mfbt/Alignment.h@97036ab72558 (annotated)
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.
| michael@0 | 1 | /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ |
| michael@0 | 2 | /* vim: set ts=8 sts=2 et sw=2 tw=80: */ |
| michael@0 | 3 | /* This Source Code Form is subject to the terms of the Mozilla Public |
| michael@0 | 4 | * License, v. 2.0. If a copy of the MPL was not distributed with this |
| michael@0 | 5 | * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ |
| michael@0 | 6 | |
| michael@0 | 7 | /* Functionality related to memory alignment. */ |
| michael@0 | 8 | |
| michael@0 | 9 | #ifndef mozilla_Alignment_h |
| michael@0 | 10 | #define mozilla_Alignment_h |
| michael@0 | 11 | |
| michael@0 | 12 | #include <stddef.h> |
| michael@0 | 13 | #include <stdint.h> |
| michael@0 | 14 | |
| michael@0 | 15 | namespace mozilla { |
| michael@0 | 16 | |
| michael@0 | 17 | /* |
| michael@0 | 18 | * This class, and the corresponding macro MOZ_ALIGNOF, figures out how many |
| michael@0 | 19 | * bytes of alignment a given type needs. |
| michael@0 | 20 | */ |
| michael@0 | 21 | template<typename T> |
| michael@0 | 22 | class AlignmentFinder |
| michael@0 | 23 | { |
| michael@0 | 24 | struct Aligner |
| michael@0 | 25 | { |
| michael@0 | 26 | char c; |
| michael@0 | 27 | T t; |
| michael@0 | 28 | }; |
| michael@0 | 29 | |
| michael@0 | 30 | public: |
| michael@0 | 31 | static const size_t alignment = sizeof(Aligner) - sizeof(T); |
| michael@0 | 32 | }; |
| michael@0 | 33 | |
| michael@0 | 34 | #define MOZ_ALIGNOF(T) mozilla::AlignmentFinder<T>::alignment |
| michael@0 | 35 | |
| michael@0 | 36 | /* |
| michael@0 | 37 | * Declare the MOZ_ALIGNED_DECL macro for declaring aligned types. |
| michael@0 | 38 | * |
| michael@0 | 39 | * For instance, |
| michael@0 | 40 | * |
| michael@0 | 41 | * MOZ_ALIGNED_DECL(char arr[2], 8); |
| michael@0 | 42 | * |
| michael@0 | 43 | * will declare a two-character array |arr| aligned to 8 bytes. |
| michael@0 | 44 | */ |
| michael@0 | 45 | |
| michael@0 | 46 | #if defined(__GNUC__) |
| michael@0 | 47 | # define MOZ_ALIGNED_DECL(_type, _align) \ |
| michael@0 | 48 | _type __attribute__((aligned(_align))) |
| michael@0 | 49 | #elif defined(_MSC_VER) |
| michael@0 | 50 | # define MOZ_ALIGNED_DECL(_type, _align) \ |
| michael@0 | 51 | __declspec(align(_align)) _type |
| michael@0 | 52 | #else |
| michael@0 | 53 | # warning "We don't know how to align variables on this compiler." |
| michael@0 | 54 | # define MOZ_ALIGNED_DECL(_type, _align) _type |
| michael@0 | 55 | #endif |
| michael@0 | 56 | |
| michael@0 | 57 | /* |
| michael@0 | 58 | * AlignedElem<N> is a structure whose alignment is guaranteed to be at least N |
| michael@0 | 59 | * bytes. |
| michael@0 | 60 | * |
| michael@0 | 61 | * We support 1, 2, 4, 8, and 16-bit alignment. |
| michael@0 | 62 | */ |
| michael@0 | 63 | template<size_t Align> |
| michael@0 | 64 | struct AlignedElem; |
| michael@0 | 65 | |
| michael@0 | 66 | /* |
| michael@0 | 67 | * We have to specialize this template because GCC doesn't like __attribute__((aligned(foo))) where |
| michael@0 | 68 | * foo is a template parameter. |
| michael@0 | 69 | */ |
| michael@0 | 70 | |
| michael@0 | 71 | template<> |
| michael@0 | 72 | struct AlignedElem<1> |
| michael@0 | 73 | { |
| michael@0 | 74 | MOZ_ALIGNED_DECL(uint8_t elem, 1); |
| michael@0 | 75 | }; |
| michael@0 | 76 | |
| michael@0 | 77 | template<> |
| michael@0 | 78 | struct AlignedElem<2> |
| michael@0 | 79 | { |
| michael@0 | 80 | MOZ_ALIGNED_DECL(uint8_t elem, 2); |
| michael@0 | 81 | }; |
| michael@0 | 82 | |
| michael@0 | 83 | template<> |
| michael@0 | 84 | struct AlignedElem<4> |
| michael@0 | 85 | { |
| michael@0 | 86 | MOZ_ALIGNED_DECL(uint8_t elem, 4); |
| michael@0 | 87 | }; |
| michael@0 | 88 | |
| michael@0 | 89 | template<> |
| michael@0 | 90 | struct AlignedElem<8> |
| michael@0 | 91 | { |
| michael@0 | 92 | MOZ_ALIGNED_DECL(uint8_t elem, 8); |
| michael@0 | 93 | }; |
| michael@0 | 94 | |
| michael@0 | 95 | template<> |
| michael@0 | 96 | struct AlignedElem<16> |
| michael@0 | 97 | { |
| michael@0 | 98 | MOZ_ALIGNED_DECL(uint8_t elem, 16); |
| michael@0 | 99 | }; |
| michael@0 | 100 | |
| michael@0 | 101 | /* |
| michael@0 | 102 | * This utility pales in comparison to Boost's aligned_storage. The utility |
| michael@0 | 103 | * simply assumes that uint64_t is enough alignment for anyone. This may need |
| michael@0 | 104 | * to be extended one day... |
| michael@0 | 105 | * |
| michael@0 | 106 | * As an important side effect, pulling the storage into this template is |
| michael@0 | 107 | * enough obfuscation to confuse gcc's strict-aliasing analysis into not giving |
| michael@0 | 108 | * false negatives when we cast from the char buffer to whatever type we've |
| michael@0 | 109 | * constructed using the bytes. |
| michael@0 | 110 | */ |
| michael@0 | 111 | template<size_t Nbytes> |
| michael@0 | 112 | struct AlignedStorage |
| michael@0 | 113 | { |
| michael@0 | 114 | union U { |
| michael@0 | 115 | char bytes[Nbytes]; |
| michael@0 | 116 | uint64_t _; |
| michael@0 | 117 | } u; |
| michael@0 | 118 | |
| michael@0 | 119 | const void* addr() const { return u.bytes; } |
| michael@0 | 120 | void* addr() { return u.bytes; } |
| michael@0 | 121 | }; |
| michael@0 | 122 | |
| michael@0 | 123 | template<typename T> |
| michael@0 | 124 | struct AlignedStorage2 |
| michael@0 | 125 | { |
| michael@0 | 126 | union U { |
| michael@0 | 127 | char bytes[sizeof(T)]; |
| michael@0 | 128 | uint64_t _; |
| michael@0 | 129 | } u; |
| michael@0 | 130 | |
| michael@0 | 131 | const T* addr() const { return reinterpret_cast<const T*>(u.bytes); } |
| michael@0 | 132 | T* addr() { return static_cast<T*>(static_cast<void*>(u.bytes)); } |
| michael@0 | 133 | }; |
| michael@0 | 134 | |
| michael@0 | 135 | } /* namespace mozilla */ |
| michael@0 | 136 | |
| michael@0 | 137 | #endif /* mozilla_Alignment_h */ |
