[libc++] Further improve the contiguous-iterator story, and fix some bugs.
- Quality-of-implementation: Avoid calling __unwrap_iter in constexpr contexts.
The user might conceivably write a contiguous iterator where normal iterator
arithmetic is constexpr-friendly but `std::to_address(it)` isn't.
- Bugfix: When you pass contiguous iterators to `std::copy`, you should get
back your contiguous iterator type, not a raw pointer. That means that
libc++ can't `__unwrap_iter` unless it also does `__rewrap_iter`.
Fortunately, this is implementable.
- Improve test coverage of the new `contiguous_iterator` test iterator.
This catches the bug described above.
- Tests: Stop testing that we can `std::copy` //into// an `input_iterator`.
Our test iterators may currently support that, but it seems nonsensical to me.
Differential Revision: https://reviews.llvm.org/D95983
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 04126a1..4c13929 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -1639,12 +1639,22 @@
__value_, __equal_to<__v, _Tp>());
}
-// __unwrap_iter
+// __unwrap_iter, __rewrap_iter
-// The job of __unwrap_iter is to lower iterators-that-are-tantamount-to-pointers
-// (such as vector<T>::iterator) into pointers, to reduce the number of template
+// The job of __unwrap_iter is to lower contiguous iterators (such as
+// vector<T>::iterator) into pointers, to reduce the number of template
// instantiations and to enable pointer-based optimizations e.g. in std::copy.
+// For iterators that are not contiguous, it must be a no-op.
// In debug mode, we don't do this.
+//
+// __unwrap_iter is non-constexpr for user-defined iterators whose
+// `to_address` and/or `operator->` is non-constexpr. This is okay; but we
+// try to avoid doing __unwrap_iter in constant-evaluated contexts anyway.
+//
+// Some algorithms (e.g. std::copy, but not std::sort) need to convert an
+// "unwrapped" result back into a contiguous iterator. Since contiguous iterators
+// are random-access, we can do this portably using iterator arithmetic; this
+// is the job of __rewrap_iter.
template <class _Iter, bool = __is_cpp17_contiguous_iterator<_Iter>::value>
struct __unwrap_iter_impl {
@@ -1674,6 +1684,20 @@
return _Impl::__apply(__i);
}
+template<class _OrigIter>
+_OrigIter __rewrap_iter(_OrigIter, _OrigIter __result)
+{
+ return __result;
+}
+
+template<class _OrigIter, class _UnwrappedIter>
+_OrigIter __rewrap_iter(_OrigIter __first, _UnwrappedIter __result)
+{
+ // Precondition: __result is reachable from __first
+ // Precondition: _OrigIter is a contiguous iterator
+ return __first + (__result - _VSTD::__unwrap_iter(__first));
+}
+
// copy
template <class _InputIterator, class _OutputIterator>
@@ -1716,11 +1740,12 @@
copy(_InputIterator __first, _InputIterator __last, _OutputIterator __result)
{
if (__libcpp_is_constant_evaluated()) {
- return _VSTD::__copy_constexpr(
- _VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _VSTD::__unwrap_iter(__result));
+ return _VSTD::__copy_constexpr(__first, __last, __result);
} else {
- return _VSTD::__copy(
- _VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _VSTD::__unwrap_iter(__result));
+ return _VSTD::__rewrap_iter(__result,
+ _VSTD::__copy(_VSTD::__unwrap_iter(__first),
+ _VSTD::__unwrap_iter(__last),
+ _VSTD::__unwrap_iter(__result)));
}
}
@@ -1770,13 +1795,12 @@
_BidirectionalIterator2 __result)
{
if (__libcpp_is_constant_evaluated()) {
- return _VSTD::__copy_backward_constexpr(_VSTD::__unwrap_iter(__first),
- _VSTD::__unwrap_iter(__last),
- _VSTD::__unwrap_iter(__result));
+ return _VSTD::__copy_backward_constexpr(__first, __last, __result);
} else {
- return _VSTD::__copy_backward(_VSTD::__unwrap_iter(__first),
- _VSTD::__unwrap_iter(__last),
- _VSTD::__unwrap_iter(__result));
+ return _VSTD::__rewrap_iter(__result,
+ _VSTD::__copy_backward(_VSTD::__unwrap_iter(__first),
+ _VSTD::__unwrap_iter(__last),
+ _VSTD::__unwrap_iter(__result)));
}
}
@@ -1843,8 +1867,6 @@
// move
-// __move_constexpr exists so that __move doesn't call itself when delegating to the constexpr
-// version of __move.
template <class _InputIterator, class _OutputIterator>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
_OutputIterator
@@ -1873,8 +1895,6 @@
>::type
__move(_Tp* __first, _Tp* __last, _Up* __result)
{
- if (__libcpp_is_constant_evaluated())
- return _VSTD::__move_constexpr(__first, __last, __result);
const size_t __n = static_cast<size_t>(__last - __first);
if (__n > 0)
_VSTD::memmove(__result, __first, __n * sizeof(_Up));
@@ -1886,13 +1906,18 @@
_OutputIterator
move(_InputIterator __first, _InputIterator __last, _OutputIterator __result)
{
- return _VSTD::__move(_VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _VSTD::__unwrap_iter(__result));
+ if (__libcpp_is_constant_evaluated()) {
+ return _VSTD::__move_constexpr(__first, __last, __result);
+ } else {
+ return _VSTD::__rewrap_iter(__result,
+ _VSTD::__move(_VSTD::__unwrap_iter(__first),
+ _VSTD::__unwrap_iter(__last),
+ _VSTD::__unwrap_iter(__result)));
+ }
}
// move_backward
-// __move_backward_constexpr exists so that __move_backward doesn't call itself when delegating to
-// the constexpr version of __move_backward.
template <class _InputIterator, class _OutputIterator>
inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
_OutputIterator
@@ -1921,8 +1946,6 @@
>::type
__move_backward(_Tp* __first, _Tp* __last, _Up* __result)
{
- if (__libcpp_is_constant_evaluated())
- return _VSTD::__move_backward_constexpr(__first, __last, __result);
const size_t __n = static_cast<size_t>(__last - __first);
if (__n > 0)
{
@@ -1938,7 +1961,14 @@
move_backward(_BidirectionalIterator1 __first, _BidirectionalIterator1 __last,
_BidirectionalIterator2 __result)
{
- return _VSTD::__move_backward(_VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _VSTD::__unwrap_iter(__result));
+ if (__libcpp_is_constant_evaluated()) {
+ return _VSTD::__move_backward_constexpr(__first, __last, __result);
+ } else {
+ return _VSTD::__rewrap_iter(__result,
+ _VSTD::__move_backward(_VSTD::__unwrap_iter(__first),
+ _VSTD::__unwrap_iter(__last),
+ _VSTD::__unwrap_iter(__result)));
+ }
}
// iter_swap
