layout/base/StackArena.h

   1 /* This Source Code Form is subject to the terms of the Mozilla Public
   2  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
   3  * You can obtain one at http://mozilla.org/MPL/2.0/. */
   4
   5 #ifndef StackArena_h
   6 #define StackArena_h
   7
   8 #include "nsError.h"
   9 #include "mozilla/Assertions.h"
  10 #include "mozilla/MemoryReporting.h"
  11 #include "mozilla/NullPtr.h"
  12
  13 namespace mozilla {
  14
  15 struct StackBlock;
  16 struct StackMark;
  17 class AutoStackArena;
  18
  19 // Private helper class for AutoStackArena.
  20 class StackArena {
  21 private:
  22   friend class AutoStackArena;
  23   StackArena();
  24   ~StackArena();
  25
  26   nsresult Init() { return mBlocks ? NS_OK : NS_ERROR_OUT_OF_MEMORY; }
  27
  28   // Memory management functions.
  29   void* Allocate(size_t aSize);
  30   void Push();
  31   void Pop();
  32
  33   size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
  34
  35   // Our current position in memory.
  36   size_t mPos;
  37
  38   // A list of memory blocks. Usually there is only one
  39   // but if we overrun our stack size we can get more memory.
  40   StackBlock* mBlocks;
  41
  42   // The current block.
  43   StackBlock* mCurBlock;
  44
  45   // Our stack of mark where push has been called.
  46   StackMark* mMarks;
  47
  48   // The current top of the mark list.
  49   uint32_t mStackTop;
  50
  51   // The size of the mark array.
  52   uint32_t mMarkLength;
  53 };
  54
  55 // Class for stack scoped arena memory allocations.
  56 //
  57 // Callers who wish to allocate memory whose lifetime corresponds to the
  58 // lifetime of a stack-allocated object can use this class.  First,
  59 // declare an AutoStackArena object on the stack.  Then all subsequent
  60 // calls to Allocate will allocate memory from an arena pool that will
  61 // be freed when that variable goes out of scope.  Nesting is allowed.
  62 //
  63 // Individual allocations cannot exceed StackBlock::MAX_USABLE_SIZE
  64 // bytes.
  65 //
  66 class MOZ_STACK_CLASS AutoStackArena {
  67 public:
  68   AutoStackArena()
  69     : mOwnsStackArena(false)
  70   {
  71     if (!gStackArena) {
  72       gStackArena = new StackArena();
  73       mOwnsStackArena = true;
  74       gStackArena->Init();
  75     }
  76     gStackArena->Push();
  77   }
  78
  79   ~AutoStackArena() {
  80     gStackArena->Pop();
  81     if (mOwnsStackArena) {
  82       delete gStackArena;
  83       gStackArena = nullptr;
  84     }
  85   }
  86
  87   static void* Allocate(size_t aSize) {
  88     return gStackArena->Allocate(aSize);
  89   }
  90
  91 private:
  92   static StackArena* gStackArena;
  93   bool mOwnsStackArena;
  94 };
  95
  96 } // namespace mozilla
  97
  98 #endif