absl/container/internal/raw_hash_set.cc

// Copyright 2018 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "absl/container/internal/raw_hash_set.h"

#include <atomic>
#include <cstddef>

#include "absl/base/config.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace container_internal {

// A single block of empty control bytes for tables without any slots allocated.
// This enables removing a branch in the hot path of find().
alignas(16) ABSL_CONST_INIT ABSL_DLL const ctrl_t kEmptyGroup[16] = {
    ctrl_t::kSentinel, ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
    ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
    ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
    ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty};

#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
constexpr size_t Group::kWidth;
#endif

// Returns "random" seed.
inline size_t RandomSeed() {
#ifdef ABSL_HAVE_THREAD_LOCAL
  static thread_local size_t counter = 0;
  size_t value = ++counter;
#else   // ABSL_HAVE_THREAD_LOCAL
  static std::atomic<size_t> counter(0);
  size_t value = counter.fetch_add(1, std::memory_order_relaxed);
#endif  // ABSL_HAVE_THREAD_LOCAL
  return value ^ static_cast<size_t>(reinterpret_cast<uintptr_t>(&counter));
}

bool ShouldInsertBackwards(size_t hash, const ctrl_t* ctrl) {
  // To avoid problems with weak hashes and single bit tests, we use % 13.
  // TODO(kfm,sbenza): revisit after we do unconditional mixing
  return (H1(hash, ctrl) ^ RandomSeed()) % 13 > 6;
}

void ConvertDeletedToEmptyAndFullToDeleted(ctrl_t* ctrl, size_t capacity) {
  assert(ctrl[capacity] == ctrl_t::kSentinel);
  assert(IsValidCapacity(capacity));
  for (ctrl_t* pos = ctrl; pos < ctrl + capacity; pos += Group::kWidth) {
    Group{pos}.ConvertSpecialToEmptyAndFullToDeleted(pos);
  }
  // Copy the cloned ctrl bytes.
  std::memcpy(ctrl + capacity + 1, ctrl, NumClonedBytes());
  ctrl[capacity] = ctrl_t::kSentinel;
}
// Extern template instantiation for inline function.
template FindInfo find_first_non_full(const CommonFields&, size_t);

FindInfo find_first_non_full_outofline(const CommonFields& common,
                                       size_t hash) {
  return find_first_non_full(common, hash);
}

// Return address of the ith slot in slots where each slot occupies slot_size.
static inline void* SlotAddress(void* slot_array, size_t slot,
                                size_t slot_size) {
  return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(slot_array) +
                                 (slot * slot_size));
}

// Return the address of the slot just after slot assuming each slot
// has the specified size.
static inline void* NextSlot(void* slot, size_t slot_size) {
  return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(slot) + slot_size);
}

// Return the address of the slot just before slot assuming each slot
// has the specified size.
static inline void* PrevSlot(void* slot, size_t slot_size) {
  return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(slot) - slot_size);
}

void DropDeletesWithoutResize(CommonFields& common, size_t& growth_left,
                              const PolicyFunctions& policy, void* tmp_space) {
  void* set = &common;
  void* slot_array = common.slots_;
  const size_t capacity = common.capacity_;
  assert(IsValidCapacity(capacity));
  assert(!is_small(capacity));
  // Algorithm:
  // - mark all DELETED slots as EMPTY
  // - mark all FULL slots as DELETED
  // - for each slot marked as DELETED
  //     hash = Hash(element)
  //     target = find_first_non_full(hash)
  //     if target is in the same group
  //       mark slot as FULL
  //     else if target is EMPTY
  //       transfer element to target
  //       mark slot as EMPTY
  //       mark target as FULL
  //     else if target is DELETED
  //       swap current element with target element
  //       mark target as FULL
  //       repeat procedure for current slot with moved from element (target)
  ctrl_t* ctrl = common.control_;
  ConvertDeletedToEmptyAndFullToDeleted(ctrl, capacity);
  auto hasher = policy.hash_slot;
  auto transfer = policy.transfer;
  const size_t slot_size = policy.slot_size;

  size_t total_probe_length = 0;
  void* slot_ptr = SlotAddress(slot_array, 0, slot_size);
  for (size_t i = 0; i != capacity;
       ++i, slot_ptr = NextSlot(slot_ptr, slot_size)) {
    assert(slot_ptr == SlotAddress(slot_array, i, slot_size));
    if (!IsDeleted(ctrl[i])) continue;
    const size_t hash = (*hasher)(set, slot_ptr);
    const FindInfo target = find_first_non_full(common, hash);
    const size_t new_i = target.offset;
    total_probe_length += target.probe_length;

    // Verify if the old and new i fall within the same group wrt the hash.
    // If they do, we don't need to move the object as it falls already in the
    // best probe we can.
    const size_t probe_offset = probe(common, hash).offset();
    const auto probe_index = [probe_offset, capacity](size_t pos) {
      return ((pos - probe_offset) & capacity) / Group::kWidth;
    };

    // Element doesn't move.
    if (ABSL_PREDICT_TRUE(probe_index(new_i) == probe_index(i))) {
      SetCtrl(common, i, H2(hash), slot_size);
      continue;
    }

    void* new_slot_ptr = SlotAddress(slot_array, new_i, slot_size);
    if (IsEmpty(ctrl[new_i])) {
      // Transfer element to the empty spot.
      // SetCtrl poisons/unpoisons the slots so we have to call it at the
      // right time.
      SetCtrl(common, new_i, H2(hash), slot_size);
      (*transfer)(set, new_slot_ptr, slot_ptr);
      SetCtrl(common, i, ctrl_t::kEmpty, slot_size);
    } else {
      assert(IsDeleted(ctrl[new_i]));
      SetCtrl(common, new_i, H2(hash), slot_size);
      // Until we are done rehashing, DELETED marks previously FULL slots.

      // Swap i and new_i elements.
      (*transfer)(set, tmp_space, new_slot_ptr);
      (*transfer)(set, new_slot_ptr, slot_ptr);
      (*transfer)(set, slot_ptr, tmp_space);

      // repeat the processing of the ith slot
      --i;
      slot_ptr = PrevSlot(slot_ptr, slot_size);
    }
  }
  ResetGrowthLeft(common, growth_left);
  common.infoz().RecordRehash(total_probe_length);
}

void EraseMetaOnly(CommonFields& c, size_t& growth_left, ctrl_t* it,
                   size_t slot_size) {
  assert(IsFull(*it) && "erasing a dangling iterator");
  --c.size_;
  const auto index = static_cast<size_t>(it - c.control_);
  const size_t index_before = (index - Group::kWidth) & c.capacity_;
  const auto empty_after = Group(it).MaskEmpty();
  const auto empty_before = Group(c.control_ + index_before).MaskEmpty();

  // We count how many consecutive non empties we have to the right and to the
  // left of `it`. If the sum is >= kWidth then there is at least one probe
  // window that might have seen a full group.
  bool was_never_full =
      empty_before && empty_after &&
      static_cast<size_t>(empty_after.TrailingZeros() +
                          empty_before.LeadingZeros()) < Group::kWidth;

  SetCtrl(c, index, was_never_full ? ctrl_t::kEmpty : ctrl_t::kDeleted,
          slot_size);
  growth_left += (was_never_full ? 1 : 0);
  c.infoz().RecordErase();
}

void ClearBackingArray(CommonFields& c, size_t& growth_left,
                       const PolicyFunctions& policy, bool reuse) {
  if (reuse) {
    c.size_ = 0;
    ResetCtrl(c, growth_left, policy.slot_size);
    c.infoz().RecordStorageChanged(0, c.capacity_);
  } else {
    void* set = &c;
    (*policy.dealloc)(set, policy, c.control_, c.slots_, c.capacity_);
    c.control_ = EmptyGroup();
    c.slots_ = nullptr;
    c.size_ = 0;
    c.capacity_ = 0;
    growth_left = 0;
    c.infoz().RecordClearedReservation();
    assert(c.size_ == 0);
    c.infoz().RecordStorageChanged(0, 0);
  }
}

}  // namespace container_internal
ABSL_NAMESPACE_END
}  // namespace absl