[libc++] Optimize __hash_table::erase(iterator, iterator)

philnik777 · philnik777 · commit ff5a8ec10da8 · 2025-08-18T15:49:02.000+02:00
diff --git a/libcxx/docs/ReleaseNotes/22.rst b/libcxx/docs/ReleaseNotes/22.rst
@@ -49,6 +49,7 @@ Improvements and New Features
 - The performance of ``unordered_set::operator=(const unordered_set&)`` has been improved by up to 5x.
 - The performance of ``map::erase`` and ``set::erase`` has been improved by up to 2x
 - The performance of ``find(key)`` in ``map``, ``set``, ``multimap`` and ``multiset`` has been improved by up to 2.3x
+- The performance of ``erase(iterator, iterator)`` in the unordered containers has been improved by up to 1.9x
 
 Deprecations and Removals
 -------------------------
diff --git a/libcxx/include/__hash_table b/libcxx/include/__hash_table
@@ -1024,7 +1024,21 @@ private:
   }
   _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(__hash_table&, false_type) _NOEXCEPT {}
 
-  _LIBCPP_HIDE_FROM_ABI void __deallocate_node(__next_pointer __np) _NOEXCEPT;
+  _LIBCPP_HIDE_FROM_ABI void __deallocate_node(__node_pointer __nd) _NOEXCEPT {
+    auto& __alloc = __node_alloc();
+    __node_traits::destroy(__alloc, std::addressof(__nd->__get_value()));
+    std::__destroy_at(std::__to_address(__nd));
+    __node_traits::deallocate(__alloc, __nd, 1);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void __deallocate_node_list(__next_pointer __np) _NOEXCEPT {
+    while (__np != nullptr) {
+      __next_pointer __next = __np->__next_;
+      __deallocate_node(__np->__upcast());
+      __np = __next;
+    }
+  }
+
   _LIBCPP_HIDE_FROM_ABI __next_pointer __detach() _NOEXCEPT;
 
   template <class _From, class _ValueT = _Tp, __enable_if_t<__is_hash_value_type<_ValueT>::value, int> = 0>
@@ -1162,7 +1176,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::~__hash_table() {
   static_assert(is_copy_constructible<hasher>::value, "Hasher must be copy-constructible.");
 #endif
 
-  __deallocate_node(__first_node_.__next_);
+  __deallocate_node_list(__first_node_.__next_);
 }
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
@@ -1238,7 +1252,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::operator=(const __hash_table& __other)
   // At this point we either have consumed the whole incoming hash table, or we don't have any more nodes to reuse in
   // the destination. Either continue with constructing new nodes, or deallocate the left over nodes.
   if (__own_iter->__next_) {
-    __deallocate_node(__own_iter->__next_);
+    __deallocate_node_list(__own_iter->__next_);
     __own_iter->__next_ = nullptr;
   } else {
     __copy_construct(__other_iter, __own_iter, __current_chash);
@@ -1249,19 +1263,6 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::operator=(const __hash_table& __other)
   return *this;
 }
 
-template <class _Tp, class _Hash, class _Equal, class _Alloc>
-void __hash_table<_Tp, _Hash, _Equal, _Alloc>::__deallocate_node(__next_pointer __np) _NOEXCEPT {
-  __node_allocator& __na = __node_alloc();
-  while (__np != nullptr) {
-    __next_pointer __next    = __np->__next_;
-    __node_pointer __real_np = __np->__upcast();
-    __node_traits::destroy(__na, std::addressof(__real_np->__get_value()));
-    std::__destroy_at(std::addressof(*__real_np));
-    __node_traits::deallocate(__na, __real_np, 1);
-    __np = __next;
-  }
-}
-
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
 typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::__next_pointer
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::__detach() _NOEXCEPT {
@@ -1317,11 +1318,11 @@ void __hash_table<_Tp, _Hash, _Equal, _Alloc>::__move_assign(__hash_table& __u,
         }
 #if _LIBCPP_HAS_EXCEPTIONS
       } catch (...) {
-        __deallocate_node(__cache);
+        __deallocate_node_list(__cache);
         throw;
       }
 #endif // _LIBCPP_HAS_EXCEPTIONS
-      __deallocate_node(__cache);
+      __deallocate_node_list(__cache);
     }
     const_iterator __i = __u.begin();
     while (__u.size() != 0)
@@ -1360,11 +1361,11 @@ void __hash_table<_Tp, _Hash, _Equal, _Alloc>::__assign_unique(_InputIterator __
       }
 #if _LIBCPP_HAS_EXCEPTIONS
     } catch (...) {
-      __deallocate_node(__cache);
+      __deallocate_node_list(__cache);
       throw;
     }
 #endif // _LIBCPP_HAS_EXCEPTIONS
-    __deallocate_node(__cache);
+    __deallocate_node_list(__cache);
   }
   for (; __first != __last; ++__first)
     __emplace_unique(*__first);
@@ -1390,11 +1391,11 @@ void __hash_table<_Tp, _Hash, _Equal, _Alloc>::__assign_multi(_InputIterator __f
       }
 #if _LIBCPP_HAS_EXCEPTIONS
     } catch (...) {
-      __deallocate_node(__cache);
+      __deallocate_node_list(__cache);
       throw;
     }
 #endif // _LIBCPP_HAS_EXCEPTIONS
-    __deallocate_node(__cache);
+    __deallocate_node_list(__cache);
   }
   for (; __first != __last; ++__first)
     __emplace_multi(*__first);
@@ -1427,7 +1428,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::end() const _NOEXCEPT {
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
 void __hash_table<_Tp, _Hash, _Equal, _Alloc>::clear() _NOEXCEPT {
   if (size() > 0) {
-    __deallocate_node(__first_node_.__next_);
+    __deallocate_node_list(__first_node_.__next_);
     __first_node_.__next_ = nullptr;
     size_type __bc        = bucket_count();
     for (size_type __i = 0; __i < __bc; ++__i)
@@ -1955,12 +1956,57 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::erase(const_iterator __p) {
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
 typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::erase(const_iterator __first, const_iterator __last) {
-  for (const_iterator __p = __first; __first != __last; __p = __first) {
-    ++__first;
-    erase(__p);
+  if (__first == __last)
+    return iterator(__last.__node_);
+
+  // current node
+  __next_pointer __current = __first.__node_;
+  size_type __bucket_count = bucket_count();
+  size_t __chash           = std::__constrain_hash(__current->__hash(), __bucket_count);
+  // find previous node
+  __next_pointer __before_first = __bucket_list_[__chash];
+  for (; __before_first->__next_ != __current; __before_first = __before_first->__next_)
+    ;
+
+  __next_pointer __last_node = __last.__node_;
+
+  // If __before_first is in the same bucket (i.e. the first element we erase is not the first in the bucket), clear
+  // this bucket first without re-linking it
+  if (__before_first != __first_node_.__ptr() &&
+      std::__constrain_hash(__before_first->__hash(), __bucket_count) == __chash) {
+    while (__current != __last_node) {
+      if (auto __next_chash = std::__constrain_hash(__current->__hash(), __bucket_count); __next_chash != __chash) {
+        __chash = __next_chash;
+        break;
+      }
+      auto __next = __current->__next_;
+      __deallocate_node(__current->__upcast());
+      __current = __next;
+      --__size_;
+    }
   }
-  __next_pointer __np = __last.__node_;
-  return iterator(__np);
+
+  while (__current != __last_node) {
+    auto __next = __current->__next_;
+    __deallocate_node(__current->__upcast());
+    __current = __next;
+    --__size_;
+
+    // When switching buckets, set the old bucket to be empty and update the next bucket to have __before_first as its
+    // before-first element
+    if (__next) {
+      if (auto __next_chash = std::__constrain_hash(__next->__hash(), __bucket_count); __next_chash != __chash) {
+        __bucket_list_[__chash] = nullptr;
+        __chash                 = __next_chash;
+        __bucket_list_[__chash] = __before_first;
+      }
+    }
+  }
+
+  // re-link __before_first with __last
+  __before_first->__next_ = __current;
+
+  return iterator(__last.__node_);
 }
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
diff --git a/libcxx/test/std/containers/unord/unord.multimap/unord.multimap.modifiers/erase_range.pass.cpp b/libcxx/test/std/containers/unord/unord.multimap/unord.multimap.modifiers/erase_range.pass.cpp
@@ -91,6 +91,20 @@ int main(int, char**) {
     assert(c.size() == 0);
     assert(k == c.end());
   }
+  { // Make sure we're properly destroying the elements when erasing
+    { // When erasing part of a bucket
+      std::unordered_multimap<int, std::string> map;
+      map.insert(std::make_pair(1, "This is a long string to make sure ASan can detect a memory leak"));
+      map.insert(std::make_pair(1, "This is another long string to make sure ASan can detect a memory leak"));
+      map.erase(++map.begin(), map.end());
+    }
+    { // When erasing the whole bucket
+      std::unordered_multimap<int, std::string> map;
+      map.insert(std::make_pair(1, "This is a long string to make sure ASan can detect a memory leak"));
+      map.insert(std::make_pair(1, "This is another long string to make sure ASan can detect a memory leak"));
+      map.erase(map.begin(), map.end());
+    }
+  }
 #if TEST_STD_VER >= 11
   {
     typedef std::unordered_multimap<int,
