1 // Copyright 2014 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef BASE_SAFE_CONVERSIONS_IMPL_H_ 6 #define BASE_SAFE_CONVERSIONS_IMPL_H_ 7 8 #include <limits> 9 10 #include "base/macros.h" 11 #include "base/template_util.h" 12 13 namespace base { 14 namespace internal { 15 16 // The std library doesn't provide a binary max_exponent for integers, however 17 // we can compute one by adding one to the number of non-sign bits. This allows 18 // for accurate range comparisons between floating point and integer types. 19 template <typename NumericType> 20 struct MaxExponent { 21 static const int value = std::numeric_limits<NumericType>::is_iec559 22 ? std::numeric_limits<NumericType>::max_exponent 23 : (sizeof(NumericType) * 8 + 1 - 24 std::numeric_limits<NumericType>::is_signed); 25 }; 26 27 enum IntegerRepresentation { 28 INTEGER_REPRESENTATION_UNSIGNED, 29 INTEGER_REPRESENTATION_SIGNED 30 }; 31 32 // A range for a given nunmeric Src type is contained for a given numeric Dst 33 // type if both numeric_limits<Src>::max() <= numeric_limits<Dst>::max() and 34 // numeric_limits<Src>::min() >= numeric_limits<Dst>::min() are true. 35 // We implement this as template specializations rather than simple static 36 // comparisons to ensure type correctness in our comparisons. 37 enum NumericRangeRepresentation { 38 NUMERIC_RANGE_NOT_CONTAINED, 39 NUMERIC_RANGE_CONTAINED 40 }; 41 42 // Helper templates to statically determine if our destination type can contain 43 // maximum and minimum values represented by the source type. 44 45 template < 46 typename Dst, 47 typename Src, 48 IntegerRepresentation DstSign = std::numeric_limits<Dst>::is_signed 49 ? INTEGER_REPRESENTATION_SIGNED 50 : INTEGER_REPRESENTATION_UNSIGNED, 51 IntegerRepresentation SrcSign = 52 std::numeric_limits<Src>::is_signed 53 ? INTEGER_REPRESENTATION_SIGNED 54 : INTEGER_REPRESENTATION_UNSIGNED > 55 struct StaticDstRangeRelationToSrcRange; 56 57 // Same sign: Dst is guaranteed to contain Src only if its range is equal or 58 // larger. 59 template <typename Dst, typename Src, IntegerRepresentation Sign> 60 struct StaticDstRangeRelationToSrcRange<Dst, Src, Sign, Sign> { 61 static const NumericRangeRepresentation value = 62 MaxExponent<Dst>::value >= MaxExponent<Src>::value 63 ? NUMERIC_RANGE_CONTAINED 64 : NUMERIC_RANGE_NOT_CONTAINED; 65 }; 66 67 // Unsigned to signed: Dst is guaranteed to contain source only if its range is 68 // larger. 69 template <typename Dst, typename Src> 70 struct StaticDstRangeRelationToSrcRange<Dst, 71 Src, 72 INTEGER_REPRESENTATION_SIGNED, 73 INTEGER_REPRESENTATION_UNSIGNED> { 74 static const NumericRangeRepresentation value = 75 MaxExponent<Dst>::value > MaxExponent<Src>::value 76 ? NUMERIC_RANGE_CONTAINED 77 : NUMERIC_RANGE_NOT_CONTAINED; 78 }; 79 80 // Signed to unsigned: Dst cannot be statically determined to contain Src. 81 template <typename Dst, typename Src> 82 struct StaticDstRangeRelationToSrcRange<Dst, 83 Src, 84 INTEGER_REPRESENTATION_UNSIGNED, 85 INTEGER_REPRESENTATION_SIGNED> { 86 static const NumericRangeRepresentation value = NUMERIC_RANGE_NOT_CONTAINED; 87 }; 88 89 enum RangeConstraint { 90 RANGE_VALID = 0x0, // Value can be represented by the destination type. 91 RANGE_UNDERFLOW = 0x1, // Value would overflow. 92 RANGE_OVERFLOW = 0x2, // Value would underflow. 93 RANGE_INVALID = RANGE_UNDERFLOW | RANGE_OVERFLOW // Invalid (i.e. NaN). 94 }; 95 96 // Helper function for coercing an int back to a RangeContraint. 97 inline RangeConstraint GetRangeConstraint(int integer_range_constraint) { 98 DCHECK(integer_range_constraint >= RANGE_VALID && 99 integer_range_constraint <= RANGE_INVALID); 100 return static_cast<RangeConstraint>(integer_range_constraint); 101 } 102 103 // This function creates a RangeConstraint from an upper and lower bound 104 // check by taking advantage of the fact that only NaN can be out of range in 105 // both directions at once. 106 inline RangeConstraint GetRangeConstraint(bool is_in_upper_bound, 107 bool is_in_lower_bound) { 108 return GetRangeConstraint((is_in_upper_bound ? 0 : RANGE_OVERFLOW) | 109 (is_in_lower_bound ? 0 : RANGE_UNDERFLOW)); 110 } 111 112 template < 113 typename Dst, 114 typename Src, 115 IntegerRepresentation DstSign = std::numeric_limits<Dst>::is_signed 116 ? INTEGER_REPRESENTATION_SIGNED 117 : INTEGER_REPRESENTATION_UNSIGNED, 118 IntegerRepresentation SrcSign = std::numeric_limits<Src>::is_signed 119 ? INTEGER_REPRESENTATION_SIGNED 120 : INTEGER_REPRESENTATION_UNSIGNED, 121 NumericRangeRepresentation DstRange = 122 StaticDstRangeRelationToSrcRange<Dst, Src>::value > 123 struct DstRangeRelationToSrcRangeImpl; 124 125 // The following templates are for ranges that must be verified at runtime. We 126 // split it into checks based on signedness to avoid confusing casts and 127 // compiler warnings on signed an unsigned comparisons. 128 129 // Dst range is statically determined to contain Src: Nothing to check. 130 template <typename Dst, 131 typename Src, 132 IntegerRepresentation DstSign, 133 IntegerRepresentation SrcSign> 134 struct DstRangeRelationToSrcRangeImpl<Dst, 135 Src, 136 DstSign, 137 SrcSign, 138 NUMERIC_RANGE_CONTAINED> { 139 static RangeConstraint Check(Src value) { return RANGE_VALID; } 140 }; 141 142 // Signed to signed narrowing: Both the upper and lower boundaries may be 143 // exceeded. 144 template <typename Dst, typename Src> 145 struct DstRangeRelationToSrcRangeImpl<Dst, 146 Src, 147 INTEGER_REPRESENTATION_SIGNED, 148 INTEGER_REPRESENTATION_SIGNED, 149 NUMERIC_RANGE_NOT_CONTAINED> { 150 static RangeConstraint Check(Src value) { 151 return std::numeric_limits<Dst>::is_iec559 152 ? GetRangeConstraint(value <= std::numeric_limits<Dst>::max(), 153 value >= -std::numeric_limits<Dst>::max()) 154 : GetRangeConstraint(value <= std::numeric_limits<Dst>::max(), 155 value >= std::numeric_limits<Dst>::min()); 156 } 157 }; 158 159 // Unsigned to unsigned narrowing: Only the upper boundary can be exceeded. 160 template <typename Dst, typename Src> 161 struct DstRangeRelationToSrcRangeImpl<Dst, 162 Src, 163 INTEGER_REPRESENTATION_UNSIGNED, 164 INTEGER_REPRESENTATION_UNSIGNED, 165 NUMERIC_RANGE_NOT_CONTAINED> { 166 static RangeConstraint Check(Src value) { 167 return GetRangeConstraint(value <= std::numeric_limits<Dst>::max(), true); 168 } 169 }; 170 171 // Unsigned to signed: The upper boundary may be exceeded. 172 template <typename Dst, typename Src> 173 struct DstRangeRelationToSrcRangeImpl<Dst, 174 Src, 175 INTEGER_REPRESENTATION_SIGNED, 176 INTEGER_REPRESENTATION_UNSIGNED, 177 NUMERIC_RANGE_NOT_CONTAINED> { 178 static RangeConstraint Check(Src value) { 179 return sizeof(Dst) > sizeof(Src) 180 ? RANGE_VALID 181 : GetRangeConstraint( 182 value <= static_cast<Src>(std::numeric_limits<Dst>::max()), 183 true); 184 } 185 }; 186 187 // Signed to unsigned: The upper boundary may be exceeded for a narrower Dst, 188 // and any negative value exceeds the lower boundary. 189 template <typename Dst, typename Src> 190 struct DstRangeRelationToSrcRangeImpl<Dst, 191 Src, 192 INTEGER_REPRESENTATION_UNSIGNED, 193 INTEGER_REPRESENTATION_SIGNED, 194 NUMERIC_RANGE_NOT_CONTAINED> { 195 static RangeConstraint Check(Src value) { 196 return (MaxExponent<Dst>::value >= MaxExponent<Src>::value) 197 ? GetRangeConstraint(true, value >= static_cast<Src>(0)) 198 : GetRangeConstraint( 199 value <= static_cast<Src>(std::numeric_limits<Dst>::max()), 200 value >= static_cast<Src>(0)); 201 } 202 }; 203 204 template <typename Dst, typename Src> 205 inline RangeConstraint DstRangeRelationToSrcRange(Src value) { 206 COMPILE_ASSERT(std::numeric_limits<Src>::is_specialized, 207 argument_must_be_numeric); 208 COMPILE_ASSERT(std::numeric_limits<Dst>::is_specialized, 209 result_must_be_numeric); 210 return DstRangeRelationToSrcRangeImpl<Dst, Src>::Check(value); 211 } 212 213 } // namespace internal 214 } // namespace base 215 216 #endif // BASE_SAFE_CONVERSIONS_IMPL_H_ 217 218
