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