1 /* 2 * Copyright 2013 Google Inc. 3 * 4 * Use of this source code is governed by a BSD-style license that can be 5 * found in the LICENSE file. 6 */ 7 8 #ifndef SkTFitsIn_DEFINED 9 #define SkTFitsIn_DEFINED 10 11 #include "include/private/base/SkDebug.h" 12 13 #include <cstdint> 14 #include <limits> 15 #include <type_traits> 16 17 /** 18 * std::underlying_type is only defined for enums. For integral types, we just want the type. 19 */ 20 template <typename T, class Enable = void> 21 struct sk_strip_enum { 22 typedef T type; 23 }; 24 25 template <typename T> 26 struct sk_strip_enum<T, typename std::enable_if<std::is_enum<T>::value>::type> { 27 typedef typename std::underlying_type<T>::type type; 28 }; 29 30 31 /** 32 * In C++ an unsigned to signed cast where the source value cannot be represented in the destination 33 * type results in an implementation defined destination value. Unlike C, C++ does not allow a trap. 34 * This makes "(S)(D)s == s" a possibly useful test. However, there are two cases where this is 35 * incorrect: 36 * 37 * when testing if a value of a smaller signed type can be represented in a larger unsigned type 38 * (int8_t)(uint16_t)-1 == -1 => (int8_t)0xFFFF == -1 => [implementation defined] == -1 39 * 40 * when testing if a value of a larger unsigned type can be represented in a smaller signed type 41 * (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true. 42 * 43 * Consider the cases: 44 * u = unsigned, less digits 45 * U = unsigned, more digits 46 * s = signed, less digits 47 * S = signed, more digits 48 * v is the value we're considering. 49 * 50 * u -> U: (u)(U)v == v, trivially true 51 * U -> u: (U)(u)v == v, both casts well defined, test works 52 * s -> S: (s)(S)v == v, trivially true 53 * S -> s: (S)(s)v == v, first cast implementation value, second cast defined, test works 54 * s -> U: (s)(U)v == v, *this is bad*, the second cast results in implementation defined value 55 * S -> u: (S)(u)v == v, the second cast is required to prevent promotion of rhs to unsigned 56 * u -> S: (u)(S)v == v, trivially true 57 * U -> s: (U)(s)v == v, *this is bad*, 58 * first cast results in implementation defined value, 59 * second cast is defined. However, this creates false positives 60 * uint16_t x = 0xFFFF 61 * (uint16_t)(int8_t)x == x 62 * => (uint16_t)-1 == x 63 * => 0xFFFF == x 64 * => true 65 * 66 * So for the eight cases three are trivially true, three more are valid casts, and two are special. 67 * The two 'full' checks which otherwise require two comparisons are valid cast checks. 68 * The two remaining checks s -> U [v >= 0] and U -> s [v <= max(s)] can be done with one op. 69 */ 70 71 template <typename D, typename S> 72 static constexpr inline 73 typename std::enable_if<(std::is_integral<S>::value || std::is_enum<S>::value) && 74 (std::is_integral<D>::value || std::is_enum<D>::value), bool>::type 75 /*bool*/ SkTFitsIn(S src) { 76 // Ensure that is_signed and is_unsigned are passed the arithmetic underlyng types of enums. 77 using Sa = typename sk_strip_enum<S>::type; 78 using Da = typename sk_strip_enum<D>::type; 79 80 // SkTFitsIn() is used in public headers, so needs to be written targeting at most C++11. 81 return 82 83 // E.g. (int8_t)(uint8_t) int8_t(-1) == -1, but the uint8_t == 255, not -1. 84 (std::is_signed<Sa>::value && std::is_unsigned<Da>::value && sizeof(Sa) <= sizeof(Da)) ? 85 (S)0 <= src : 86 87 // E.g. (uint8_t)(int8_t) uint8_t(255) == 255, but the int8_t == -1. 88 (std::is_signed<Da>::value && std::is_unsigned<Sa>::value && sizeof(Da) <= sizeof(Sa)) ? 89 src <= (S)std::numeric_limits<Da>::max() : 90 91 #if !defined(SK_DEBUG) && !defined(__MSVC_RUNTIME_CHECKS ) 92 // Correct (simple) version. This trips up MSVC's /RTCc run-time checking. 93 (S)(D)src == src; 94 #else 95 // More complex version that's safe with /RTCc. Used in all debug builds, for coverage. 96 (std::is_signed<Sa>::value) ? 97 (intmax_t)src >= (intmax_t)std::numeric_limits<Da>::min() && 98 (intmax_t)src <= (intmax_t)std::numeric_limits<Da>::max() : 99 100 // std::is_unsigned<S> ? 101 (uintmax_t)src <= (uintmax_t)std::numeric_limits<Da>::max(); 102 #endif 103 } 104 105 #endif 106