[libcxx-commits] [libcxx] [libc++] Add `__find_end` optimizations back (#100685)" (PR #171374)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/171374

This essentially reverts #100685 and fixes the bidirectional and random
access specializations to be actually used.


>From 39ed63597850dc5f010074f8e75766bcfec0cc8c Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Tue, 9 Dec 2025 10:21:57 +0100
Subject: [PATCH] [libc++] Add `__find_end` optimizations back (#100685)"

This essentially reverts #100685 and fixes the bidirectional and random
access specializations to be actually used.
---
 libcxx/include/__algorithm/find_end.h | 105 ++++++++++++++++++++++++++
 1 file changed, 105 insertions(+)

diff --git a/libcxx/include/__algorithm/find_end.h b/libcxx/include/__algorithm/find_end.h
index 86b4a3e2e3689..61b7a6f07e653 100644
--- a/libcxx/include/__algorithm/find_end.h
+++ b/libcxx/include/__algorithm/find_end.h
@@ -76,6 +76,111 @@ _LIBCPP_HIDE_FROM_ABI inline _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_Iter1, _Iter1>
   }
 }
 
+template <class _AlgPolicy,
+          class _Pred,
+          class _Iter1,
+          class _Sent1,
+          class _Iter2,
+          class _Sent2,
+          class _Proj1,
+          class _Proj2>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair<_Iter1, _Iter1> __find_end_impl(
+    _Iter1 __first1,
+    _Sent1 __sent1,
+    _Iter2 __first2,
+    _Sent2 __sent2,
+    _Pred& __pred,
+    _Proj1& __proj1,
+    _Proj2& __proj2,
+    bidirectional_iterator_tag,
+    bidirectional_iterator_tag) {
+  auto __last1 = _IterOps<_AlgPolicy>::next(__first1, __sent1);
+  auto __last2 = _IterOps<_AlgPolicy>::next(__first2, __sent2);
+  // modeled after search algorithm (in reverse)
+  if (__first2 == __last2)
+    return std::make_pair(__last1, __last1); // Everything matches an empty sequence
+  _Iter1 __l1 = __last1;
+  _Iter2 __l2 = __last2;
+  --__l2;
+  while (true) {
+    // Find last element in sequence 1 that matches *(__last2-1), with a mininum of loop checks
+    while (true) {
+      if (__first1 == __l1) // return __last1 if no element matches *__first2
+        return std::make_pair(__last1, __last1);
+      if (std::__invoke(__pred, std::__invoke(__proj1, *--__l1), std::__invoke(__proj2, *__l2)))
+        break;
+    }
+    // *__l1 matches *__l2, now match elements before here
+    _Iter1 __match_last = __l1;
+    _Iter1 __m1 = __l1;
+    _Iter2 __m2 = __l2;
+    while (true) {
+      if (__m2 == __first2) // If pattern exhausted, __m1 is the answer (works for 1 element pattern)
+        return std::make_pair(__m1, ++__match_last);
+      if (__m1 == __first1) // Otherwise if source exhaused, pattern not found
+        return std::make_pair(__last1, __last1);
+
+      // if there is a mismatch, restart with a new __l1
+      if (!std::__invoke(__pred, std::__invoke(__proj1, *--__m1), std::__invoke(__proj2, *--__m2))) {
+        break;
+      } // else there is a match, check next elements
+    }
+  }
+}
+
+template <class _AlgPolicy,
+          class _Pred,
+          class _Iter1,
+          class _Sent1,
+          class _Iter2,
+          class _Sent2,
+          class _Proj1,
+          class _Proj2>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_Iter1, _Iter1> __find_end_impl(
+    _Iter1 __first1,
+    _Sent1 __sent1,
+    _Iter2 __first2,
+    _Sent2 __sent2,
+    _Pred& __pred,
+    _Proj1& __proj1,
+    _Proj2& __proj2,
+    random_access_iterator_tag,
+    random_access_iterator_tag) {
+  typedef typename iterator_traits<_Iter1>::difference_type _D1;
+  auto __last1 = _IterOps<_AlgPolicy>::next(__first1, __sent1);
+  auto __last2 = _IterOps<_AlgPolicy>::next(__first2, __sent2);
+  // Take advantage of knowing source and pattern lengths.  Stop short when source is smaller than pattern
+  auto __len2 = __last2 - __first2;
+  if (__len2 == 0)
+    return std::make_pair(__last1, __last1);
+  auto __len1 = __last1 - __first1;
+  if (__len1 < __len2)
+    return std::make_pair(__last1, __last1);
+  const _Iter1 __s = __first1 + _D1(__len2 - 1); // End of pattern match can't go before here
+  _Iter1 __l1      = __last1;
+  _Iter2 __l2      = __last2;
+  --__l2;
+  while (true) {
+    while (true) {
+      if (__s == __l1)
+        return std::make_pair(__last1, __last1);
+      if (std::__invoke(__pred, std::__invoke(__proj1, *--__l1), std::__invoke(__proj2, *__l2)))
+        break;
+    }
+    _Iter1 __last_match = __l1;
+    _Iter1 __m1         = __l1;
+    _Iter2 __m2         = __l2;
+    while (true) {
+      if (__m2 == __first2)
+        return std::make_pair(__m1, ++__last_match);
+      // no need to check range on __m1 because __s guarantees we have enough source
+      if (!std::__invoke(__pred, std::__invoke(__proj1, *--__m1), std::__invoke(__proj2, *--__m2))) {
+        break;
+      }
+    }
+  }
+}
+
 template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 [[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _ForwardIterator1 __find_end_classic(
     _ForwardIterator1 __first1,