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 <algorithm>
  23 #include <stdint.h>
  24 #include <string.h>
  25
  26 namespace mozilla {
  27
  28 template <typename T>
  29 class NotNull;
  30
  31 /** Set the contents of |aT| to 0. */
  32 template <typename T>
  33 static MOZ_ALWAYS_INLINE void PodZero(T* aT) {
  34   memset(aT, 0, sizeof(T));
  35 }
  36
  37 /** Set the contents of |aNElem| elements starting at |aT| to 0. */
  38 template <typename T>
  39 static MOZ_ALWAYS_INLINE void PodZero(T* aT, size_t aNElem) {
  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 /** Set the contents of |aNElem| elements starting at |aT| to 0. */
  51 template <typename T>
  52 static MOZ_ALWAYS_INLINE void PodZero(NotNull<T*> aT, size_t aNElem) {
  53   PodZero(aT.get(), aNElem);
  54 }
  55
  56 /*
  57  * Arrays implicitly convert to pointers to their first element, which is
  58  * dangerous when combined with the above PodZero definitions.  Adding an
  59  * overload for arrays is ambiguous, so we need another identifier.  The
  60  * ambiguous overload is left to catch mistaken uses of PodZero; if you get a
  61  * compile error involving PodZero and array types, use PodArrayZero instead.
  62  */
  63 template <typename T, size_t N>
  64 static void PodZero(T (&aT)[N]) = delete;
  65 template <typename T, size_t N>
  66 static void PodZero(T (&aT)[N], size_t aNElem) = delete;
  67
  68 /** Set the contents of the array |aT| to zero. */
  69 template <class T, size_t N>
  70 static MOZ_ALWAYS_INLINE void PodArrayZero(T (&aT)[N]) {
  71   memset(aT, 0, N * sizeof(T));
  72 }
  73
  74 template <typename T, size_t N>
  75 static MOZ_ALWAYS_INLINE void PodArrayZero(Array<T, N>& aArr) {
  76   memset(&aArr[0], 0, N * sizeof(T));
  77 }
  78
  79 /**
  80  * Assign |*aSrc| to |*aDst|.  The locations must not be the same and must not
  81  * overlap.
  82  */
  83 template <typename T>
  84 static MOZ_ALWAYS_INLINE void PodAssign(T* aDst, const T* aSrc) {
  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 PodCopy(T* aDst, const T* aSrc, size_t aNElem) {
  97   MOZ_ASSERT(aDst + aNElem <= aSrc || aSrc + aNElem <= aDst,
  98              "destination and source must not overlap");
  99   if (aNElem < 128) {
 100     /*
 101      * Avoid using operator= in this loop, as it may have been
 102      * intentionally deleted by the POD type.
 103      */
 104     for (const T* srcend = aSrc + aNElem; aSrc < srcend; aSrc++, aDst++) {
 105       PodAssign(aDst, aSrc);
 106     }
 107   } else {
 108     memcpy(aDst, aSrc, aNElem * sizeof(T));
 109   }
 110 }
 111
 112 template <typename T>
 113 static MOZ_ALWAYS_INLINE void PodCopy(volatile T* aDst, const volatile T* aSrc,
 114                                       size_t aNElem) {
 115   MOZ_ASSERT(aDst + aNElem <= aSrc || aSrc + aNElem <= aDst,
 116              "destination and source must not overlap");
 117
 118   /*
 119    * Volatile |aDst| requires extra work, because it's undefined behavior to
 120    * modify volatile objects using the mem* functions.  Just write out the
 121    * loops manually, using operator= rather than memcpy for the same reason,
 122    * and let the compiler optimize to the extent it can.
 123    */
 124   for (const volatile T* srcend = aSrc + aNElem; aSrc < srcend;
 125        aSrc++, aDst++) {
 126     *aDst = *aSrc;
 127   }
 128 }
 129
 130 /*
 131  * Copy the contents of the array |aSrc| into the array |aDst|, both of size N.
 132  * The arrays must not overlap!
 133  */
 134 template <class T, size_t N>
 135 static MOZ_ALWAYS_INLINE void PodArrayCopy(T (&aDst)[N], const T (&aSrc)[N]) {
 136   PodCopy(aDst, aSrc, N);
 137 }
 138
 139 /**
 140  * Copy the memory for |aNElem| T elements from |aSrc| to |aDst|.  If the two
 141  * memory ranges overlap, then the effect is as if the |aNElem| elements are
 142  * first copied from |aSrc| to a temporary array, and then from the temporary
 143  * array to |aDst|.
 144  */
 145 template <typename T>
 146 static MOZ_ALWAYS_INLINE void PodMove(T* aDst, const T* aSrc, size_t aNElem) {
 147   MOZ_ASSERT(aNElem <= SIZE_MAX / sizeof(T),
 148              "trying to move an impossible number of elements");
 149   memmove(aDst, aSrc, aNElem * sizeof(T));
 150 }
 151
 152 /**
 153  * Looking for a PodEqual? Use ArrayEqual from ArrayUtils.h.
 154  * Note that we *cannot* use memcmp for this, due to padding bytes, etc..
 155  */
 156
 157 }  // namespace mozilla
 158
 159 #endif /* mozilla_PodOperations_h */