mfbt/PodOperations.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  * Operations for zeroing POD types, arrays, and so on.
   9  *
  10  * These operations are preferable to memset, memcmp, and the like because they
  11  * don't require remembering to multiply by sizeof(T), array lengths, and so on
  12  * everywhere.
  13  */
  14
  15 #ifndef mozilla_PodOperations_h
  16 #define mozilla_PodOperations_h
  17
  18 #include "mozilla/Array.h"
  19 #include "mozilla/ArrayUtils.h"
  20 #include "mozilla/Attributes.h"
  21
  22 #include <stdint.h>
  23 #include <string.h>
  24
  25 namespace mozilla {
  26
  27 /** Set the contents of |aT| to 0. */
  28 template<typename T>
  29 static MOZ_ALWAYS_INLINE void
  30 PodZero(T* aT)
  31 {
  32   memset(aT, 0, sizeof(T));
  33 }
  34
  35 /** Set the contents of |aNElem| elements starting at |aT| to 0. */
  36 template<typename T>
  37 static MOZ_ALWAYS_INLINE void
  38 PodZero(T* aT, size_t aNElem)
  39 {
  40   /*
  41    * This function is often called with 'aNElem' small; we use an inline loop
  42    * instead of calling 'memset' with a non-constant length.  The compiler
  43    * should inline the memset call with constant size, though.
  44    */
  45   for (T* end = aT + aNElem; aT < end; aT++) {
  46     memset(aT, 0, sizeof(T));
  47   }
  48 }
  49
  50 /*
  51  * Arrays implicitly convert to pointers to their first element, which is
  52  * dangerous when combined with the above PodZero definitions.  Adding an
  53  * overload for arrays is ambiguous, so we need another identifier.  The
  54  * ambiguous overload is left to catch mistaken uses of PodZero; if you get a
  55  * compile error involving PodZero and array types, use PodArrayZero instead.
  56  */
  57 template<typename T, size_t N>
  58 static void PodZero(T (&aT)[N]) = delete;
  59 template<typename T, size_t N>
  60 static void PodZero(T (&aT)[N], size_t aNElem) = delete;
  61
  62 /** Set the contents of the array |aT| to zero. */
  63 template <class T, size_t N>
  64 static MOZ_ALWAYS_INLINE void
  65 PodArrayZero(T (&aT)[N])
  66 {
  67   memset(aT, 0, N * sizeof(T));
  68 }
  69
  70 template <typename T, size_t N>
  71 static MOZ_ALWAYS_INLINE void
  72 PodArrayZero(Array<T, N>& aArr)
  73 {
  74   memset(&aArr[0], 0, N * sizeof(T));
  75 }
  76
  77 /**
  78  * Assign |*aSrc| to |*aDst|.  The locations must not be the same and must not
  79  * overlap.
  80  */
  81 template<typename T>
  82 static MOZ_ALWAYS_INLINE void
  83 PodAssign(T* aDst, const T* aSrc)
  84 {
  85   MOZ_ASSERT(aDst + 1 <= aSrc || aSrc + 1 <= aDst,
  86              "destination and source must not overlap");
  87   memcpy(reinterpret_cast<char*>(aDst), reinterpret_cast<const char*>(aSrc),
  88          sizeof(T));
  89 }
  90
  91 /**
  92  * Copy |aNElem| T elements from |aSrc| to |aDst|.  The two memory ranges must
  93  * not overlap!
  94  */
  95 template<typename T>
  96 static MOZ_ALWAYS_INLINE void
  97 PodCopy(T* aDst, const T* aSrc, size_t aNElem)
  98 {
  99   MOZ_ASSERT(aDst + aNElem <= aSrc || aSrc + aNElem <= aDst,
 100              "destination and source must not overlap");
 101   if (aNElem < 128) {
 102     /*
 103      * Avoid using operator= in this loop, as it may have been
 104      * intentionally deleted by the POD type.
 105      */
 106     for (const T* srcend = aSrc + aNElem; aSrc < srcend; aSrc++, aDst++) {
 107       PodAssign(aDst, aSrc);
 108     }
 109   } else {
 110     memcpy(aDst, aSrc, aNElem * sizeof(T));
 111   }
 112 }
 113
 114 template<typename T>
 115 static MOZ_ALWAYS_INLINE void
 116 PodCopy(volatile T* aDst, const volatile T* aSrc, size_t aNElem)
 117 {
 118   MOZ_ASSERT(aDst + aNElem <= aSrc || aSrc + aNElem <= aDst,
 119              "destination and source must not overlap");
 120
 121   /*
 122    * Volatile |aDst| requires extra work, because it's undefined behavior to
 123    * modify volatile objects using the mem* functions.  Just write out the
 124    * loops manually, using operator= rather than memcpy for the same reason,
 125    * and let the compiler optimize to the extent it can.
 126    */
 127   for (const volatile T* srcend = aSrc + aNElem;
 128        aSrc < srcend;
 129        aSrc++, aDst++) {
 130     *aDst = *aSrc;
 131   }
 132 }
 133
 134 /*
 135  * Copy the contents of the array |aSrc| into the array |aDst|, both of size N.
 136  * The arrays must not overlap!
 137  */
 138 template <class T, size_t N>
 139 static MOZ_ALWAYS_INLINE void
 140 PodArrayCopy(T (&aDst)[N], const T (&aSrc)[N])
 141 {
 142   PodCopy(aDst, aSrc, N);
 143 }
 144
 145 /**
 146  * Copy the memory for |aNElem| T elements from |aSrc| to |aDst|.  If the two
 147  * memory ranges overlap, then the effect is as if the |aNElem| elements are
 148  * first copied from |aSrc| to a temporary array, and then from the temporary
 149  * array to |aDst|.
 150  */
 151 template<typename T>
 152 static MOZ_ALWAYS_INLINE void
 153 PodMove(T* aDst, const T* aSrc, size_t aNElem)
 154 {
 155   MOZ_ASSERT(aNElem <= SIZE_MAX / sizeof(T),
 156              "trying to move an impossible number of elements");
 157   memmove(aDst, aSrc, aNElem * sizeof(T));
 158 }
 159
 160 /**
 161  * Determine whether the |len| elements at |one| are memory-identical to the
 162  * |len| elements at |two|.
 163  */
 164 template<typename T>
 165 static MOZ_ALWAYS_INLINE bool
 166 PodEqual(const T* one, const T* two, size_t len)
 167 {
 168   if (len < 128) {
 169     const T* p1end = one + len;
 170     const T* p1 = one;
 171     const T* p2 = two;
 172     for (; p1 < p1end; p1++, p2++) {
 173       if (*p1 != *p2) {
 174         return false;
 175       }
 176     }
 177     return true;
 178   }
 179
 180   return !memcmp(one, two, len * sizeof(T));
 181 }
 182
 183 } // namespace mozilla
 184
 185 #endif /* mozilla_PodOperations_h */