Revert "Revert "[libc++] [P0879] constexpr std::nth_element, and rewrite its tests.""
This reverts commit b6ffece32035a90d181101f356bd9c04ea1d3122.
The bug is now fixed (it was a stupid cut-and-paste kind of error),
and the regression test added. The new patch is also simpler than the old one!
Differential Revision: https://reviews.llvm.org/D96084
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index a6fceaa..04126a1 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -385,11 +385,11 @@
RandomAccessIterator result_first, RandomAccessIterator result_last, Compare comp);
template <class RandomAccessIterator>
- void
+ constexpr void // constexpr in C++20
nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last);
template <class RandomAccessIterator, class Compare>
- void
+ constexpr void // constexpr in C++20
nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare comp);
template <class ForwardIterator, class T>
@@ -3807,7 +3807,7 @@
// stable, 2-3 compares, 0-2 swaps
template <class _Compare, class _ForwardIterator>
-unsigned
+_LIBCPP_CONSTEXPR_AFTER_CXX11 unsigned
__sort3(_ForwardIterator __x, _ForwardIterator __y, _ForwardIterator __z, _Compare __c)
{
unsigned __r = 0;
@@ -3901,7 +3901,7 @@
// Assumes size > 0
template <class _Compare, class _BidirectionalIterator>
-void
+_LIBCPP_CONSTEXPR_AFTER_CXX11 void
__selection_sort(_BidirectionalIterator __first, _BidirectionalIterator __last, _Compare __comp)
{
_BidirectionalIterator __lm1 = __last;
@@ -5218,8 +5218,24 @@
// nth_element
+template<class _Compare, class _RandomAccessIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX11 bool
+__nth_element_find_guard(_RandomAccessIterator& __i, _RandomAccessIterator& __j,
+ _RandomAccessIterator __m, _Compare __comp)
+{
+ // manually guard downward moving __j against __i
+ while (true) {
+ if (__i == --__j) {
+ return false;
+ }
+ if (__comp(*__j, *__m)) {
+ return true; // found guard for downward moving __j, now use unguarded partition
+ }
+ }
+}
+
template <class _Compare, class _RandomAccessIterator>
-void
+_LIBCPP_CONSTEXPR_AFTER_CXX11 void
__nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
{
// _Compare is known to be a reference type
@@ -5227,7 +5243,6 @@
const difference_type __limit = 7;
while (true)
{
- __restart:
if (__nth == __last)
return;
difference_type __len = __last - __first;
@@ -5267,61 +5282,51 @@
if (!__comp(*__i, *__m)) // if *__first == *__m
{
// *__first == *__m, *__first doesn't go in first part
- // manually guard downward moving __j against __i
- while (true)
- {
- if (__i == --__j)
- {
- // *__first == *__m, *__m <= all other elements
- // Partition instead into [__first, __i) == *__first and *__first < [__i, __last)
- ++__i; // __first + 1
- __j = __last;
- if (!__comp(*__first, *--__j)) // we need a guard if *__first == *(__last-1)
- {
- while (true)
- {
- if (__i == __j)
- return; // [__first, __last) all equivalent elements
- if (__comp(*__first, *__i))
- {
- swap(*__i, *__j);
- ++__n_swaps;
- ++__i;
- break;
- }
+ if (_VSTD::__nth_element_find_guard<_Compare>(__i, __j, __m, __comp)) {
+ swap(*__i, *__j);
+ ++__n_swaps;
+ } else {
+ // *__first == *__m, *__m <= all other elements
+ // Partition instead into [__first, __i) == *__first and *__first < [__i, __last)
+ ++__i; // __first + 1
+ __j = __last;
+ if (!__comp(*__first, *--__j)) { // we need a guard if *__first == *(__last-1)
+ while (true) {
+ if (__i == __j) {
+ return; // [__first, __last) all equivalent elements
+ } else if (__comp(*__first, *__i)) {
+ swap(*__i, *__j);
+ ++__n_swaps;
++__i;
- }
- }
- // [__first, __i) == *__first and *__first < [__j, __last) and __j == __last - 1
- if (__i == __j)
- return;
- while (true)
- {
- while (!__comp(*__first, *__i))
- ++__i;
- while (__comp(*__first, *--__j))
- ;
- if (__i >= __j)
break;
- swap(*__i, *__j);
- ++__n_swaps;
+ }
++__i;
}
- // [__first, __i) == *__first and *__first < [__i, __last)
- // The first part is sorted,
- if (__nth < __i)
- return;
- // __nth_element the second part
- // _VSTD::__nth_element<_Compare>(__i, __nth, __last, __comp);
- __first = __i;
- goto __restart;
}
- if (__comp(*__j, *__m))
- {
+ // [__first, __i) == *__first and *__first < [__j, __last) and __j == __last - 1
+ if (__i == __j) {
+ return;
+ }
+ while (true) {
+ while (!__comp(*__first, *__i))
+ ++__i;
+ while (__comp(*__first, *--__j))
+ ;
+ if (__i >= __j)
+ break;
swap(*__i, *__j);
++__n_swaps;
- break; // found guard for downward moving __j, now use unguarded partition
+ ++__i;
}
+ // [__first, __i) == *__first and *__first < [__i, __last)
+ // The first part is sorted,
+ if (__nth < __i) {
+ return;
+ }
+ // __nth_element the second part
+ // _VSTD::__nth_element<_Compare>(__i, __nth, __last, __comp);
+ __first = __i;
+ continue;
}
}
++__i;
@@ -5365,32 +5370,35 @@
{
// Check for [__first, __i) already sorted
__j = __m = __first;
- while (++__j != __i)
- {
- if (__comp(*__j, *__m))
+ while (true) {
+ if (++__j == __i) {
+ // [__first, __i) sorted
+ return;
+ }
+ if (__comp(*__j, *__m)) {
// not yet sorted, so sort
- goto not_sorted;
+ break;
+ }
__m = __j;
}
- // [__first, __i) sorted
- return;
}
else
{
// Check for [__i, __last) already sorted
__j = __m = __i;
- while (++__j != __last)
- {
- if (__comp(*__j, *__m))
+ while (true) {
+ if (++__j == __last) {
+ // [__i, __last) sorted
+ return;
+ }
+ if (__comp(*__j, *__m)) {
// not yet sorted, so sort
- goto not_sorted;
+ break;
+ }
__m = __j;
}
- // [__i, __last) sorted
- return;
}
}
-not_sorted:
// __nth_element on range containing __nth
if (__nth < __i)
{
@@ -5406,7 +5414,7 @@
}
template <class _RandomAccessIterator, class _Compare>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
void
nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
{
@@ -5415,7 +5423,7 @@
}
template <class _RandomAccessIterator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
void
nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last)
{
