mfbt/Alignment.h@97036ab72558
mfbt/Alignment.h
Tue, 06 Jan 2015 21:39:09 +0100
- author
- Michael Schloh von Bennewitz <michael@schloh.com>
- date
- Tue, 06 Jan 2015 21:39:09 +0100
- branch
- TOR_BUG_9701
- changeset 8
- 97036ab72558
- permissions
- -rw-r--r--
Conditionally force memory storage according to privacy.thirdparty.isolate;
This solves Tor bug #9701, complying with disk avoidance documented in
https://www.torproject.org/projects/torbrowser/design/#disk-avoidance.
1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
3 /* This Source Code Form is subject to the terms of the Mozilla Public
4 * License, v. 2.0. If a copy of the MPL was not distributed with this
5 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
7 /* Functionality related to memory alignment. */
9 #ifndef mozilla_Alignment_h
10 #define mozilla_Alignment_h
12 #include <stddef.h>
13 #include <stdint.h>
15 namespace mozilla {
17 /*
18 * This class, and the corresponding macro MOZ_ALIGNOF, figures out how many
19 * bytes of alignment a given type needs.
20 */
21 template<typename T>
22 class AlignmentFinder
23 {
24 struct Aligner
25 {
26 char c;
27 T t;
28 };
30 public:
31 static const size_t alignment = sizeof(Aligner) - sizeof(T);
32 };
34 #define MOZ_ALIGNOF(T) mozilla::AlignmentFinder<T>::alignment
36 /*
37 * Declare the MOZ_ALIGNED_DECL macro for declaring aligned types.
38 *
39 * For instance,
40 *
41 * MOZ_ALIGNED_DECL(char arr[2], 8);
42 *
43 * will declare a two-character array |arr| aligned to 8 bytes.
44 */
46 #if defined(__GNUC__)
47 # define MOZ_ALIGNED_DECL(_type, _align) \
48 _type __attribute__((aligned(_align)))
49 #elif defined(_MSC_VER)
50 # define MOZ_ALIGNED_DECL(_type, _align) \
51 __declspec(align(_align)) _type
52 #else
53 # warning "We don't know how to align variables on this compiler."
54 # define MOZ_ALIGNED_DECL(_type, _align) _type
55 #endif
57 /*
58 * AlignedElem<N> is a structure whose alignment is guaranteed to be at least N
59 * bytes.
60 *
61 * We support 1, 2, 4, 8, and 16-bit alignment.
62 */
63 template<size_t Align>
64 struct AlignedElem;
66 /*
67 * We have to specialize this template because GCC doesn't like __attribute__((aligned(foo))) where
68 * foo is a template parameter.
69 */
71 template<>
72 struct AlignedElem<1>
73 {
74 MOZ_ALIGNED_DECL(uint8_t elem, 1);
75 };
77 template<>
78 struct AlignedElem<2>
79 {
80 MOZ_ALIGNED_DECL(uint8_t elem, 2);
81 };
83 template<>
84 struct AlignedElem<4>
85 {
86 MOZ_ALIGNED_DECL(uint8_t elem, 4);
87 };
89 template<>
90 struct AlignedElem<8>
91 {
92 MOZ_ALIGNED_DECL(uint8_t elem, 8);
93 };
95 template<>
96 struct AlignedElem<16>
97 {
98 MOZ_ALIGNED_DECL(uint8_t elem, 16);
99 };
101 /*
102 * This utility pales in comparison to Boost's aligned_storage. The utility
103 * simply assumes that uint64_t is enough alignment for anyone. This may need
104 * to be extended one day...
105 *
106 * As an important side effect, pulling the storage into this template is
107 * enough obfuscation to confuse gcc's strict-aliasing analysis into not giving
108 * false negatives when we cast from the char buffer to whatever type we've
109 * constructed using the bytes.
110 */
111 template<size_t Nbytes>
112 struct AlignedStorage
113 {
114 union U {
115 char bytes[Nbytes];
116 uint64_t _;
117 } u;
119 const void* addr() const { return u.bytes; }
120 void* addr() { return u.bytes; }
121 };
123 template<typename T>
124 struct AlignedStorage2
125 {
126 union U {
127 char bytes[sizeof(T)];
128 uint64_t _;
129 } u;
131 const T* addr() const { return reinterpret_cast<const T*>(u.bytes); }
132 T* addr() { return static_cast<T*>(static_cast<void*>(u.bytes)); }
133 };
135 } /* namespace mozilla */
137 #endif /* mozilla_Alignment_h */
