heap-profiler: Clean up some comments and C++.

base/trace_event/heap_profiler_allocation_register.h

 // 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 <stddef.h>
 #include <stdint.h>
 
@@ skipping 34 matching lines @@
  public:
   using KVPair = std::pair<const Key, Value>;
 
   // For implementation simplicity API uses integer index instead
   // of iterators. Most operations (except Find) on KVIndex are O(1).
   using KVIndex = size_t;
   enum : KVIndex { kInvalidKVIndex = static_cast<KVIndex>(-1) };
 
   // Capacity controls how many items this hash map can hold, and largely
   // affects memory footprint.
-  FixedHashMap(size_t capacity)
+  explicit FixedHashMap(size_t capacity)
       : num_cells_(capacity),
         num_inserts_dropped_(0),
         cells_(static_cast<Cell*>(
             AllocateGuardedVirtualMemory(num_cells_ * sizeof(Cell)))),
         buckets_(static_cast<Bucket*>(
             AllocateGuardedVirtualMemory(NumBuckets * sizeof(Bucket)))),
         free_list_(nullptr),
         next_unused_cell_(0) {}
 
   ~FixedHashMap() {
@@ skipping 179 matching lines @@
 };
 
 }  // namespace internal
 
 class TraceEventMemoryOverhead;
 
 // The allocation register keeps track of all allocations that have not been
 // freed. Internally it has two hashtables: one for Backtraces and one for
 // actual allocations. Sizes of both hashtables are fixed, and this class
 // allocates (mmaps) only in its constructor.
+//
+// When either hash table hits max size, new inserts are dropped.
 class BASE_EXPORT AllocationRegister {
  public:
   // Details about an allocation.
   struct Allocation {
     const void* address;
     size_t size;
     AllocationContext context;
   };
 
   // An iterator that iterates entries in no particular order.
@@ skipping 94 matching lines @@
       AddressHasher>;
 
   BacktraceMap::KVIndex InsertBacktrace(const Backtrace& backtrace);
   void RemoveBacktrace(BacktraceMap::KVIndex index);
 
   Allocation GetAllocation(AllocationMap::KVIndex) const;
 
   AllocationMap allocations_;
   BacktraceMap backtraces_;
 
-  // Sentinel used when we run out of backtraces_ storage.
-  BacktraceMap::KVIndex out_of_storage_backtrace_index_;
+  // Sentinel used when the |backtraces_| table is full.
+  //
+  // This is a slightly abstraction to allow for constant propagation. It
+  // knows that the sentinel will be the first item inserted into the table
+  // and that the first index retuned will be 0. The constructor DCHECKs
+  // this assumption.
+  enum : BacktraceMap::KVIndex { kOutOfStorageBacktraceIndex = 0 };
 
   DISALLOW_COPY_AND_ASSIGN(AllocationRegister);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_REGISTER_H_