mfbt/ArrayUtils.h

   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/. */
   6
   7 /*
   8  * Implements various helper functions related to arrays.
   9  */
  10
  11 #ifndef mozilla_ArrayUtils_h
  12 #define mozilla_ArrayUtils_h
  13
  14 #include "mozilla/Assertions.h"
  15 #include "mozilla/Attributes.h"
  16
  17 #include <stddef.h>
  18
  19 #ifdef __cplusplus
  20
  21 #include "mozilla/Alignment.h"
  22 #include "mozilla/Array.h"
  23 #include "mozilla/TypeTraits.h"
  24
  25 namespace mozilla {
  26
  27 /*
  28  * Safely subtract two pointers when it is known that aEnd >= aBegin.  This
  29  * avoids the common compiler bug that if (size_t(aEnd) - size_t(aBegin)) has
  30  * the MSB set, the unsigned subtraction followed by right shift will produce
  31  * -1, or size_t(-1), instead of the real difference.
  32  */
  33 template<class T>
  34 MOZ_ALWAYS_INLINE size_t
  35 PointerRangeSize(T* aBegin, T* aEnd)
  36 {
  37   MOZ_ASSERT(aEnd >= aBegin);
  38   return (size_t(aEnd) - size_t(aBegin)) / sizeof(T);
  39 }
  40
  41 /*
  42  * Compute the length of an array with constant length.  (Use of this method
  43  * with a non-array pointer will not compile.)
  44  *
  45  * Beware of the implicit trailing '\0' when using this with string constants.
  46  */
  47 template<typename T, size_t N>
  48 MOZ_CONSTEXPR size_t
  49 ArrayLength(T (&aArr)[N])
  50 {
  51   return N;
  52 }
  53
  54 template<typename T, size_t N>
  55 MOZ_CONSTEXPR size_t
  56 ArrayLength(const Array<T, N>& aArr)
  57 {
  58   return N;
  59 }
  60
  61 /*
  62  * Compute the address one past the last element of a constant-length array.
  63  *
  64  * Beware of the implicit trailing '\0' when using this with string constants.
  65  */
  66 template<typename T, size_t N>
  67 MOZ_CONSTEXPR T*
  68 ArrayEnd(T (&aArr)[N])
  69 {
  70   return aArr + ArrayLength(aArr);
  71 }
  72
  73 template<typename T, size_t N>
  74 MOZ_CONSTEXPR T*
  75 ArrayEnd(Array<T, N>& aArr)
  76 {
  77   return &aArr[0] + ArrayLength(aArr);
  78 }
  79
  80 template<typename T, size_t N>
  81 MOZ_CONSTEXPR const T*
  82 ArrayEnd(const Array<T, N>& aArr)
  83 {
  84   return &aArr[0] + ArrayLength(aArr);
  85 }
  86
  87 namespace detail {
  88
  89 template<typename AlignType, typename Pointee>
  90 struct AlignedChecker
  91 {
  92   static void
  93   test(Pointee* aPtr)
  94   {
  95     MOZ_ASSERT((uintptr_t(aPtr) % MOZ_ALIGNOF(AlignType)) == 0,
  96                "performing a range-check with a misaligned pointer");
  97   }
  98 };
  99
 100 template<typename Pointee>
 101 struct AlignedChecker<void, Pointee>
 102 {
 103   static void
 104   test(Pointee* aPtr)
 105   {
 106   }
 107 };
 108
 109 } // namespace detail
 110
 111 /**
 112  * Determines whether |aPtr| points at an object in the range [aBegin, aEnd).
 113  *
 114  * |aPtr| must have the same alignment as |aBegin| and |aEnd|.  This usually
 115  * should be achieved by ensuring |aPtr| points at a |U|, not just that it
 116  * points at a |T|.
 117  *
 118  * It is a usage error for any argument to be misaligned.
 119  *
 120  * It's okay for T* to be void*, and if so U* may also be void*.  In the latter
 121  * case no argument is required to be aligned (obviously, as void* implies no
 122  * particular alignment).
 123  */
 124 template<typename T, typename U>
 125 inline typename EnableIf<IsSame<T, U>::value ||
 126                          IsBaseOf<T, U>::value ||
 127                          IsVoid<T>::value,
 128                          bool>::Type
 129 IsInRange(T* aPtr, U* aBegin, U* aEnd)
 130 {
 131   MOZ_ASSERT(aBegin <= aEnd);
 132   detail::AlignedChecker<U, T>::test(aPtr);
 133   detail::AlignedChecker<U, U>::test(aBegin);
 134   detail::AlignedChecker<U, U>::test(aEnd);
 135   return aBegin <= static_cast<U*>(aPtr) && static_cast<U*>(aPtr) < aEnd;
 136 }
 137
 138 /**
 139  * Convenience version of the above method when the valid range is specified as
 140  * uintptr_t values.  As above, |aPtr| must be aligned, and |aBegin| and |aEnd|
 141  * must be aligned with respect to |T|.
 142  */
 143 template<typename T>
 144 inline bool
 145 IsInRange(T* aPtr, uintptr_t aBegin, uintptr_t aEnd)
 146 {
 147   return IsInRange(aPtr,
 148                    reinterpret_cast<T*>(aBegin), reinterpret_cast<T*>(aEnd));
 149 }
 150
 151 namespace detail {
 152
 153 /*
 154  * Helper for the MOZ_ARRAY_LENGTH() macro to make the length a typesafe
 155  * compile-time constant even on compilers lacking constexpr support.
 156  */
 157 template <typename T, size_t N>
 158 char (&ArrayLengthHelper(T (&array)[N]))[N];
 159
 160 } /* namespace detail */
 161
 162 } /* namespace mozilla */
 163
 164 #endif /* __cplusplus */
 165
 166 /*
 167  * MOZ_ARRAY_LENGTH() is an alternative to mozilla::ArrayLength() for C files
 168  * that can't use C++ template functions and for static_assert() calls that
 169  * can't call ArrayLength() when it is not a C++11 constexpr function.
 170  */
 171 #ifdef __cplusplus
 172 #  define MOZ_ARRAY_LENGTH(array)   sizeof(mozilla::detail::ArrayLengthHelper(array))
 173 #else
 174 #  define MOZ_ARRAY_LENGTH(array)   (sizeof(array)/sizeof((array)[0]))
 175 #endif
 176
 177 #endif /* mozilla_ArrayUtils_h */