[Tracing] Add allocation register for heap profiling

base/trace_event/memory_profiler_allocation_register.cc

+// 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/memory_profiler_allocation_register.h"
+
+namespace base {
+namespace trace_event {
+
+AllocationRegister::AllocationRegister()
+    // Reserve enough address space to store |kNumCells| entries if necessary,
+    // with a guard page after it to crash the program when attempting to store
+    // more entries.
+    : cells_(static_cast<Cell*>(AllocateVirtualMemory(kNumCells *
+                                                      sizeof(Cell)))),
+      buckets_(static_cast<CellIndex*>(
+          AllocateVirtualMemory(kNumBuckets * sizeof(CellIndex)))),
+
+      // The free list is empty. The first unused cell is cell 1, because index
+      // 0 is used as list terminator.
+      free_list_(0),
+      next_unused_cell_(1) {}
+
+AllocationRegister::~AllocationRegister() {
+  FreeVirtualMemory(buckets_, kNumBuckets * sizeof(CellIndex));
+  FreeVirtualMemory(cells_, kNumCells * sizeof(Cell));
+}
+
+void AllocationRegister::Insert(void* address,
+                                size_t size,
+                                AllocationContext context) {
+  DCHECK(address != nullptr);
+
+  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();
+
+    cells_[*idx_ptr].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)
+    return;
+
+  // 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.
+  freed_cell->allocation.address = nullptr;
+}
+
+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.
+  ++iterator;
+  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_);
+}
+
+AllocationRegister::ConstIterator::ConstIterator(
+    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 {
+    index_++;
+  } 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 |key| 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_;
+  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_, kNumCells + 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;
+}
+
+}  // namespace trace_event
+}  // namespace base