[libcxx-commits] [libcxx] [libc++] Optimize make_heap() and sift_down() (PR #121480)
Yang Kun via libcxx-commits
libcxx-commits at lists.llvm.org
Sun Jan 19 03:59:56 PST 2025
https://github.com/omikrun updated https://github.com/llvm/llvm-project/pull/121480
>From 1b61da143d85fb3dc26007fee338957059c497ba Mon Sep 17 00:00:00 2001
From: Yang Kun <193369907+omikrun at users.noreply.github.com>
Date: Sat, 4 Jan 2025 19:44:44 +0800
Subject: [PATCH] [libc++] Optimize make_heap() and sift_down()
---
libcxx/include/__algorithm/make_heap.h | 8 ++--
libcxx/include/__algorithm/pop_heap.h | 3 +-
libcxx/include/__algorithm/push_heap.h | 16 +++----
libcxx/include/__algorithm/sift_down.h | 64 +++++++++++++++-----------
4 files changed, 50 insertions(+), 41 deletions(-)
diff --git a/libcxx/include/__algorithm/make_heap.h b/libcxx/include/__algorithm/make_heap.h
index e8f0cdb27333a4..746c2621fa70e6 100644
--- a/libcxx/include/__algorithm/make_heap.h
+++ b/libcxx/include/__algorithm/make_heap.h
@@ -34,10 +34,12 @@ __make_heap(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compar
using difference_type = typename iterator_traits<_RandomAccessIterator>::difference_type;
difference_type __n = __last - __first;
if (__n > 1) {
- // start from the first parent, there is no need to consider children
- for (difference_type __start = (__n - 2) / 2; __start >= 0; --__start) {
+ difference_type __start = __n / 2;
+ do {
+ // start from the first parent, there is no need to consider children
+ --__start;
std::__sift_down<_AlgPolicy>(__first, __comp_ref, __n, __first + __start);
- }
+ } while (__start != 0);
}
}
diff --git a/libcxx/include/__algorithm/pop_heap.h b/libcxx/include/__algorithm/pop_heap.h
index 6d23830097ff96..60481938c4bc57 100644
--- a/libcxx/include/__algorithm/pop_heap.h
+++ b/libcxx/include/__algorithm/pop_heap.h
@@ -51,9 +51,8 @@ __pop_heap(_RandomAccessIterator __first,
*__hole = std::move(__top);
} else {
*__hole = _IterOps<_AlgPolicy>::__iter_move(__last);
- ++__hole;
*__last = std::move(__top);
- std::__sift_up<_AlgPolicy>(__first, __hole, __comp_ref, __hole - __first);
+ std::__sift_up<_AlgPolicy>(__first, __hole, __comp_ref);
}
}
}
diff --git a/libcxx/include/__algorithm/push_heap.h b/libcxx/include/__algorithm/push_heap.h
index ec0b445f2b70f3..3a1f95c8651ad6 100644
--- a/libcxx/include/__algorithm/push_heap.h
+++ b/libcxx/include/__algorithm/push_heap.h
@@ -29,17 +29,16 @@ _LIBCPP_BEGIN_NAMESPACE_STD
template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
-__sift_up(_RandomAccessIterator __first,
- _RandomAccessIterator __last,
- _Compare&& __comp,
- typename iterator_traits<_RandomAccessIterator>::difference_type __len) {
- using value_type = typename iterator_traits<_RandomAccessIterator>::value_type;
+__sift_up(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare&& __comp) {
+ using difference_type = typename iterator_traits<_RandomAccessIterator>::difference_type;
+ using value_type = typename iterator_traits<_RandomAccessIterator>::value_type;
+ difference_type __len = __last - __first;
if (__len > 1) {
- __len = (__len - 2) / 2;
+ __len = (__len - 1) / 2;
_RandomAccessIterator __ptr = __first + __len;
- if (__comp(*__ptr, *--__last)) {
+ if (__comp(*__ptr, *__last)) {
value_type __t(_IterOps<_AlgPolicy>::__iter_move(__last));
do {
*__last = _IterOps<_AlgPolicy>::__iter_move(__ptr);
@@ -58,8 +57,7 @@ __sift_up(_RandomAccessIterator __first,
template <class _AlgPolicy, class _RandomAccessIterator, class _Compare>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
__push_heap(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare& __comp) {
- typename iterator_traits<_RandomAccessIterator>::difference_type __len = __last - __first;
- std::__sift_up<_AlgPolicy, __comp_ref_type<_Compare> >(std::move(__first), std::move(__last), __comp, __len);
+ std::__sift_up<_AlgPolicy, __comp_ref_type<_Compare> >(std::move(__first), std::move(--__last), __comp);
}
template <class _RandomAccessIterator, class _Compare>
diff --git a/libcxx/include/__algorithm/sift_down.h b/libcxx/include/__algorithm/sift_down.h
index 42803e30631fb1..0cd5aa32556f3d 100644
--- a/libcxx/include/__algorithm/sift_down.h
+++ b/libcxx/include/__algorithm/sift_down.h
@@ -34,20 +34,22 @@ __sift_down(_RandomAccessIterator __first,
typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
- // left-child of __start is at 2 * __start + 1
- // right-child of __start is at 2 * __start + 2
- difference_type __child = __start - __first;
- if (__len < 2 || (__len - 2) / 2 < __child)
+ if (__len < 2)
return;
- __child = 2 * __child + 1;
+ // left-child of __start is at 2 * __start + 1
+ // right-child of __start is at 2 * __start + 2
+ difference_type __child = 2 * (__start - __first) + 1;
_RandomAccessIterator __child_i = __first + __child;
- if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
- // right-child exists and is greater than left-child
- ++__child_i;
- ++__child;
+ if ((__child + 1) < __len) {
+ _RandomAccessIterator __right_i = _Ops::next(__child_i);
+ if (__comp(*__child_i, *__right_i)) {
+ // right-child exists and is greater than left-child
+ __child_i = __right_i;
+ ++__child;
+ }
}
// check if we are in heap-order
@@ -68,10 +70,13 @@ __sift_down(_RandomAccessIterator __first,
__child = 2 * __child + 1;
__child_i = __first + __child;
- if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
- // right-child exists and is greater than left-child
- ++__child_i;
- ++__child;
+ if ((__child + 1) < __len) {
+ _RandomAccessIterator __right_i = _Ops::next(__child_i);
+ if (__comp(*__child_i, *__right_i)) {
+ // right-child exists and is greater than left-child
+ __child_i = __right_i;
+ ++__child;
+ }
}
// check if we are in heap-order
@@ -84,29 +89,34 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _RandomAccessIterator __floy
_RandomAccessIterator __first,
_Compare&& __comp,
typename iterator_traits<_RandomAccessIterator>::difference_type __len) {
- using difference_type = typename iterator_traits<_RandomAccessIterator>::difference_type;
- _LIBCPP_ASSERT_INTERNAL(__len >= 2, "shouldn't be called unless __len >= 2");
+ _LIBCPP_ASSERT_INTERNAL(__len > 1, "shouldn't be called unless __len > 1");
- _RandomAccessIterator __hole = __first;
- _RandomAccessIterator __child_i = __first;
- difference_type __child = 0;
+ using _Ops = _IterOps<_AlgPolicy>;
- while (true) {
- __child_i += difference_type(__child + 1);
- __child = 2 * __child + 1;
+ typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
- if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
- // right-child exists and is greater than left-child
- ++__child_i;
- ++__child;
+ difference_type __child = 1;
+ _RandomAccessIterator __hole = __first, __child_i = __first;
+
+ while (true) {
+ __child_i += __child;
+ __child *= 2;
+
+ if (__child < __len) {
+ _RandomAccessIterator __right_i = _Ops::next(__child_i);
+ if (__comp(*__child_i, *__right_i)) {
+ // right-child exists and is greater than left-child
+ __child_i = __right_i;
+ ++__child;
+ }
}
// swap __hole with its largest child
- *__hole = _IterOps<_AlgPolicy>::__iter_move(__child_i);
+ *__hole = _Ops::__iter_move(__child_i);
__hole = __child_i;
// if __hole is now a leaf, we're done
- if (__child > (__len - 2) / 2)
+ if (__child > __len / 2)
return __hole;
}
}
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