xpcom/string/nsStringBuffer.cpp

   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 file,
   5  * You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #include "nsStringBuffer.h"
   8
   9 #include "mozilla/MemoryReporting.h"
  10 #include "nsISupportsImpl.h"
  11 #include "nsString.h"
  12
  13 #ifdef DEBUG
  14 #  include "nsStringStats.h"
  15 #else
  16 #  define STRING_STAT_INCREMENT(_s)
  17 #endif
  18
  19 void nsStringBuffer::AddRef() {
  20   // Memory synchronization is not required when incrementing a
  21   // reference count.  The first increment of a reference count on a
  22   // thread is not important, since the first use of the object on a
  23   // thread can happen before it.  What is important is the transfer
  24   // of the pointer to that thread, which may happen prior to the
  25   // first increment on that thread.  The necessary memory
  26   // synchronization is done by the mechanism that transfers the
  27   // pointer between threads.
  28 #ifdef NS_BUILD_REFCNT_LOGGING
  29   uint32_t count =
  30 #endif
  31       mRefCount.fetch_add(1, std::memory_order_relaxed)
  32 #ifdef NS_BUILD_REFCNT_LOGGING
  33       + 1
  34 #endif
  35       ;
  36   STRING_STAT_INCREMENT(Share);
  37   NS_LOG_ADDREF(this, count, "nsStringBuffer", sizeof(*this));
  38 }
  39
  40 void nsStringBuffer::Release() {
  41   // Since this may be the last release on this thread, we need
  42   // release semantics so that prior writes on this thread are visible
  43   // to the thread that destroys the object when it reads mValue with
  44   // acquire semantics.
  45   uint32_t count = mRefCount.fetch_sub(1, std::memory_order_release) - 1;
  46   NS_LOG_RELEASE(this, count, "nsStringBuffer");
  47   if (count == 0) {
  48     // We're going to destroy the object on this thread, so we need
  49     // acquire semantics to synchronize with the memory released by
  50     // the last release on other threads, that is, to ensure that
  51     // writes prior to that release are now visible on this thread.
  52     count = mRefCount.load(std::memory_order_acquire);
  53
  54     STRING_STAT_INCREMENT(Free);
  55     free(this);  // we were allocated with |malloc|
  56   }
  57 }
  58
  59 /**
  60  * Alloc returns a pointer to a new string header with set capacity.
  61  */
  62 already_AddRefed<nsStringBuffer> nsStringBuffer::Alloc(size_t aSize) {
  63   NS_ASSERTION(aSize != 0, "zero capacity allocation not allowed");
  64   NS_ASSERTION(sizeof(nsStringBuffer) + aSize <= size_t(uint32_t(-1)) &&
  65                    sizeof(nsStringBuffer) + aSize > aSize,
  66                "mStorageSize will truncate");
  67
  68   nsStringBuffer* hdr = (nsStringBuffer*)malloc(sizeof(nsStringBuffer) + aSize);
  69   if (hdr) {
  70     STRING_STAT_INCREMENT(Alloc);
  71
  72     hdr->mRefCount = 1;
  73     hdr->mStorageSize = aSize;
  74     NS_LOG_ADDREF(hdr, 1, "nsStringBuffer", sizeof(*hdr));
  75   }
  76   return already_AddRefed(hdr);
  77 }
  78
  79 nsStringBuffer* nsStringBuffer::Realloc(nsStringBuffer* aHdr, size_t aSize) {
  80   STRING_STAT_INCREMENT(Realloc);
  81
  82   NS_ASSERTION(aSize != 0, "zero capacity allocation not allowed");
  83   NS_ASSERTION(sizeof(nsStringBuffer) + aSize <= size_t(uint32_t(-1)) &&
  84                    sizeof(nsStringBuffer) + aSize > aSize,
  85                "mStorageSize will truncate");
  86
  87   // no point in trying to save ourselves if we hit this assertion
  88   NS_ASSERTION(!aHdr->IsReadonly(), "|Realloc| attempted on readonly string");
  89
  90   // Treat this as a release and addref for refcounting purposes, since we
  91   // just asserted that the refcount is 1.  If we don't do that, refcount
  92   // logging will claim we've leaked all sorts of stuff.
  93   NS_LOG_RELEASE(aHdr, 0, "nsStringBuffer");
  94
  95   aHdr = (nsStringBuffer*)realloc(aHdr, sizeof(nsStringBuffer) + aSize);
  96   if (aHdr) {
  97     NS_LOG_ADDREF(aHdr, 1, "nsStringBuffer", sizeof(*aHdr));
  98     aHdr->mStorageSize = aSize;
  99   }
 100
 101   return aHdr;
 102 }
 103
 104 nsStringBuffer* nsStringBuffer::FromString(const nsAString& aStr) {
 105   if (!(aStr.mDataFlags & nsAString::DataFlags::REFCOUNTED)) {
 106     return nullptr;
 107   }
 108
 109   return FromData(aStr.mData);
 110 }
 111
 112 nsStringBuffer* nsStringBuffer::FromString(const nsACString& aStr) {
 113   if (!(aStr.mDataFlags & nsACString::DataFlags::REFCOUNTED)) {
 114     return nullptr;
 115   }
 116
 117   return FromData(aStr.mData);
 118 }
 119
 120 void nsStringBuffer::ToString(uint32_t aLen, nsAString& aStr,
 121                               bool aMoveOwnership) {
 122   char16_t* data = static_cast<char16_t*>(Data());
 123
 124   MOZ_DIAGNOSTIC_ASSERT(data[aLen] == char16_t(0),
 125                         "data should be null terminated");
 126
 127   nsAString::DataFlags flags =
 128       nsAString::DataFlags::REFCOUNTED | nsAString::DataFlags::TERMINATED;
 129
 130   if (!aMoveOwnership) {
 131     AddRef();
 132   }
 133   aStr.Finalize();
 134   aStr.SetData(data, aLen, flags);
 135 }
 136
 137 void nsStringBuffer::ToString(uint32_t aLen, nsACString& aStr,
 138                               bool aMoveOwnership) {
 139   char* data = static_cast<char*>(Data());
 140
 141   MOZ_DIAGNOSTIC_ASSERT(data[aLen] == char(0),
 142                         "data should be null terminated");
 143
 144   nsACString::DataFlags flags =
 145       nsACString::DataFlags::REFCOUNTED | nsACString::DataFlags::TERMINATED;
 146
 147   if (!aMoveOwnership) {
 148     AddRef();
 149   }
 150   aStr.Finalize();
 151   aStr.SetData(data, aLen, flags);
 152 }
 153
 154 size_t nsStringBuffer::SizeOfIncludingThisIfUnshared(
 155     mozilla::MallocSizeOf aMallocSizeOf) const {
 156   return IsReadonly() ? 0 : aMallocSizeOf(this);
 157 }
 158
 159 size_t nsStringBuffer::SizeOfIncludingThisEvenIfShared(
 160     mozilla::MallocSizeOf aMallocSizeOf) const {
 161   return aMallocSizeOf(this);
 162 }