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// 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.
#include "base/trace_event/heap_profiler_allocation_register.h"
#include "base/trace_event/trace_event_memory_overhead.h"
namespace base {
namespace trace_event {
: AllocationRegister(kNumBuckets * kNumCellsPerBucket) {}
AllocationRegister::AllocationRegister(uint32_t num_cells)
// Reserve enough address space to store |num_cells_| entries if necessary,
// with a guard page after it to crash the program when attempting to store
// more entries.
: num_cells_(num_cells),
cells_(static_cast<Cell*>(AllocateVirtualMemory(num_cells_ *
AllocateVirtualMemory(kNumBuckets * sizeof(CellIndex)))),
// The free list is empty. The first unused cell is cell 1, because index
// 0 is used as list terminator.
next_unused_cell_(1) {}
AllocationRegister::~AllocationRegister() {
FreeVirtualMemory(buckets_, kNumBuckets * sizeof(CellIndex));
FreeVirtualMemory(cells_, num_cells_ * sizeof(Cell));
void AllocationRegister::Insert(void* address,
size_t size,
AllocationContext context) {
DCHECK(address != nullptr);
if (size == 0)
CellIndex* idx_ptr = Lookup(address);
// If the index is 0, the address is not yet present, so insert it.
if (*idx_ptr == 0) {
*idx_ptr = GetFreeCell();
// The address stored in a cell is const as long as it is exposed (via the
// iterators or |Get|), but because cells are re-used, a const cast is
// required to set it on insert and remove.
void* const& allocation_address = cells_[*idx_ptr].allocation.address;
const_cast<void*&>(allocation_address) = address;
cells_[*idx_ptr].next = 0;
cells_[*idx_ptr].allocation.size = size;
cells_[*idx_ptr].allocation.context = context;
void AllocationRegister::Remove(void* address) {
// Get a pointer to the index of the cell that stores |address|. The index can
// be an element of |buckets_| or the |next| member of a cell.
CellIndex* idx_ptr = Lookup(address);
CellIndex freed_idx = *idx_ptr;
// If the index is 0, the address was not there in the first place.
if (freed_idx == 0)
// The cell at the index is now free, remove it from the linked list for
// |Hash(address)|.
Cell* freed_cell = &cells_[freed_idx];
*idx_ptr = freed_cell->next;
// Put the free cell at the front of the free list.
freed_cell->next = free_list_;
free_list_ = freed_idx;
// Reset the address, so that on iteration the free cell is ignored.
const_cast<void*&>(freed_cell->allocation.address) = nullptr;
AllocationRegister::Allocation* AllocationRegister::Get(void* address) {
CellIndex* idx_ptr = Lookup(address);
// If the index is 0, the address is not present in the table.
return *idx_ptr == 0 ? nullptr : &cells_[*idx_ptr].allocation;
AllocationRegister::ConstIterator AllocationRegister::begin() const {
// Initialize the iterator's index to 0. Cell 0 never stores an entry.
ConstIterator iterator(*this, 0);
// Incrementing will advance the iterator to the first used cell.
return iterator;
AllocationRegister::ConstIterator AllocationRegister::end() const {
// Cell |next_unused_cell_ - 1| is the last cell that could contain an entry,
// so index |next_unused_cell_| is an iterator past the last element, in line
// with the STL iterator conventions.
return ConstIterator(*this, next_unused_cell_);
const AllocationRegister& alloc_register,
CellIndex index)
: register_(alloc_register), index_(index) {}
void AllocationRegister::ConstIterator::operator++() {
// Find the next cell with a non-null address until all cells that could
// possibly be used have been iterated. A null address indicates a free cell.
do {
} while (index_ < register_.next_unused_cell_ &&
register_.cells_[index_].allocation.address == nullptr);
bool AllocationRegister::ConstIterator::operator!=(
const ConstIterator& other) const {
return index_ != other.index_;
const AllocationRegister::Allocation& AllocationRegister::ConstIterator::
operator*() const {
return register_.cells_[index_].allocation;
AllocationRegister::CellIndex* AllocationRegister::Lookup(void* address) {
// The list head is in |buckets_| at the hash offset.
CellIndex* idx_ptr = &buckets_[Hash(address)];
// Chase down the list until the cell that holds |address| is found,
// or until the list ends.
while (*idx_ptr != 0 && cells_[*idx_ptr].allocation.address != address)
idx_ptr = &cells_[*idx_ptr].next;
return idx_ptr;
AllocationRegister::CellIndex AllocationRegister::GetFreeCell() {
// First try to re-use a cell from the freelist.
if (free_list_) {
CellIndex idx = free_list_;
free_list_ = cells_[idx].next;
return idx;
// Otherwise pick the next cell that has not been touched before.
CellIndex idx = next_unused_cell_;
// If the hash table has too little capacity (when too little address space
// was reserved for |cells_|), |next_unused_cell_| can be an index outside of
// the allocated storage. A guard page is allocated there to crash the
// program in that case. There are alternative solutions:
// - Deal with it, increase capacity by reallocating |cells_|.
// - Refuse to insert and let the caller deal with it.
// Because free cells are re-used before accessing fresh cells with a higher
// index, and because reserving address space without touching it is cheap,
// the simplest solution is to just allocate a humongous chunk of address
// space.
DCHECK_LT(next_unused_cell_, num_cells_ + 1);
return idx;
// static
uint32_t AllocationRegister::Hash(void* address) {
// The multiplicative hashing scheme from [Knuth 1998]. The value of |a| has
// been chosen carefully based on measurements with real-word data (addresses
// recorded from a Chrome trace run). It is the first prime after 2^17. For
// |shift|, 13, 14 and 15 yield good results. These values are tuned to 2^18
// buckets. Microbenchmarks show that this simple scheme outperforms fancy
// hashes like Murmur3 by 20 to 40 percent.
const uintptr_t key = reinterpret_cast<uintptr_t>(address);
const uintptr_t a = 131101;
const uintptr_t shift = 14;
const uintptr_t h = (key * a) >> shift;
return static_cast<uint32_t>(h) & kNumBucketsMask;
void AllocationRegister::EstimateTraceMemoryOverhead(
TraceEventMemoryOverhead* overhead) const {
// Estimate memory overhead by counting all of the cells that have ever been
// touched. Don't report mmapped memory as allocated, because it has not been
// allocated by malloc.
size_t allocated = sizeof(AllocationRegister);
size_t resident = sizeof(AllocationRegister)
// Include size of touched cells (size of |*cells_|).
+ sizeof(Cell) * next_unused_cell_
// Size of |*buckets_|.
+ sizeof(CellIndex) * kNumBuckets;
overhead->Add("AllocationRegister", allocated, resident);
} // namespace trace_event
} // namespace base