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// 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 <cstring>
#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().
// We have 17 bytes because there may be a generation counter. Any constant is
// fine for the generation counter.
alignas(16) ABSL_CONST_INIT ABSL_DLL const ctrl_t kEmptyGroup[17] = {
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,
static_cast<ctrl_t>(0)};
#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,
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);
common.infoz().RecordRehash(total_probe_length);
}
void EraseMetaOnly(CommonFields& c, 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);
c.growth_left() += (was_never_full ? 1 : 0);
c.infoz().RecordErase();
}
void ClearBackingArray(CommonFields& c, const PolicyFunctions& policy,
bool reuse) {
c.size_ = 0;
if (reuse) {
ResetCtrl(c, 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.set_generation_ptr(EmptyGeneration());
c.slots_ = nullptr;
c.capacity_ = 0;
c.growth_left() = 0;
c.infoz().RecordClearedReservation();
assert(c.size_ == 0);
c.infoz().RecordStorageChanged(0, 0);
}
}
} // namespace container_internal
ABSL_NAMESPACE_END
} // namespace absl
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