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diff --git a/layout/base/StackArena.h b/layout/base/StackArena.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef StackArena_h
+#define StackArena_h
+
+#include "nsError.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/MemoryReporting.h"
+
+namespace mozilla {
+
+struct StackBlock;
+struct StackMark;
+class AutoStackArena;
+
+// Private helper class for AutoStackArena.
+class StackArena {
+ private:
+ friend class AutoStackArena;
+ StackArena();
+ ~StackArena();
+
+ // Memory management functions.
+ void* Allocate(size_t aSize);
+ void Push();
+ void Pop();
+
+ size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+ // Our current position in memory.
+ size_t mPos;
+
+ // A list of memory blocks. Usually there is only one
+ // but if we overrun our stack size we can get more memory.
+ StackBlock* mBlocks;
+
+ // The current block.
+ StackBlock* mCurBlock;
+
+ // Our stack of mark where push has been called.
+ StackMark* mMarks;
+
+ // The current top of the mark list.
+ uint32_t mStackTop;
+
+ // The size of the mark array.
+ uint32_t mMarkLength;
+};
+
+// Class for stack scoped arena memory allocations.
+//
+// Callers who wish to allocate memory whose lifetime corresponds to the
+// lifetime of a stack-allocated object can use this class. First,
+// declare an AutoStackArena object on the stack. Then all subsequent
+// calls to Allocate will allocate memory from an arena pool that will
+// be freed when that variable goes out of scope. Nesting is allowed.
+//
+// Individual allocations cannot exceed StackBlock::MAX_USABLE_SIZE
+// bytes.
+//
+class MOZ_RAII AutoStackArena {
+ public:
+ AutoStackArena() : mOwnsStackArena(false) {
+ if (!gStackArena) {
+ gStackArena = new StackArena();
+ mOwnsStackArena = true;
+ }
+ gStackArena->Push();
+ }
+
+ ~AutoStackArena() {
+ gStackArena->Pop();
+ if (mOwnsStackArena) {
+ delete gStackArena;
+ gStackArena = nullptr;
+ }
+ }
+
+ static void* Allocate(size_t aSize) { return gStackArena->Allocate(aSize); }
+
+ private:
+ static StackArena* gStackArena;
+ bool mOwnsStackArena;
+};
+
+} // namespace mozilla
+
+#endif