xpcom/base/SizeOfState.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
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef SizeOfState_h
   8 #define SizeOfState_h
   9
  10 #include "mozilla/fallible.h"
  11 #include "mozilla/MemoryReporting.h"
  12 #include "mozilla/Unused.h"
  13 #include "nsTHashtable.h"
  14 #include "nsHashKeys.h"
  15
  16 // This file includes types that are useful during memory reporting, but which
  17 // cannot be put into mfbt/MemoryReporting.h because they depend on things that
  18 // are not in MFBT.
  19
  20 namespace mozilla {
  21
  22 // A table of seen pointers. Useful when measuring structures that contain
  23 // nodes that may be pointed to from multiple places, e.g. via RefPtr (in C++
  24 // code) or Arc (in Rust code).
  25 class SeenPtrs : public nsTHashtable<nsPtrHashKey<const void>> {
  26  public:
  27   // Returns true if we have seen this pointer before, false otherwise. Also
  28   // remembers this pointer for later queries.
  29   bool HaveSeenPtr(const void* aPtr) {
  30     uint32_t oldCount = Count();
  31
  32     mozilla::Unused << PutEntry(aPtr, fallible);
  33
  34     // If the counts match, there are two possibilities.
  35     //
  36     // - Lookup succeeded: we've seen the pointer before, and didn't need to
  37     //   add a new entry.
  38     //
  39     // - PutEntry() tried to add the entry and failed due to lack of memory. In
  40     //   this case we can't tell if this pointer has been seen before (because
  41     //   the table is in an unreliable state and may have dropped previous
  42     //   insertions). When doing memory reporting it's better to err on the
  43     //   side of under-reporting rather than over-reporting, so we assume we've
  44     //   seen the pointer before.
  45     //
  46     return oldCount == Count();
  47   }
  48 };
  49
  50 // Memory reporting state. Some memory measuring functions
  51 // (SizeOfIncludingThis(), etc.) just need a MallocSizeOf parameter, but some
  52 // also need a record of pointers that have been seen and should not be
  53 // re-measured. This class encapsulates both of those things.
  54 class SizeOfState {
  55  public:
  56   explicit SizeOfState(MallocSizeOf aMallocSizeOf)
  57       : mMallocSizeOf(aMallocSizeOf) {}
  58
  59   bool HaveSeenPtr(const void* aPtr) { return mSeenPtrs.HaveSeenPtr(aPtr); }
  60
  61   MallocSizeOf mMallocSizeOf;
  62   SeenPtrs mSeenPtrs;
  63 };
  64
  65 }  // namespace mozilla
  66
  67 #endif  // SizeOfState_h