base/numerics/safe_conversions_impl.h

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