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diff --git a/xpcom/base/CountingAllocatorBase.h b/xpcom/base/CountingAllocatorBase.h
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+++ b/xpcom/base/CountingAllocatorBase.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 CountingAllocatorBase_h
+#define CountingAllocatorBase_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "nsIMemoryReporter.h"
+
+namespace mozilla {
+
+// This CRTP class handles several details of wrapping allocators and should
+// be preferred to manually counting with MOZ_DEFINE_MALLOC_SIZE_OF_ON_ALLOC
+// and MOZ_DEFINE_MALLOC_SIZE_OF_ON_FREE.  The typical use is in a memory
+// reporter for a particular third party library:
+//
+//   class MyMemoryReporter : public CountingAllocatorBase<MyMemoryReporter>
+//   {
+//     ...
+//     NS_IMETHOD
+//     CollectReports(nsIHandleReportCallback* aHandleReport,
+//                    nsISupports* aData, bool aAnonymize) override
+//     {
+//        MOZ_COLLECT_REPORT(
+//          "explicit/path/to/somewhere", KIND_HEAP, UNITS_BYTES,
+//          MemoryAllocated(),
+//          "A description of what we are reporting.");
+//
+//        return NS_OK;
+//     }
+//   };
+//
+//   ...somewhere later in the code...
+//   SetThirdPartyMemoryFunctions(MyMemoryReporter::CountingAlloc,
+//                                MyMemoryReporter::CountingFree);
+template<typename T>
+class CountingAllocatorBase
+{
+public:
+  CountingAllocatorBase()
+  {
+#ifdef DEBUG
+    // There must be only one instance of this class, due to |sAmount| being
+    // static.
+    static bool hasRun = false;
+    MOZ_ASSERT(!hasRun);
+    hasRun = true;
+#endif
+  }
+
+  static size_t
+  MemoryAllocated()
+  {
+    return sAmount;
+  }
+
+  static void*
+  CountingMalloc(size_t size)
+  {
+    void* p = malloc(size);
+    sAmount += MallocSizeOfOnAlloc(p);
+    return p;
+  }
+
+  static void*
+  CountingCalloc(size_t nmemb, size_t size)
+  {
+    void* p = calloc(nmemb, size);
+    sAmount += MallocSizeOfOnAlloc(p);
+    return p;
+  }
+
+  static void*
+  CountingRealloc(void* p, size_t size)
+  {
+    size_t oldsize = MallocSizeOfOnFree(p);
+    void *pnew = realloc(p, size);
+    if (pnew) {
+      size_t newsize = MallocSizeOfOnAlloc(pnew);
+      sAmount += newsize - oldsize;
+    } else if (size == 0) {
+      // We asked for a 0-sized (re)allocation of some existing pointer
+      // and received NULL in return.  0-sized allocations are permitted
+      // to either return NULL or to allocate a unique object per call (!).
+      // For a malloc implementation that chooses the second strategy,
+      // that allocation may fail (unlikely, but possible).
+      //
+      // Given a NULL return value and an allocation size of 0, then, we
+      // don't know if that means the original pointer was freed or if
+      // the allocation of the unique object failed.  If the original
+      // pointer was freed, then we have nothing to do here.  If the
+      // allocation of the unique object failed, the original pointer is
+      // still valid and we ought to undo the decrement from above.
+      // However, we have no way of knowing how the underlying realloc
+      // implementation is behaving.  Assuming that the original pointer
+      // was freed is the safest course of action.  We do, however, need
+      // to note that we freed memory.
+      sAmount -= oldsize;
+    } else {
+      // realloc failed.  The amount allocated hasn't changed.
+    }
+    return pnew;
+  }
+
+  // Some library code expects that realloc(x, 0) will free x, which is not
+  // the behavior of the version of jemalloc we're using, so this wrapped
+  // version of realloc is needed.
+  static void*
+  CountingFreeingRealloc(void* p, size_t size)
+  {
+    if (size == 0) {
+      CountingFree(p);
+      return nullptr;
+    }
+    return CountingRealloc(p, size);
+  }
+
+  static void
+  CountingFree(void* p)
+  {
+    sAmount -= MallocSizeOfOnFree(p);
+    free(p);
+  }
+
+private:
+  // |sAmount| can be (implicitly) accessed by multiple threads, so it
+  // must be thread-safe.
+  static Atomic<size_t> sAmount;
+
+  MOZ_DEFINE_MALLOC_SIZE_OF_ON_ALLOC(MallocSizeOfOnAlloc)
+  MOZ_DEFINE_MALLOC_SIZE_OF_ON_FREE(MallocSizeOfOnFree)
+};
+
+} // namespace mozilla
+
+#endif // CountingAllocatorBase_h