// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_REGISTER_H_
#define BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_REGISTER_H_

#include <stdint.h>

#include "base/logging.h"
#include "base/macros.h"
#include "base/trace_event/heap_profiler_allocation_context.h"

namespace base {
namespace trace_event {

class TraceEventMemoryOverhead;

// The allocation register keeps track of all allocations that have not been
// freed. It is a memory map-backed hash table that stores size and context
// indexed by address. The hash table is tailored specifically for this use
// case. The common case is that an entry is inserted and removed after a
// while, lookup without modifying the table is not an intended use case. The
// hash table is implemented as an array of linked lists. The size of this
// array is fixed, but it does not limit the amount of entries that can be
// stored.
//
// Replaying a recording of Chrome's allocations and frees against this hash
// table takes about 15% of the time that it takes to replay them against
// |std::map|.
class BASE_EXPORT AllocationRegister {
 public:
  // The data stored in the hash table;
  // contains the details about an allocation.
  struct Allocation {
    void* address;
    size_t size;
    AllocationContext context;
  };

  // An iterator that iterates entries in the hash table efficiently, but in no
  // particular order. It can do this by iterating the cells and ignoring the
  // linked lists altogether. Instead of checking whether a cell is in the free
  // list to see if it should be skipped, a null address is used to indicate
  // that a cell is free.
  class BASE_EXPORT ConstIterator {
   public:
    void operator++();
    bool operator!=(const ConstIterator& other) const;
    const Allocation& operator*() const;

   private:
    friend class AllocationRegister;
    using CellIndex = uint32_t;

    ConstIterator(const AllocationRegister& alloc_register, CellIndex index);

    const AllocationRegister& register_;
    CellIndex index_;
  };

  AllocationRegister();
  ~AllocationRegister();

  // Inserts allocation details into the table. If the address was present
  // already, its details are updated. |address| must not be null. (This is
  // because null is used to mark free cells, to allow efficient iteration of
  // the hash table.)
  void Insert(void* address, size_t size, AllocationContext context);

  // Removes the address from the table if it is present. It is ok to call this
  // with a null pointer.
  void Remove(void* address);

  ConstIterator begin() const;
  ConstIterator end() const;

  // Estimates memory overhead including |sizeof(AllocationRegister)|.
  void EstimateTraceMemoryOverhead(TraceEventMemoryOverhead* overhead) const;

 private:
  friend class AllocationRegisterTest;
  using CellIndex = uint32_t;

  // A cell can store allocation details (size and context) by address. Cells
  // are part of a linked list via the |next| member. This list is either the
  // list for a particular hash, or the free list. All cells are contiguous in
  // memory in one big array. Therefore, on 64-bit systems, space can be saved
  // by storing 32-bit indices instead of pointers as links. Index 0 is used as
  // the list terminator.
  struct Cell {
    CellIndex next;
    Allocation allocation;
  };

  // The number of buckets, 2^18, approximately 260 000, has been tuned for
  // Chrome's typical number of outstanding allocations. (This number varies
  // between processes. Most processes have a sustained load of ~30k unfreed
  // allocations, but some processes have peeks around 100k-400k allocations.)
  // Because of the size of the table, it is likely that every |buckets_|
  // access and every |cells_| access will incur a cache miss. Microbenchmarks
  // suggest that it is worthwile to use more memory for the table to avoid
  // chasing down the linked list, until the size is 2^18. The number of buckets
  // is a power of two so modular indexing can be done with bitwise and.
  static const uint32_t kNumBuckets = 0x40000;
  static const uint32_t kNumBucketsMask = kNumBuckets - 1;

  // Reserve address space to store at most this number of entries. High
  // capacity does not imply high memory usage due to the access pattern. The
  // only constraint on the number of cells is that on 32-bit systems address
  // space is scarce (i.e. reserving 2GiB of address space for the entries is
  // not an option). A value of ~3M entries is large enough to handle spikes in
  // the number of allocations, and modest enough to require no more than a few
  // dozens of MiB of address space.
  static const uint32_t kNumCells = kNumBuckets * 10;

  // Returns a value in the range [0, kNumBuckets - 1] (inclusive).
  static uint32_t Hash(void* address);

  // Allocates a region of virtual address space of |size| rounded up to the
  // system page size. The memory is zeroed by the system. A guard page is
  // added after the end.
  static void* AllocateVirtualMemory(size_t size);

  // Frees a region of virtual address space allocated by a call to
  // |AllocateVirtualMemory|.
  static void FreeVirtualMemory(void* address, size_t allocated_size);

  // Returns a pointer to the variable that contains or should contain the
  // index of the cell that stores the entry for |address|. The pointer may
  // point at an element of |buckets_| or at the |next| member of an element of
  // |cells_|. If the value pointed at is 0, |address| is not in the table.
  CellIndex* Lookup(void* address);

  // Takes a cell that is not being used to store an entry (either by recycling
  // from the free list or by taking a fresh cell) and returns its index.
  CellIndex GetFreeCell();

  // The array of cells. This array is backed by mmapped memory. Lower indices
  // are accessed first, higher indices are only accessed when required. In
  // this way, even if a huge amount of address space has been mmapped, only
  // the cells that are actually used will be backed by physical memory.
  Cell* const cells_;

  // The array of indices into |cells_|. |buckets_[Hash(address)]| will contain
  // the index of the head of the linked list for |Hash(address)|. A value of 0
  // indicates an empty list. This array is backed by mmapped memory.
  CellIndex* const buckets_;

  // The head of the free list. This is the index of the cell. A value of 0
  // means that the free list is empty.
  CellIndex free_list_;

  // The index of the first element of |cells_| that has not been used before.
  // If the free list is empty and a new cell is needed, the cell at this index
  // is used. This is the high water mark for the number of entries stored.
  CellIndex next_unused_cell_;

  DISALLOW_COPY_AND_ASSIGN(AllocationRegister);
};

}  // namespace trace_event
}  // namespace base

#endif  // BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_REGISTER_H_