1/*
2 * Copyright 2016-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#pragma once
18
19#include <cstddef>
20#include <utility>
21
22#include <folly/lang/Align.h>
23
24namespace folly {
25
26/**
27 * Holds a type T, in addition to enough padding to ensure that it isn't subject
28 * to false sharing within the range used by folly.
29 *
30 * If `sizeof(T) <= alignof(T)` then the inner `T` will be entirely within one
31 * false sharing range (AKA cache line).
32 *
33 * CachelinePadded may add padding both before and after the value. Consider
34 * whether alignas(folly::hardware_destructive_interference_size) suffices.
35 */
36template <typename T>
37class CachelinePadded {
38 static_assert(
39 alignof(T) <= max_align_v,
40 "CachelinePadded does not support over-aligned types.");
41
42 public:
43 template <typename... Args>
44 explicit CachelinePadded(Args&&... args)
45 : inner_(std::forward<Args>(args)...) {}
46
47 T* get() {
48 return &inner_;
49 }
50
51 const T* get() const {
52 return &inner_;
53 }
54
55 T* operator->() {
56 return get();
57 }
58
59 const T* operator->() const {
60 return get();
61 }
62
63 T& operator*() {
64 return *get();
65 }
66
67 const T& operator*() const {
68 return *get();
69 }
70
71 private:
72 static constexpr size_t paddingSize() noexcept {
73 return hardware_destructive_interference_size -
74 (alignof(T) % hardware_destructive_interference_size);
75 }
76 char paddingPre_[paddingSize()];
77 T inner_;
78 char paddingPost_[paddingSize()];
79};
80} // namespace folly
81