mfbt/TemplateLib.h@97036ab72558 (annotated)
mfbt/TemplateLib.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 | /* |
| michael@0 | 8 | * Reusable template meta-functions on types and compile-time values. Meta- |
| michael@0 | 9 | * functions are placed inside the 'tl' namespace to avoid conflict with non- |
| michael@0 | 10 | * meta functions of the same name (e.g., mozilla::tl::FloorLog2 vs. |
| michael@0 | 11 | * mozilla::FloorLog2). |
| michael@0 | 12 | * |
| michael@0 | 13 | * When constexpr support becomes universal, we should probably use that instead |
| michael@0 | 14 | * of some of these templates, for simplicity. |
| michael@0 | 15 | */ |
| michael@0 | 16 | |
| michael@0 | 17 | #ifndef mozilla_TemplateLib_h |
| michael@0 | 18 | #define mozilla_TemplateLib_h |
| michael@0 | 19 | |
| michael@0 | 20 | #include <limits.h> |
| michael@0 | 21 | #include <stddef.h> |
| michael@0 | 22 | |
| michael@0 | 23 | namespace mozilla { |
| michael@0 | 24 | |
| michael@0 | 25 | namespace tl { |
| michael@0 | 26 | |
| michael@0 | 27 | /** Compute min/max. */ |
| michael@0 | 28 | template<size_t I, size_t J> |
| michael@0 | 29 | struct Min |
| michael@0 | 30 | { |
| michael@0 | 31 | static const size_t value = I < J ? I : J; |
| michael@0 | 32 | }; |
| michael@0 | 33 | template<size_t I, size_t J> |
| michael@0 | 34 | struct Max |
| michael@0 | 35 | { |
| michael@0 | 36 | static const size_t value = I > J ? I : J; |
| michael@0 | 37 | }; |
| michael@0 | 38 | |
| michael@0 | 39 | /** Compute floor(log2(i)). */ |
| michael@0 | 40 | template<size_t I> |
| michael@0 | 41 | struct FloorLog2 |
| michael@0 | 42 | { |
| michael@0 | 43 | static const size_t value = 1 + FloorLog2<I / 2>::value; |
| michael@0 | 44 | }; |
| michael@0 | 45 | template<> struct FloorLog2<0> { /* Error */ }; |
| michael@0 | 46 | template<> struct FloorLog2<1> { static const size_t value = 0; }; |
| michael@0 | 47 | |
| michael@0 | 48 | /** Compute ceiling(log2(i)). */ |
| michael@0 | 49 | template<size_t I> |
| michael@0 | 50 | struct CeilingLog2 |
| michael@0 | 51 | { |
| michael@0 | 52 | static const size_t value = FloorLog2<2 * I - 1>::value; |
| michael@0 | 53 | }; |
| michael@0 | 54 | |
| michael@0 | 55 | /** Round up to the nearest power of 2. */ |
| michael@0 | 56 | template<size_t I> |
| michael@0 | 57 | struct RoundUpPow2 |
| michael@0 | 58 | { |
| michael@0 | 59 | static const size_t value = size_t(1) << CeilingLog2<I>::value; |
| michael@0 | 60 | }; |
| michael@0 | 61 | template<> |
| michael@0 | 62 | struct RoundUpPow2<0> |
| michael@0 | 63 | { |
| michael@0 | 64 | static const size_t value = 1; |
| michael@0 | 65 | }; |
| michael@0 | 66 | |
| michael@0 | 67 | /** Compute the number of bits in the given unsigned type. */ |
| michael@0 | 68 | template<typename T> |
| michael@0 | 69 | struct BitSize |
| michael@0 | 70 | { |
| michael@0 | 71 | static const size_t value = sizeof(T) * CHAR_BIT; |
| michael@0 | 72 | }; |
| michael@0 | 73 | |
| michael@0 | 74 | /** |
| michael@0 | 75 | * Produce an N-bit mask, where N <= BitSize<size_t>::value. Handle the |
| michael@0 | 76 | * language-undefined edge case when N = BitSize<size_t>::value. |
| michael@0 | 77 | */ |
| michael@0 | 78 | template<size_t N> |
| michael@0 | 79 | struct NBitMask |
| michael@0 | 80 | { |
| michael@0 | 81 | // Assert the precondition. On success this evaluates to 0. Otherwise it |
| michael@0 | 82 | // triggers divide-by-zero at compile time: a guaranteed compile error in |
| michael@0 | 83 | // C++11, and usually one in C++98. Add this value to |value| to assure |
| michael@0 | 84 | // its computation. |
| michael@0 | 85 | static const size_t checkPrecondition = 0 / size_t(N < BitSize<size_t>::value); |
| michael@0 | 86 | static const size_t value = (size_t(1) << N) - 1 + checkPrecondition; |
| michael@0 | 87 | }; |
| michael@0 | 88 | template<> |
| michael@0 | 89 | struct NBitMask<BitSize<size_t>::value> |
| michael@0 | 90 | { |
| michael@0 | 91 | static const size_t value = size_t(-1); |
| michael@0 | 92 | }; |
| michael@0 | 93 | |
| michael@0 | 94 | /** |
| michael@0 | 95 | * For the unsigned integral type size_t, compute a mask M for N such that |
| michael@0 | 96 | * for all X, !(X & M) implies X * N will not overflow (w.r.t size_t) |
| michael@0 | 97 | */ |
| michael@0 | 98 | template<size_t N> |
| michael@0 | 99 | struct MulOverflowMask |
| michael@0 | 100 | { |
| michael@0 | 101 | static const size_t value = |
| michael@0 | 102 | ~NBitMask<BitSize<size_t>::value - CeilingLog2<N>::value>::value; |
| michael@0 | 103 | }; |
| michael@0 | 104 | template<> struct MulOverflowMask<0> { /* Error */ }; |
| michael@0 | 105 | template<> struct MulOverflowMask<1> { static const size_t value = 0; }; |
| michael@0 | 106 | |
| michael@0 | 107 | } // namespace tl |
| michael@0 | 108 | |
| michael@0 | 109 | } // namespace mozilla |
| michael@0 | 110 | |
| michael@0 | 111 | #endif /* mozilla_TemplateLib_h */ |
