up*, _Pred&, _Proj1&, _Proj2&) {
  return std::__constexpr_memcmp_equal(__first1, __first2, __element_count(__last1 - __first1));
}
```

These lines allows the user to pass the `std::ranges::equal_to` predicate wrapped in a `std::reference_wrapper`.

https://github.com/llvm/llvm-project/pull/84570