Ben Murdoch | b8a8cc1 | 2014-11-26 15:28:44 +0000 | [diff] [blame] | 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 | // Slightly adapted for inclusion in V8. |
| 6 | // Copyright 2014 the V8 project authors. All rights reserved. |
| 7 | |
| 8 | #ifndef V8_BASE_SAFE_CONVERSIONS_IMPL_H_ |
| 9 | #define V8_BASE_SAFE_CONVERSIONS_IMPL_H_ |
| 10 | |
| 11 | #include <limits> |
| 12 | |
| 13 | #include "src/base/logging.h" |
| 14 | #include "src/base/macros.h" |
| 15 | |
| 16 | namespace v8 { |
| 17 | namespace base { |
| 18 | namespace internal { |
| 19 | |
| 20 | // The std library doesn't provide a binary max_exponent for integers, however |
| 21 | // we can compute one by adding one to the number of non-sign bits. This allows |
| 22 | // for accurate range comparisons between floating point and integer types. |
| 23 | template <typename NumericType> |
| 24 | struct MaxExponent { |
| 25 | static const int value = std::numeric_limits<NumericType>::is_iec559 |
| 26 | ? std::numeric_limits<NumericType>::max_exponent |
| 27 | : (sizeof(NumericType) * 8 + 1 - |
| 28 | std::numeric_limits<NumericType>::is_signed); |
| 29 | }; |
| 30 | |
| 31 | enum IntegerRepresentation { |
| 32 | INTEGER_REPRESENTATION_UNSIGNED, |
| 33 | INTEGER_REPRESENTATION_SIGNED |
| 34 | }; |
| 35 | |
| 36 | // A range for a given nunmeric Src type is contained for a given numeric Dst |
| 37 | // type if both numeric_limits<Src>::max() <= numeric_limits<Dst>::max() and |
| 38 | // numeric_limits<Src>::min() >= numeric_limits<Dst>::min() are true. |
| 39 | // We implement this as template specializations rather than simple static |
| 40 | // comparisons to ensure type correctness in our comparisons. |
| 41 | enum NumericRangeRepresentation { |
| 42 | NUMERIC_RANGE_NOT_CONTAINED, |
| 43 | NUMERIC_RANGE_CONTAINED |
| 44 | }; |
| 45 | |
| 46 | // Helper templates to statically determine if our destination type can contain |
| 47 | // maximum and minimum values represented by the source type. |
| 48 | |
| 49 | template < |
| 50 | typename Dst, |
| 51 | typename Src, |
| 52 | IntegerRepresentation DstSign = std::numeric_limits<Dst>::is_signed |
| 53 | ? INTEGER_REPRESENTATION_SIGNED |
| 54 | : INTEGER_REPRESENTATION_UNSIGNED, |
| 55 | IntegerRepresentation SrcSign = |
| 56 | std::numeric_limits<Src>::is_signed |
| 57 | ? INTEGER_REPRESENTATION_SIGNED |
| 58 | : INTEGER_REPRESENTATION_UNSIGNED > |
| 59 | struct StaticDstRangeRelationToSrcRange; |
| 60 | |
| 61 | // Same sign: Dst is guaranteed to contain Src only if its range is equal or |
| 62 | // larger. |
| 63 | template <typename Dst, typename Src, IntegerRepresentation Sign> |
| 64 | struct StaticDstRangeRelationToSrcRange<Dst, Src, Sign, Sign> { |
| 65 | static const NumericRangeRepresentation value = |
| 66 | MaxExponent<Dst>::value >= MaxExponent<Src>::value |
| 67 | ? NUMERIC_RANGE_CONTAINED |
| 68 | : NUMERIC_RANGE_NOT_CONTAINED; |
| 69 | }; |
| 70 | |
| 71 | // Unsigned to signed: Dst is guaranteed to contain source only if its range is |
| 72 | // larger. |
| 73 | template <typename Dst, typename Src> |
| 74 | struct StaticDstRangeRelationToSrcRange<Dst, |
| 75 | Src, |
| 76 | INTEGER_REPRESENTATION_SIGNED, |
| 77 | INTEGER_REPRESENTATION_UNSIGNED> { |
| 78 | static const NumericRangeRepresentation value = |
| 79 | MaxExponent<Dst>::value > MaxExponent<Src>::value |
| 80 | ? NUMERIC_RANGE_CONTAINED |
| 81 | : NUMERIC_RANGE_NOT_CONTAINED; |
| 82 | }; |
| 83 | |
| 84 | // Signed to unsigned: Dst cannot be statically determined to contain Src. |
| 85 | template <typename Dst, typename Src> |
| 86 | struct StaticDstRangeRelationToSrcRange<Dst, |
| 87 | Src, |
| 88 | INTEGER_REPRESENTATION_UNSIGNED, |
| 89 | INTEGER_REPRESENTATION_SIGNED> { |
| 90 | static const NumericRangeRepresentation value = NUMERIC_RANGE_NOT_CONTAINED; |
| 91 | }; |
| 92 | |
| 93 | enum RangeConstraint { |
| 94 | RANGE_VALID = 0x0, // Value can be represented by the destination type. |
| 95 | RANGE_UNDERFLOW = 0x1, // Value would overflow. |
| 96 | RANGE_OVERFLOW = 0x2, // Value would underflow. |
| 97 | RANGE_INVALID = RANGE_UNDERFLOW | RANGE_OVERFLOW // Invalid (i.e. NaN). |
| 98 | }; |
| 99 | |
| 100 | // Helper function for coercing an int back to a RangeContraint. |
| 101 | inline RangeConstraint GetRangeConstraint(int integer_range_constraint) { |
| 102 | DCHECK(integer_range_constraint >= RANGE_VALID && |
| 103 | integer_range_constraint <= RANGE_INVALID); |
| 104 | return static_cast<RangeConstraint>(integer_range_constraint); |
| 105 | } |
| 106 | |
| 107 | // This function creates a RangeConstraint from an upper and lower bound |
| 108 | // check by taking advantage of the fact that only NaN can be out of range in |
| 109 | // both directions at once. |
| 110 | inline RangeConstraint GetRangeConstraint(bool is_in_upper_bound, |
| 111 | bool is_in_lower_bound) { |
| 112 | return GetRangeConstraint((is_in_upper_bound ? 0 : RANGE_OVERFLOW) | |
| 113 | (is_in_lower_bound ? 0 : RANGE_UNDERFLOW)); |
| 114 | } |
| 115 | |
| 116 | template < |
| 117 | typename Dst, |
| 118 | typename Src, |
| 119 | IntegerRepresentation DstSign = std::numeric_limits<Dst>::is_signed |
| 120 | ? INTEGER_REPRESENTATION_SIGNED |
| 121 | : INTEGER_REPRESENTATION_UNSIGNED, |
| 122 | IntegerRepresentation SrcSign = std::numeric_limits<Src>::is_signed |
| 123 | ? INTEGER_REPRESENTATION_SIGNED |
| 124 | : INTEGER_REPRESENTATION_UNSIGNED, |
| 125 | NumericRangeRepresentation DstRange = |
| 126 | StaticDstRangeRelationToSrcRange<Dst, Src>::value > |
| 127 | struct DstRangeRelationToSrcRangeImpl; |
| 128 | |
| 129 | // The following templates are for ranges that must be verified at runtime. We |
| 130 | // split it into checks based on signedness to avoid confusing casts and |
| 131 | // compiler warnings on signed an unsigned comparisons. |
| 132 | |
| 133 | // Dst range is statically determined to contain Src: Nothing to check. |
| 134 | template <typename Dst, |
| 135 | typename Src, |
| 136 | IntegerRepresentation DstSign, |
| 137 | IntegerRepresentation SrcSign> |
| 138 | struct DstRangeRelationToSrcRangeImpl<Dst, |
| 139 | Src, |
| 140 | DstSign, |
| 141 | SrcSign, |
| 142 | NUMERIC_RANGE_CONTAINED> { |
| 143 | static RangeConstraint Check(Src value) { return RANGE_VALID; } |
| 144 | }; |
| 145 | |
| 146 | // Signed to signed narrowing: Both the upper and lower boundaries may be |
| 147 | // exceeded. |
| 148 | template <typename Dst, typename Src> |
| 149 | struct DstRangeRelationToSrcRangeImpl<Dst, |
| 150 | Src, |
| 151 | INTEGER_REPRESENTATION_SIGNED, |
| 152 | INTEGER_REPRESENTATION_SIGNED, |
| 153 | NUMERIC_RANGE_NOT_CONTAINED> { |
| 154 | static RangeConstraint Check(Src value) { |
| 155 | return std::numeric_limits<Dst>::is_iec559 |
| 156 | ? GetRangeConstraint(value <= std::numeric_limits<Dst>::max(), |
| 157 | value >= -std::numeric_limits<Dst>::max()) |
| 158 | : GetRangeConstraint(value <= std::numeric_limits<Dst>::max(), |
| 159 | value >= std::numeric_limits<Dst>::min()); |
| 160 | } |
| 161 | }; |
| 162 | |
| 163 | // Unsigned to unsigned narrowing: Only the upper boundary can be exceeded. |
| 164 | template <typename Dst, typename Src> |
| 165 | struct DstRangeRelationToSrcRangeImpl<Dst, |
| 166 | Src, |
| 167 | INTEGER_REPRESENTATION_UNSIGNED, |
| 168 | INTEGER_REPRESENTATION_UNSIGNED, |
| 169 | NUMERIC_RANGE_NOT_CONTAINED> { |
| 170 | static RangeConstraint Check(Src value) { |
| 171 | return GetRangeConstraint(value <= std::numeric_limits<Dst>::max(), true); |
| 172 | } |
| 173 | }; |
| 174 | |
| 175 | // Unsigned to signed: The upper boundary may be exceeded. |
| 176 | template <typename Dst, typename Src> |
| 177 | struct DstRangeRelationToSrcRangeImpl<Dst, |
| 178 | Src, |
| 179 | INTEGER_REPRESENTATION_SIGNED, |
| 180 | INTEGER_REPRESENTATION_UNSIGNED, |
| 181 | NUMERIC_RANGE_NOT_CONTAINED> { |
| 182 | static RangeConstraint Check(Src value) { |
| 183 | return sizeof(Dst) > sizeof(Src) |
| 184 | ? RANGE_VALID |
| 185 | : GetRangeConstraint( |
| 186 | value <= static_cast<Src>(std::numeric_limits<Dst>::max()), |
| 187 | true); |
| 188 | } |
| 189 | }; |
| 190 | |
| 191 | // Signed to unsigned: The upper boundary may be exceeded for a narrower Dst, |
| 192 | // and any negative value exceeds the lower boundary. |
| 193 | template <typename Dst, typename Src> |
| 194 | struct DstRangeRelationToSrcRangeImpl<Dst, |
| 195 | Src, |
| 196 | INTEGER_REPRESENTATION_UNSIGNED, |
| 197 | INTEGER_REPRESENTATION_SIGNED, |
| 198 | NUMERIC_RANGE_NOT_CONTAINED> { |
| 199 | static RangeConstraint Check(Src value) { |
| 200 | return (MaxExponent<Dst>::value >= MaxExponent<Src>::value) |
| 201 | ? GetRangeConstraint(true, value >= static_cast<Src>(0)) |
| 202 | : GetRangeConstraint( |
| 203 | value <= static_cast<Src>(std::numeric_limits<Dst>::max()), |
| 204 | value >= static_cast<Src>(0)); |
| 205 | } |
| 206 | }; |
| 207 | |
| 208 | template <typename Dst, typename Src> |
| 209 | inline RangeConstraint DstRangeRelationToSrcRange(Src value) { |
| 210 | // Both source and destination must be numeric. |
| 211 | STATIC_ASSERT(std::numeric_limits<Src>::is_specialized); |
| 212 | STATIC_ASSERT(std::numeric_limits<Dst>::is_specialized); |
| 213 | return DstRangeRelationToSrcRangeImpl<Dst, Src>::Check(value); |
| 214 | } |
| 215 | |
| 216 | } // namespace internal |
| 217 | } // namespace base |
| 218 | } // namespace v8 |
| 219 | |
| 220 | #endif // V8_BASE_SAFE_CONVERSIONS_IMPL_H_ |