layout/base/StackArena.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 file,
   5  * You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef StackArena_h
   8 #define StackArena_h
   9
  10 #include "nsError.h"
  11 #include "mozilla/Assertions.h"
  12 #include "mozilla/MemoryReporting.h"
  13
  14 namespace mozilla {
  15
  16 struct StackBlock;
  17 struct StackMark;
  18 class AutoStackArena;
  19
  20 // Private helper class for AutoStackArena.
  21 class StackArena {
  22  private:
  23   friend class AutoStackArena;
  24   StackArena();
  25   ~StackArena();
  26
  27   // Memory management functions.
  28   void* Allocate(size_t aSize);
  29   void Push();
  30   void Pop();
  31
  32   size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
  33
  34   // Our current position in memory.
  35   size_t mPos;
  36
  37   // A list of memory blocks. Usually there is only one
  38   // but if we overrun our stack size we can get more memory.
  39   StackBlock* mBlocks;
  40
  41   // The current block.
  42   StackBlock* mCurBlock;
  43
  44   // Our stack of mark where push has been called.
  45   StackMark* mMarks;
  46
  47   // The current top of the mark list.
  48   uint32_t mStackTop;
  49
  50   // The size of the mark array.
  51   uint32_t mMarkLength;
  52 };
  53
  54 // Class for stack scoped arena memory allocations.
  55 //
  56 // Callers who wish to allocate memory whose lifetime corresponds to the
  57 // lifetime of a stack-allocated object can use this class.  First,
  58 // declare an AutoStackArena object on the stack.  Then all subsequent
  59 // calls to Allocate will allocate memory from an arena pool that will
  60 // be freed when that variable goes out of scope.  Nesting is allowed.
  61 //
  62 // Individual allocations cannot exceed StackBlock::MAX_USABLE_SIZE
  63 // bytes.
  64 //
  65 class MOZ_RAII AutoStackArena {
  66  public:
  67   AutoStackArena() : mOwnsStackArena(false) {
  68     if (!gStackArena) {
  69       gStackArena = new StackArena();
  70       mOwnsStackArena = true;
  71     }
  72     gStackArena->Push();
  73   }
  74
  75   ~AutoStackArena() {
  76     gStackArena->Pop();
  77     if (mOwnsStackArena) {
  78       delete gStackArena;
  79       gStackArena = nullptr;
  80     }
  81   }
  82
  83   static void* Allocate(size_t aSize) { return gStackArena->Allocate(aSize); }
  84
  85  private:
  86   static StackArena* gStackArena;
  87   bool mOwnsStackArena;
  88 };
  89
  90 }  // namespace mozilla
  91
  92 #endif